    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017
    018package org.apache.commons.logging;
    019
    020import java.io.FileOutputStream;
    021import java.io.IOException;
    022import java.io.InputStream;
    023import java.io.PrintStream;
    024import java.lang.ref.WeakReference;
    025import java.net.URL;
    026import java.net.URLConnection;
    027import java.nio.charset.StandardCharsets;
    028import java.security.AccessController;
    029import java.security.PrivilegedAction;
    030import java.util.Enumeration;
    031import java.util.Hashtable;
    032import java.util.Iterator;
    033import java.util.Objects;
    034import java.util.Properties;
    035import java.util.ServiceConfigurationError;
    036import java.util.ServiceLoader;
    037
    038/**
    039 * Factory for creating {@link Log} instances, with discovery and
    040 * configuration features similar to that employed by standard Java APIs
    041 * such as JAXP.
    042 * <p>
    043 * <strong>IMPLEMENTATION NOTE</strong> - This implementation is heavily
    044 * based on the SAXParserFactory and DocumentBuilderFactory implementations
    045 * (corresponding to the JAXP pluggability APIs) found in Apache Xerces.
    046 */
    047public abstract class LogFactory {
    048    // Implementation note re AccessController usage
    049    //
    050    // It is important to keep code invoked via an AccessController to small
    051    // auditable blocks. Such code must carefully evaluate all user input
    052    // (parameters, system properties, configuration file contents, etc). As an
    053    // example, a Log implementation should not write to its log file
    054    // with an AccessController anywhere in the call stack, otherwise an
    055    // insecure application could configure the log implementation to write
    056    // to a protected file using the privileges granted to JCL rather than
    057    // to the calling application.
    058    //
    059    // Under no circumstance should a non-private method return data that is
    060    // retrieved via an AccessController. That would allow an insecure application
    061    // to invoke that method and obtain data that it is not permitted to have.
    062    //
    063    // Invoking user-supplied code with an AccessController set is not a major
    064    // issue (for example, invoking the constructor of the class specified by
    065    // HASHTABLE_IMPLEMENTATION_PROPERTY). That class will be in a different
    066    // trust domain, and therefore must have permissions to do whatever it
    067    // is trying to do regardless of the permissions granted to JCL. There is
    068    // a slight issue in that untrusted code may point that environment variable
    069    // to another trusted library, in which case the code runs if both that
    070    // library and JCL have the necessary permissions even when the untrusted
    071    // caller does not. That's a pretty hard route to exploit though.
    072
    073    /**
    074     * The name ({@code priority}) of the key in the configuration file used to
    075     * specify the priority of that particular configuration file. The associated value
    076     * is a floating-point number; higher values take priority over lower values.
    077     */
    078    public static final String PRIORITY_KEY = "priority";
    079
    080    /**
    081     * The name ({@code use_tccl}) of the key in the configuration file used
    082     * to specify whether logging classes should be loaded via the thread
    083     * context class loader (TCCL), or not. By default, the TCCL is used.
    084     */
    085    public static final String TCCL_KEY = "use_tccl";
    086
    087    /**
    088     * The name ({@code org.apache.commons.logging.LogFactory}) of the property
    089     * used to identify the LogFactory implementation
    090     * class name. This can be used as a system property, or as an entry in a
    091     * configuration properties file.
    092     */
    093    public static final String FACTORY_PROPERTY = "org.apache.commons.logging.LogFactory";
    094
    095    private static final String FACTORY_LOG4J_API = "org.apache.commons.logging.impl.Log4jApiLogFactory";
    096    private static final String LOG4J_API_LOGGER = "org.apache.logging.log4j.Logger";
    097    private static final String LOG4J_TO_SLF4J_BRIDGE = "org.apache.logging.slf4j.SLF4JProvider";
    098
    099    private static final String FACTORY_SLF4J = "org.apache.commons.logging.impl.Slf4jLogFactory";
    100    private static final String SLF4J_API_LOGGER = "org.slf4j.Logger";
    101
    102    /**
    103     * The fully qualified class name of the fallback {@code LogFactory}
    104     * implementation class to use, if no other can be found.
    105     */
    106    public static final String FACTORY_DEFAULT = "org.apache.commons.logging.impl.LogFactoryImpl";
    107
    108    /**
    109     * The name ({@code commons-logging.properties}) of the properties file to search for.
    110     */
    111    public static final String FACTORY_PROPERTIES = "commons-logging.properties";
    112
    113    /**
    114     * JDK 1.3+ <a href="https://java.sun.com/j2se/1.3/docs/guide/jar/jar.html#Service%20Provider">
    115     * 'Service Provider' specification</a>.
    116     */
    117    protected static final String SERVICE_ID =
    118        "META-INF/services/org.apache.commons.logging.LogFactory";
    119
    120    /**
    121     * The name ({@code org.apache.commons.logging.diagnostics.dest})
    122     * of the property used to enable internal commons-logging
    123     * diagnostic output, in order to get information on what logging
    124     * implementations are being discovered, what class loaders they
    125     * are loaded through, etc.
    126     * <p>
    127     * If a system property of this name is set then the value is
    128     * assumed to be the name of a file. The special strings
    129     * STDOUT or STDERR (case-sensitive) indicate output to
    130     * System.out and System.err respectively.
    131     * <p>
    132     * Diagnostic logging should be used only to debug problematic
    133     * configurations and should not be set in normal production use.
    134     */
    135    public static final String DIAGNOSTICS_DEST_PROPERTY =
    136        "org.apache.commons.logging.diagnostics.dest";
    137
    138    /**
    139     * When null (the usual case), no diagnostic output will be
    140     * generated by LogFactory or LogFactoryImpl. When non-null,
    141     * interesting events will be written to the specified object.
    142     */
    143    private static final PrintStream DIAGNOSTICS_STREAM;
    144
    145    /**
    146     * A string that gets prefixed to every message output by the
    147     * logDiagnostic method, so that users can clearly see which
    148     * LogFactory class is generating the output.
    149     */
    150    private static final String diagnosticPrefix;
    151
    152    /**
    153     * Setting this system property
    154     * ({@code org.apache.commons.logging.LogFactory.HashtableImpl})
    155     * value allows the {@code Hashtable} used to store
    156     * class loaders to be substituted by an alternative implementation.
    157     * <p>
    158     * <strong>Note:</strong> {@code LogFactory} will print:
    159     * <pre>
    160     * [ERROR] LogFactory: Load of custom hashtable failed
    161     * </pre>
    162     * to system error and then continue using a standard Hashtable.
    163     * <p>
    164     * <strong>Usage:</strong> Set this property when Java is invoked
    165     * and {@code LogFactory} will attempt to load a new instance
    166     * of the given implementation class.
    167     * For example, running the following ant scriplet:
    168     * <pre>
    169     *  &lt;java classname="${test.runner}" fork="yes" failonerror="${test.failonerror}"&gt;
    170     *     ...
    171     *     &lt;sysproperty
    172     *        key="org.apache.commons.logging.LogFactory.HashtableImpl"
    173     *        value="org.apache.commons.logging.AltHashtable"/&gt;
    174     *  &lt;/java&gt;
    175     * </pre>
    176     * will mean that {@code LogFactory} will load an instance of
    177     * {@code org.apache.commons.logging.AltHashtable}.
    178     * <p>
    179     * A typical use case is to allow a custom
    180     * Hashtable implementation using weak references to be substituted.
    181     * This will allow class loaders to be garbage collected without
    182     * the need to release them (on 1.3+ JVMs only, of course ;).
    183     */
    184    public static final String HASHTABLE_IMPLEMENTATION_PROPERTY =
    185        "org.apache.commons.logging.LogFactory.HashtableImpl";
    186
    187    /** Name used to load the weak hashtable implementation by names. */
    188    private static final String WEAK_HASHTABLE_CLASSNAME =
    189        "org.apache.commons.logging.impl.WeakHashtable";
    190
    191    /**
    192     * A reference to the class loader that loaded this class. This is the
    193     * same as LogFactory.class.getClassLoader(). However computing this
    194     * value isn't quite as simple as that, as we potentially need to use
    195     * AccessControllers etc. It's more efficient to compute it once and
    196     * cache it here.
    197     */
    198    private static final WeakReference<ClassLoader> thisClassLoaderRef;
    199
    200    /**
    201     * Maximum number of {@link ServiceLoader} errors to ignore, while
    202     * looking for an implementation.
    203     */
    204    private static final int MAX_BROKEN_SERVICES = 3;
    205
    206    /**
    207     * The previously constructed {@code LogFactory} instances, keyed by
    208     * the {@code ClassLoader} with which it was created.
    209     */
    210    protected static Hashtable<ClassLoader, LogFactory> factories;
    211
    212    /**
    213     * Previously constructed {@code LogFactory} instance as in the
    214     * {@code factories} map, but for the case where
    215     * {@code getClassLoader} returns {@code null}.
    216     * This can happen when:
    217     * <ul>
    218     * <li>using JDK1.1 and the calling code is loaded via the system
    219     *  class loader (very common)</li>
    220     * <li>using JDK1.2+ and the calling code is loaded via the boot
    221     *  class loader (only likely for embedded systems work).</li>
    222     * </ul>
    223     * Note that {@code factories} is a <i>Hashtable</i> (not a HashMap),
    224     * and hashtables don't allow null as a key.
    225     * @deprecated since 1.1.2
    226     */
    227    @Deprecated
    228    protected static volatile LogFactory nullClassLoaderFactory;
    229
    230    static {
    231        // note: it's safe to call methods before initDiagnostics (though
    232        // diagnostic output gets discarded).
    233        final ClassLoader thisClassLoader = getClassLoader(LogFactory.class);
    234        thisClassLoaderRef = new WeakReference<>(thisClassLoader);
    235        // In order to avoid confusion where multiple instances of JCL are
    236        // being used via different class loaders within the same app, we
    237        // ensure each logged message has a prefix of form
    238        // [LogFactory from class loader OID]
    239        //
    240        // Note that this prefix should be kept consistent with that
    241        // in LogFactoryImpl. However here we don't need to output info
    242        // about the actual *instance* of LogFactory, as all methods that
    243        // output diagnostics from this class are static.
    244        String classLoaderName;
    245        try {
    246            if (thisClassLoader == null) {
    247                classLoaderName = "BOOTLOADER";
    248            } else {
    249                classLoaderName = objectId(thisClassLoader);
    250            }
    251        } catch (final SecurityException e) {
    252            classLoaderName = "UNKNOWN";
    253        }
    254        diagnosticPrefix = "[LogFactory from " + classLoaderName + "] ";
    255        DIAGNOSTICS_STREAM = initDiagnostics();
    256        logClassLoaderEnvironment(LogFactory.class);
    257        factories = createFactoryStore();
    258        if (isDiagnosticsEnabled()) {
    259            logDiagnostic("BOOTSTRAP COMPLETED");
    260        }
    261    }
    262
    263    /**
    264     * Remember this factory, so later calls to LogFactory.getCachedFactory
    265     * can return the previously created object (together with all its
    266     * cached Log objects).
    267     *
    268     * @param classLoader should be the current context class loader. Note that
    269     *  this can be null under some circumstances; this is ok.
    270     * @param factory should be the factory to cache. This should never be null.
    271     */
    272    private static void cacheFactory(final ClassLoader classLoader, final LogFactory factory) {
    273        // Ideally we would assert(factory != null) here. However reporting
    274        // errors from within a logging implementation is a little tricky!
    275
    276        if (factory != null) {
    277            if (classLoader == null) {
    278                nullClassLoaderFactory = factory;
    279            } else {
    280                factories.put(classLoader, factory);
    281            }
    282        }
    283    }
    284
    285    /**
    286     * Implements the operations described in the Javadoc for newFactory.
    287     *
    288     * @param factoryClassName Factory class.
    289     * @param classLoader      used to load the specified factory class. This is expected to be either the TCCL or the class loader which loaded this class.
    290     *                         Note that the class loader which loaded this class might be "null" (for example, the boot loader) for embedded systems.
    291     * @return either a LogFactory object or a LogConfigurationException object.
    292     * @since 1.1
    293     */
    294    protected static Object createFactory(final String factoryClassName, final ClassLoader classLoader) {
    295        // This will be used to diagnose bad configurations
    296        // and allow a useful message to be sent to the user
    297        Class<?> logFactoryClass = null;
    298        try {
    299            if (classLoader != null) {
    300                try {
    301                    // First the given class loader param (thread class loader)
    302
    303                    // Warning: must typecast here & allow exception
    304                    // to be generated/caught & recast properly.
    305                    logFactoryClass = classLoader.loadClass(factoryClassName);
    306                    if (LogFactory.class.isAssignableFrom(logFactoryClass)) {
    307                        if (isDiagnosticsEnabled()) {
    308                            logDiagnostic("Loaded class " + logFactoryClass.getName() + " from class loader " + objectId(classLoader));
    309                        }
    310                    } else //
    311                    // This indicates a problem with the ClassLoader tree.
    312                    // An incompatible ClassLoader was used to load the
    313                    // implementation.
    314                    // As the same classes
    315                    // must be available in multiple class loaders,
    316                    // it is very likely that multiple JCL jars are present.
    317                    // The most likely fix for this
    318                    // problem is to remove the extra JCL jars from the
    319                    // ClassLoader hierarchy.
    320                    //
    321                    if (isDiagnosticsEnabled()) {
    322                        logDiagnostic("Factory class " + logFactoryClass.getName() + " loaded from class loader " + objectId(logFactoryClass.getClassLoader())
    323                                + " does not extend '" + LogFactory.class.getName() + "' as loaded by this class loader.");
    324                        logHierarchy("[BAD CL TREE] ", classLoader);
    325                    }
    326
    327                    return logFactoryClass.getConstructor().newInstance();
    328
    329                } catch (final ClassNotFoundException ex) {
    330                    if (classLoader == thisClassLoaderRef.get()) {
    331                        // Nothing more to try, onwards.
    332                        if (isDiagnosticsEnabled()) {
    333                            logDiagnostic("Unable to locate any class called '" + factoryClassName + "' via class loader " + objectId(classLoader));
    334                        }
    335                        throw ex;
    336                    }
    337                    // ignore exception, continue
    338                } catch (final NoClassDefFoundError e) {
    339                    if (classLoader == thisClassLoaderRef.get()) {
    340                        // Nothing more to try, onwards.
    341                        if (isDiagnosticsEnabled()) {
    342                            logDiagnostic("Class '" + factoryClassName + "' cannot be loaded" + " via class loader " + objectId(classLoader)
    343                                    + " - it depends on some other class that cannot be found.");
    344                        }
    345                        throw e;
    346                    }
    347                    // ignore exception, continue
    348                } catch (final ClassCastException e) {
    349                    if (classLoader == thisClassLoaderRef.get()) {
    350                        // There's no point in falling through to the code below that
    351                        // tries again with thisClassLoaderRef, because we've just tried
    352                        // loading with that loader (not the TCCL). Just throw an
    353                        // appropriate exception here.
    354
    355                        final boolean implementsLogFactory = implementsLogFactory(logFactoryClass);
    356
    357                        //
    358                        // Construct a good message: users may not actual expect that a custom implementation
    359                        // has been specified. Several well known containers use this mechanism to adapt JCL
    360                        // to their native logging system.
    361                        //
    362                        final StringBuilder msg = new StringBuilder();
    363                        msg.append("The application has specified that a custom LogFactory implementation ");
    364                        msg.append("should be used but Class '");
    365                        msg.append(factoryClassName);
    366                        msg.append("' cannot be converted to '");
    367                        msg.append(LogFactory.class.getName());
    368                        msg.append("'. ");
    369                        if (implementsLogFactory) {
    370                            msg.append("The conflict is caused by the presence of multiple LogFactory classes ");
    371                            msg.append("in incompatible class loaders. ");
    372                            msg.append("Background can be found in https://commons.apache.org/logging/tech.html. ");
    373                            msg.append("If you have not explicitly specified a custom LogFactory then it is likely ");
    374                            msg.append("that the container has set one without your knowledge. ");
    375                            msg.append("In this case, consider using the commons-logging-adapters.jar file or ");
    376                            msg.append("specifying the standard LogFactory from the command line. ");
    377                        } else {
    378                            msg.append("Please check the custom implementation. ");
    379                        }
    380                        msg.append("Help can be found at https://commons.apache.org/logging/troubleshooting.html.");
    381
    382                        if (isDiagnosticsEnabled()) {
    383                            logDiagnostic(msg.toString());
    384                        }
    385
    386                        throw new ClassCastException(msg.toString());
    387                    }
    388
    389                    // Ignore exception, continue. Presumably the class loader was the
    390                    // TCCL; the code below will try to load the class via thisClassLoaderRef.
    391                    // This will handle the case where the original calling class is in
    392                    // a shared classpath but the TCCL has a copy of LogFactory and the
    393                    // specified LogFactory implementation; we will fall back to using the
    394                    // LogFactory implementation from the same class loader as this class.
    395                    //
    396                    // Issue: this doesn't handle the reverse case, where this LogFactory
    397                    // is in the webapp, and the specified LogFactory implementation is
    398                    // in a shared classpath. In that case:
    399                    // (a) the class really does implement LogFactory (bad log msg above)
    400                    // (b) the fallback code will result in exactly the same problem.
    401                }
    402            }
    403
    404            /*
    405             * At this point, either classLoader == null, OR classLoader was unable to load factoryClass.
    406             *
    407             * In either case, we call Class.forName, which is equivalent to LogFactory.class.getClassLoader().load(name), that is, we ignore the class loader
    408             * parameter the caller passed, and fall back to trying the class loader associated with this class. See the Javadoc for the newFactory method for
    409             * more info on the consequences of this.
    410             *
    411             * Notes: * LogFactory.class.getClassLoader() may return 'null' if LogFactory is loaded by the bootstrap class loader.
    412             */
    413            // Warning: must typecast here & allow exception
    414            // to be generated/caught & recast properly.
    415            if (isDiagnosticsEnabled()) {
    416                logDiagnostic(
    417                        "Unable to load factory class via class loader " + objectId(classLoader) + " - trying the class loader associated with this LogFactory.");
    418            }
    419            logFactoryClass = Class.forName(factoryClassName);
    420            return logFactoryClass.newInstance();
    421        } catch (final Exception e) {
    422            // Check to see if we've got a bad configuration
    423            if (isDiagnosticsEnabled()) {
    424                logDiagnostic("Unable to create LogFactory instance.");
    425            }
    426            if (logFactoryClass != null && !LogFactory.class.isAssignableFrom(logFactoryClass)) {
    427                return new LogConfigurationException("The chosen LogFactory implementation does not extend LogFactory." + " Please check your configuration.",
    428                        e);
    429            }
    430            return new LogConfigurationException(e);
    431        }
    432    }
    433
    434    /**
    435     * Create the hashtable which will be used to store a map of
    436     * (context class loader -> logfactory-object). Version 1.2+ of Java
    437     * supports "weak references", allowing a custom Hashtable class
    438     * to be used which uses only weak references to its keys. Using weak
    439     * references can fix memory leaks on webapp unload in some cases (though
    440     * not all). Version 1.1 of Java does not support weak references, so we
    441     * must dynamically determine which we are using. And just for fun, this
    442     * code also supports the ability for a system property to specify an
    443     * arbitrary Hashtable implementation name.
    444     * <p>
    445     * Note that the correct way to ensure no memory leaks occur is to ensure
    446     * that LogFactory.release(contextClassLoader) is called whenever a
    447     * webapp is undeployed.
    448     */
    449    private static Hashtable<ClassLoader, LogFactory> createFactoryStore() {
    450        Hashtable<ClassLoader, LogFactory> result = null;
    451        String storeImplementationClass;
    452        try {
    453            storeImplementationClass = getSystemProperty(HASHTABLE_IMPLEMENTATION_PROPERTY, null);
    454        } catch (final SecurityException ex) {
    455            // Permissions don't allow this to be accessed. Default to the "modern"
    456            // weak hashtable implementation if it is available.
    457            storeImplementationClass = null;
    458        }
    459
    460        if (storeImplementationClass == null) {
    461            storeImplementationClass = WEAK_HASHTABLE_CLASSNAME;
    462        }
    463        try {
    464            final Class<Hashtable<ClassLoader, LogFactory>> implementationClass = (Class<Hashtable<ClassLoader, LogFactory>>) Class
    465                    .forName(storeImplementationClass);
    466            result = implementationClass.getConstructor().newInstance();
    467        } catch (final Throwable t) {
    468            handleThrowable(t); // may re-throw t
    469
    470            // ignore
    471            if (!WEAK_HASHTABLE_CLASSNAME.equals(storeImplementationClass)) {
    472                // if the user's trying to set up a custom implementation, give a clue
    473                if (isDiagnosticsEnabled()) {
    474                    // use internal logging to issue the warning
    475                    logDiagnostic("[ERROR] LogFactory: Load of custom Hashtable failed");
    476                } else {
    477                    // we *really* want this output, even if diagnostics weren't
    478                    // explicitly enabled by the user.
    479                    System.err.println("[ERROR] LogFactory: Load of custom Hashtable failed");
    480                }
    481            }
    482        }
    483        if (result == null) {
    484            result = new Hashtable<>();
    485        }
    486        return result;
    487    }
    488
    489    /**
    490     * Gets the thread context class loader if available; otherwise return null.
    491     * <p>
    492     * Most/all code should call getContextClassLoaderInternal rather than
    493     * calling this method directly.
    494     * <p>
    495     * The thread context class loader is available for JDK 1.2
    496     * or later, if certain security conditions are met.
    497     * <p>
    498     * Note that no internal logging is done within this method because
    499     * this method is called every time LogFactory.getLogger() is called,
    500     * and we don't want too much output generated here.
    501     *
    502     * @throws LogConfigurationException if a suitable class loader
    503     *  cannot be identified.
    504     * @return the thread's context class loader or {@code null} if the Java security
    505     *  policy forbids access to the context class loader from one of the classes
    506     *  in the current call stack.
    507     * @since 1.1
    508     */
    509    protected static ClassLoader directGetContextClassLoader() throws LogConfigurationException {
    510        ClassLoader classLoader = null;
    511
    512        try {
    513            classLoader = Thread.currentThread().getContextClassLoader();
    514        } catch (final SecurityException ignore) {
    515            //
    516            // getContextClassLoader() throws SecurityException when
    517            // the context class loader isn't an ancestor of the
    518            // calling class's class loader, or if security
    519            // permissions are restricted.
    520            //
    521            // We ignore this exception to be consistent with the previous
    522            // behavior (e.g. 1.1.3 and earlier).
    523            //
    524            // ignore
    525        }
    526
    527        // Return the selected class loader
    528        return classLoader;
    529    }
    530
    531    /**
    532     * Check cached factories (keyed by contextClassLoader)
    533     *
    534     * @param contextClassLoader is the context class loader associated
    535     * with the current thread. This allows separate LogFactory objects
    536     * per component within a container, provided each component has
    537     * a distinct context class loader set. This parameter may be null
    538     * in JDK1.1, and in embedded systems where jcl-using code is
    539     * placed in the bootclasspath.
    540     *
    541     * @return the factory associated with the specified class loader if
    542     *  one has previously been created, or null if this is the first time
    543     *  we have seen this particular class loader.
    544     */
    545    private static LogFactory getCachedFactory(final ClassLoader contextClassLoader) {
    546        if (contextClassLoader == null) {
    547            // We have to handle this specially, as factories is a Hashtable
    548            // and those don't accept null as a key value.
    549            //
    550            // nb: nullClassLoaderFactory might be null. That's ok.
    551            return nullClassLoaderFactory;
    552        }
    553        return factories.get(contextClassLoader);
    554    }
    555
    556    /**
    557     * Safely get access to the class loader for the specified class.
    558     * <p>
    559     * Theoretically, calling getClassLoader can throw a security exception,
    560     * and so should be done under an AccessController in order to provide
    561     * maximum flexibility. However in practice people don't appear to use
    562     * security policies that forbid getClassLoader calls. So for the moment
    563     * all code is written to call this method rather than Class.getClassLoader,
    564     * so that we could put AccessController stuff in this method without any
    565     * disruption later if we need to.
    566     * </p>
    567     * <p>
    568     * Even when using an AccessController, however, this method can still
    569     * throw SecurityException. Commons Logging basically relies on the
    570     * ability to access class loaders. A policy that forbids all
    571     * class loader access will also prevent commons-logging from working:
    572     * currently this method will throw an exception preventing the entire app
    573     * from starting up. Maybe it would be good to detect this situation and
    574     * just disable all commons-logging? Not high priority though - as stated
    575     * above, security policies that prevent class loader access aren't common.
    576     * </p>
    577     * <p>
    578     * Note that returning an object fetched via an AccessController would
    579     * technically be a security flaw anyway; untrusted code that has access
    580     * to a trusted JCL library could use it to fetch the class loader for
    581     * a class even when forbidden to do so directly.
    582     * </p>
    583     *
    584     * @param clazz Class.
    585     * @return a ClassLoader.
    586     *
    587     * @since 1.1
    588     */
    589    protected static ClassLoader getClassLoader(final Class<?> clazz) {
    590        try {
    591            return clazz.getClassLoader();
    592        } catch (final SecurityException ex) {
    593            if (isDiagnosticsEnabled()) {
    594                logDiagnostic("Unable to get class loader for class '" + clazz + "' due to security restrictions - " + ex.getMessage());
    595            }
    596            throw ex;
    597        }
    598    }
    599
    600    /**
    601     * Locate a user-provided configuration file.
    602     * <p>
    603     * The classpath of the specified classLoader (usually the context class loader)
    604     * is searched for properties files of the specified name. If none is found,
    605     * null is returned. If more than one is found, then the file with the greatest
    606     * value for its PRIORITY property is returned. If multiple files have the
    607     * same PRIORITY value then the first in the classpath is returned.
    608     * <p>
    609     * This differs from the 1.0.x releases; those always use the first one found.
    610     * However as the priority is a new field, this change is backwards compatible.
    611     * <p>
    612     * The purpose of the priority field is to allow a webserver administrator to
    613     * override logging settings in all webapps by placing a commons-logging.properties
    614     * file in a shared classpath location with a priority > 0; this overrides any
    615     * commons-logging.properties files without priorities which are in the
    616     * webapps. Webapps can also use explicit priorities to override a configuration
    617     * file in the shared classpath if needed.
    618     */
    619    private static Properties getConfigurationFile(final ClassLoader classLoader, final String fileName) {
    620        Properties props = null;
    621        double priority = 0.0;
    622        URL propsUrl = null;
    623        try {
    624            final Enumeration<URL> urls = getResources(classLoader, fileName);
    625
    626            if (urls == null) {
    627                return null;
    628            }
    629
    630            while (urls.hasMoreElements()) {
    631                final URL url = urls.nextElement();
    632
    633                final Properties newProps = getProperties(url);
    634                if (newProps != null) {
    635                    if (props == null) {
    636                        propsUrl = url;
    637                        props = newProps;
    638                        final String priorityStr = props.getProperty(PRIORITY_KEY);
    639                        priority = 0.0;
    640                        if (priorityStr != null) {
    641                            priority = Double.parseDouble(priorityStr);
    642                        }
    643
    644                        if (isDiagnosticsEnabled()) {
    645                            logDiagnostic("[LOOKUP] Properties file found at '" + url + "'" + " with priority " + priority);
    646                        }
    647                    } else {
    648                        final String newPriorityStr = newProps.getProperty(PRIORITY_KEY);
    649                        double newPriority = 0.0;
    650                        if (newPriorityStr != null) {
    651                            newPriority = Double.parseDouble(newPriorityStr);
    652                        }
    653
    654                        if (newPriority > priority) {
    655                            if (isDiagnosticsEnabled()) {
    656                                logDiagnostic("[LOOKUP] Properties file at '" + url + "'" + " with priority " + newPriority + " overrides file at '" + propsUrl
    657                                        + "'" + " with priority " + priority);
    658                            }
    659
    660                            propsUrl = url;
    661                            props = newProps;
    662                            priority = newPriority;
    663                        } else if (isDiagnosticsEnabled()) {
    664                            logDiagnostic("[LOOKUP] Properties file at '" + url + "'" + " with priority " + newPriority + " does not override file at '"
    665                                    + propsUrl + "'" + " with priority " + priority);
    666                        }
    667                    }
    668
    669                }
    670            }
    671        } catch (final SecurityException e) {
    672            if (isDiagnosticsEnabled()) {
    673                logDiagnostic("SecurityException thrown while trying to find/read config files.");
    674            }
    675        }
    676
    677        if (isDiagnosticsEnabled()) {
    678            if (props == null) {
    679                logDiagnostic("[LOOKUP] No properties file of name '" + fileName + "' found.");
    680            } else {
    681                logDiagnostic("[LOOKUP] Properties file of name '" + fileName + "' found at '" + propsUrl + '"');
    682            }
    683        }
    684
    685        return props;
    686    }
    687
    688    /**
    689     * Returns the current context class loader.
    690     * <p>
    691     * In versions prior to 1.1, this method did not use an AccessController.
    692     * In version 1.1, an AccessController wrapper was incorrectly added to
    693     * this method, causing a minor security flaw.
    694     * <p>
    695     * In version 1.1.1 this change was reverted; this method no longer uses
    696     * an AccessController. User code wishing to obtain the context class loader
    697     * must invoke this method via AccessController.doPrivileged if it needs
    698     * support for that.
    699     *
    700     * @return the context class loader associated with the current thread,
    701     *  or null if security doesn't allow it.
    702     * @throws LogConfigurationException if there was some weird error while
    703     *  attempting to get the context class loader.
    704     */
    705    protected static ClassLoader getContextClassLoader() throws LogConfigurationException {
    706        return directGetContextClassLoader();
    707    }
    708
    709    /**
    710     * Calls LogFactory.directGetContextClassLoader under the control of an
    711     * AccessController class. This means that Java code running under a
    712     * security manager that forbids access to ClassLoaders will still work
    713     * if this class is given appropriate privileges, even when the caller
    714     * doesn't have such privileges. Without using an AccessController, the
    715     * the entire call stack must have the privilege before the call is
    716     * allowed.
    717     *
    718     * @return the context class loader associated with the current thread,
    719     *  or null if security doesn't allow it.
    720     * @throws LogConfigurationException if there was some weird error while
    721     *  attempting to get the context class loader.
    722     */
    723    private static ClassLoader getContextClassLoaderInternal() throws LogConfigurationException {
    724        return AccessController.doPrivileged((PrivilegedAction<ClassLoader>) LogFactory::directGetContextClassLoader);
    725    }
    726
    727    /**
    728     * Constructs (if necessary) and return a {@code LogFactory} instance, using the following ordered lookup procedure to determine the name of the
    729     * implementation class to be loaded.
    730     * <ul>
    731     * <li>The {@code org.apache.commons.logging.LogFactory} system property.</li>
    732     * <li>The JDK 1.3 Service Discovery mechanism</li>
    733     * <li>Use the properties file {@code commons-logging.properties} file, if found in the class path of this class. The configuration file is in standard
    734     * {@link java.util.Properties} format and contains the fully qualified name of the implementation class with the key being the system property defined
    735     * above.</li>
    736     * <li>Fall back to a default implementation class ({@code org.apache.commons.logging.impl.LogFactoryImpl}).</li>
    737     * </ul>
    738     * <p>
    739     * <em>NOTE</em> - If the properties file method of identifying the {@code LogFactory} implementation class is utilized, all of the properties defined in
    740     * this file will be set as configuration attributes on the corresponding {@code LogFactory} instance.
    741     * </p>
    742     * <p>
    743     * <em>NOTE</em> - In a multi-threaded environment it is possible that two different instances will be returned for the same class loader environment.
    744     * </p>
    745     *
    746     * @return a {@code LogFactory}.
    747     * @throws LogConfigurationException if the implementation class is not available or cannot be instantiated.
    748     */
    749    public static LogFactory getFactory() throws LogConfigurationException {
    750        // Identify the class loader we will be using
    751        final ClassLoader contextClassLoader = getContextClassLoaderInternal();
    752
    753        // This is an odd enough situation to report about. This
    754        // output will be a nuisance on JDK1.1, as the system
    755        // class loader is null in that environment.
    756        if (contextClassLoader == null && isDiagnosticsEnabled()) {
    757            logDiagnostic("Context class loader is null.");
    758        }
    759
    760        // Return any previously registered factory for this class loader
    761        LogFactory factory = getCachedFactory(contextClassLoader);
    762        if (factory != null) {
    763            return factory;
    764        }
    765
    766        if (isDiagnosticsEnabled()) {
    767            logDiagnostic(
    768                    "[LOOKUP] LogFactory implementation requested for the first time for context class loader " +
    769                    objectId(contextClassLoader));
    770            logHierarchy("[LOOKUP] ", contextClassLoader);
    771        }
    772
    773        // Load properties file.
    774        //
    775        // If the properties file exists, then its contents are used as
    776        // "attributes" on the LogFactory implementation class. One particular
    777        // property may also control which LogFactory concrete subclass is
    778        // used, but only if other discovery mechanisms fail..
    779        //
    780        // As the properties file (if it exists) will be used one way or
    781        // another in the end we may as well look for it first.
    782
    783        final Properties props = getConfigurationFile(contextClassLoader, FACTORY_PROPERTIES);
    784
    785        // Determine whether we will be using the thread context class loader to
    786        // load logging classes or not by checking the loaded properties file (if any).
    787        ClassLoader baseClassLoader = contextClassLoader;
    788        if (props != null) {
    789            final String useTCCLStr = props.getProperty(TCCL_KEY);
    790            // The Boolean.valueOf(useTCCLStr).booleanValue() formulation
    791            // is required for Java 1.2 compatibility.
    792            if (useTCCLStr != null && !Boolean.parseBoolean(useTCCLStr)) {
    793                // Don't use current context class loader when locating any
    794                // LogFactory or Log classes, just use the class that loaded
    795                // this abstract class. When this class is deployed in a shared
    796                // classpath of a container, it means webapps cannot deploy their
    797                // own logging implementations. It also means that it is up to the
    798                // implementation whether to load library-specific config files
    799                // from the TCCL or not.
    800                baseClassLoader = thisClassLoaderRef.get();
    801            }
    802        }
    803
    804        // Determine which concrete LogFactory subclass to use.
    805        // First, try a global system property
    806        if (isDiagnosticsEnabled()) {
    807            logDiagnostic("[LOOKUP] Looking for system property [" + FACTORY_PROPERTY +
    808                          "] to define the LogFactory subclass to use...");
    809        }
    810
    811        try {
    812            final String factoryClass = getSystemProperty(FACTORY_PROPERTY, null);
    813            if (factoryClass != null) {
    814                if (isDiagnosticsEnabled()) {
    815                    logDiagnostic("[LOOKUP] Creating an instance of LogFactory class '" + factoryClass +
    816                                  "' as specified by system property " + FACTORY_PROPERTY);
    817                }
    818                factory = newFactory(factoryClass, baseClassLoader, contextClassLoader);
    819            } else if (isDiagnosticsEnabled()) {
    820                logDiagnostic("[LOOKUP] No system property [" + FACTORY_PROPERTY + "] defined.");
    821            }
    822        } catch (final SecurityException e) {
    823            if (isDiagnosticsEnabled()) {
    824                logDiagnostic("[LOOKUP] A security exception occurred while trying to create an" +
    825                              " instance of the custom factory class" + ": [" + trim(e.getMessage()) +
    826                              "]. Trying alternative implementations...");
    827            }
    828            // ignore
    829        } catch (final RuntimeException e) {
    830            // This is not consistent with the behavior when a bad LogFactory class is
    831            // specified in a services file.
    832            //
    833            // One possible exception that can occur here is a ClassCastException when
    834            // the specified class wasn't castable to this LogFactory type.
    835            if (isDiagnosticsEnabled()) {
    836                logDiagnostic("[LOOKUP] An exception occurred while trying to create an" +
    837                              " instance of the custom factory class" + ": [" +
    838                              trim(e.getMessage()) +
    839                              "] as specified by a system property.");
    840            }
    841            throw e;
    842        }
    843
    844        // Second, try to find a service by using the JDK 1.3 class
    845        // discovery mechanism, which involves putting a file with the name
    846        // of an interface class in the META-INF/services directory, where the
    847        // contents of the file is a single line specifying a concrete class
    848        // that implements the desired interface.
    849
    850        if (factory == null) {
    851            if (isDiagnosticsEnabled()) {
    852                logDiagnostic("[LOOKUP] Using ServiceLoader  to define the LogFactory subclass to use...");
    853            }
    854            try {
    855                final ServiceLoader<LogFactory> serviceLoader = ServiceLoader.load(LogFactory.class);
    856                final Iterator<LogFactory> iterator = serviceLoader.iterator();
    857
    858                int i = MAX_BROKEN_SERVICES;
    859                while (factory == null && i-- > 0) {
    860                    try {
    861                        if (iterator.hasNext()) {
    862                            factory = iterator.next();
    863                        }
    864                    } catch (final ServiceConfigurationError | LinkageError ex) {
    865                        if (isDiagnosticsEnabled()) {
    866                            logDiagnostic("[LOOKUP] An exception occurred while trying to find an" +
    867                                    " instance of LogFactory" +
    868                                    ": [" + trim(ex.getMessage()) +
    869                                    "]. Trying alternative implementations...");
    870                        }
    871                    }
    872                }
    873            } catch (final Exception ex) {
    874                // note: if the specified LogFactory class wasn't compatible with LogFactory
    875                // for some reason, a ClassCastException will be caught here, and attempts will
    876                // continue to find a compatible class.
    877                if (isDiagnosticsEnabled()) {
    878                    logDiagnostic(
    879                        "[LOOKUP] A security exception occurred while trying to create an" +
    880                        " instance of the custom factory class" +
    881                        ": [" + trim(ex.getMessage()) +
    882                        "]. Trying alternative implementations...");
    883                }
    884                // ignore
    885            }
    886        }
    887
    888        // Third try looking into the properties file read earlier (if found)
    889
    890        if (factory == null) {
    891            if (props != null) {
    892                if (isDiagnosticsEnabled()) {
    893                    logDiagnostic(
    894                        "[LOOKUP] Looking in properties file for entry with key '" + FACTORY_PROPERTY +
    895                        "' to define the LogFactory subclass to use...");
    896                }
    897                final String factoryClass = props.getProperty(FACTORY_PROPERTY);
    898                if (factoryClass != null) {
    899                    if (isDiagnosticsEnabled()) {
    900                        logDiagnostic(
    901                            "[LOOKUP] Properties file specifies LogFactory subclass '" + factoryClass + "'");
    902                    }
    903                    factory = newFactory(factoryClass, baseClassLoader, contextClassLoader);
    904
    905                    // TODO: think about whether we need to handle exceptions from newFactory
    906                } else {
    907                    if (isDiagnosticsEnabled()) {
    908                        logDiagnostic("[LOOKUP] Properties file has no entry specifying LogFactory subclass.");
    909                    }
    910                }
    911            } else if (isDiagnosticsEnabled()) {
    912                logDiagnostic("[LOOKUP] No properties file available to determine" + " LogFactory subclass from..");
    913            }
    914        }
    915
    916        // Fourth, try one of the 3 provided factories
    917
    918        try {
    919            // We prefer Log4j API, since it does not stringify objects.
    920            if (factory == null && isClassAvailable(LOG4J_API_LOGGER, baseClassLoader)) {
    921                // If the Log4j API is redirected to SLF4J, we use SLF4J directly.
    922                if (isClassAvailable(LOG4J_TO_SLF4J_BRIDGE, baseClassLoader)) {
    923                    logDiagnostic(
    924                            "[LOOKUP] Log4j API to SLF4J redirection detected. Loading the SLF4J LogFactory implementation '" + FACTORY_SLF4J + "'.");
    925                    factory = newFactory(FACTORY_SLF4J, baseClassLoader, contextClassLoader);
    926                } else {
    927                    logDiagnostic("[LOOKUP] Log4j API detected. Loading the Log4j API LogFactory implementation '" + FACTORY_LOG4J_API + "'.");
    928                    factory = newFactory(FACTORY_LOG4J_API, baseClassLoader, contextClassLoader);
    929                }
    930            }
    931
    932            if (factory == null && isClassAvailable(SLF4J_API_LOGGER, baseClassLoader)) {
    933                logDiagnostic("[LOOKUP] SLF4J detected. Loading the SLF4J LogFactory implementation '" + FACTORY_SLF4J + "'.");
    934                factory = newFactory(FACTORY_SLF4J, baseClassLoader, contextClassLoader);
    935            }
    936        } catch (final Exception e) {
    937            logDiagnostic("[LOOKUP] An exception occurred while creating LogFactory: " + e.getMessage());
    938        }
    939
    940        if (factory == null) {
    941            if (isDiagnosticsEnabled()) {
    942                logDiagnostic(
    943                    "[LOOKUP] Loading the default LogFactory implementation '" + FACTORY_DEFAULT +
    944                    "' via the same class loader that loaded this LogFactory" +
    945                    " class (ie not looking in the context class loader).");
    946            }
    947
    948            // Note: unlike the above code which can try to load custom LogFactory
    949            // implementations via the TCCL, we don't try to load the default LogFactory
    950            // implementation via the context class loader because:
    951            // * that can cause problems (see comments in newFactory method)
    952            // * no-one should be customising the code of the default class
    953            // Yes, we do give up the ability for the child to ship a newer
    954            // version of the LogFactoryImpl class and have it used dynamically
    955            // by an old LogFactory class in the parent, but that isn't
    956            // necessarily a good idea anyway.
    957            factory = newFactory(FACTORY_DEFAULT, thisClassLoaderRef.get(), contextClassLoader);
    958        }
    959
    960        if (factory != null) {
    961            /**
    962             * Always cache using context class loader.
    963             */
    964            cacheFactory(contextClassLoader, factory);
    965
    966            if (props != null) {
    967                final Enumeration<?> names = props.propertyNames();
    968                while (names.hasMoreElements()) {
    969                    final String name = Objects.toString(names.nextElement(), null);
    970                    final String value = props.getProperty(name);
    971                    factory.setAttribute(name, value);
    972                }
    973            }
    974        }
    975
    976        return factory;
    977    }
    978
    979    /**
    980     * Convenience method to return a named logger, without the application having to care about factories.
    981     *
    982     * @param clazz Class from which a log name will be derived
    983     * @return a named logger.
    984     * @throws LogConfigurationException if a suitable {@code Log} instance cannot be returned
    985     */
    986    public static Log getLog(final Class<?> clazz) throws LogConfigurationException {
    987        return getFactory().getInstance(clazz);
    988    }
    989
    990    /**
    991     * Convenience method to return a named logger, without the application having to care about factories.
    992     *
    993     * @param name Logical name of the {@code Log} instance to be returned (the meaning of this name is only known to the underlying logging implementation that
    994     *             is being wrapped)
    995     * @return a named logger.
    996     * @throws LogConfigurationException if a suitable {@code Log} instance cannot be returned
    997     */
    998    public static Log getLog(final String name) throws LogConfigurationException {
    999        return getFactory().getInstance(name);
    1000    }
    1001
    1002    /**
    1003     * Given a URL that refers to a .properties file, load that file.
    1004     * This is done under an AccessController so that this method will
    1005     * succeed when this jarfile is privileged but the caller is not.
    1006     * This method must therefore remain private to avoid security issues.
    1007     * <p>
    1008     * {@code Null} is returned if the URL cannot be opened.
    1009     */
    1010    private static Properties getProperties(final URL url) {
    1011        return AccessController.doPrivileged((PrivilegedAction<Properties>) () -> {
    1012            // We must ensure that useCaches is set to false, as the
    1013            // default behavior of java is to cache file handles, and
    1014            // this "locks" files, preventing hot-redeploy on windows.
    1015            try {
    1016                final URLConnection connection = url.openConnection();
    1017                connection.setUseCaches(false);
    1018                try (InputStream stream = connection.getInputStream()) {
    1019                    if (stream != null) {
    1020                        final Properties props = new Properties();
    1021                        props.load(stream);
    1022                        return props;
    1023                    }
    1024                } catch (final IOException e) {
    1025                    if (isDiagnosticsEnabled()) {
    1026                        logDiagnostic("Unable to close stream for URL " + url);
    1027                    }
    1028                }
    1029            } catch (final IOException e) {
    1030                if (isDiagnosticsEnabled()) {
    1031                    logDiagnostic("Unable to read URL " + url);
    1032                }
    1033            }
    1034
    1035            return null;
    1036        });
    1037    }
    1038
    1039    /**
    1040     * Given a file name, return an enumeration of URLs pointing to
    1041     * all the occurrences of that file name in the classpath.
    1042     * <p>
    1043     * This is just like ClassLoader.getResources except that the
    1044     * operation is done under an AccessController so that this method will
    1045     * succeed when this jarfile is privileged but the caller is not.
    1046     * This method must therefore remain private to avoid security issues.
    1047     * <p>
    1048     * If no instances are found, an Enumeration is returned whose
    1049     * hasMoreElements method returns false (ie an "empty" enumeration).
    1050     * If resources could not be listed for some reason, null is returned.
    1051     */
    1052    private static Enumeration<URL> getResources(final ClassLoader loader, final String name) {
    1053        return AccessController.doPrivileged((PrivilegedAction<Enumeration<URL>>) () -> {
    1054            try {
    1055                if (loader != null) {
    1056                    return loader.getResources(name);
    1057                }
    1058                return ClassLoader.getSystemResources(name);
    1059            } catch (final IOException e) {
    1060                if (isDiagnosticsEnabled()) {
    1061                    logDiagnostic("Exception while trying to find configuration file " + name + ":" + e.getMessage());
    1062                }
    1063                return null;
    1064            } catch (final NoSuchMethodError e) {
    1065                // we must be running on a 1.1 JVM which doesn't support
    1066                // ClassLoader.getSystemResources; just return null in
    1067                // this case.
    1068                return null;
    1069            }
    1070        });
    1071    }
    1072
    1073    /**
    1074     * Read the specified system property, using an AccessController so that
    1075     * the property can be read if JCL has been granted the appropriate
    1076     * security rights even if the calling code has not.
    1077     * <p>
    1078     * Take care not to expose the value returned by this method to the
    1079     * calling application in any way; otherwise the calling app can use that
    1080     * info to access data that should not be available to it.
    1081     */
    1082    private static String getSystemProperty(final String key, final String def)
    1083            throws SecurityException {
    1084        return AccessController.doPrivileged((PrivilegedAction<String>) () -> System.getProperty(key, def));
    1085    }
    1086
    1087    /**
    1088     * Checks whether the supplied Throwable is one that needs to be
    1089     * re-thrown and ignores all others.
    1090     *
    1091     * The following errors are re-thrown:
    1092     * <ul>
    1093     *   <li>ThreadDeath</li>
    1094     *   <li>VirtualMachineError</li>
    1095     * </ul>
    1096     *
    1097     * @param t the Throwable to check
    1098     */
    1099    protected static void handleThrowable(final Throwable t) {
    1100        if (t instanceof ThreadDeath) {
    1101            throw (ThreadDeath) t;
    1102        }
    1103        if (t instanceof VirtualMachineError) {
    1104            throw (VirtualMachineError) t;
    1105        }
    1106        // All other instances of Throwable will be silently ignored
    1107    }
    1108
    1109    /**
    1110     * Determines whether the given class actually implements {@code LogFactory}.
    1111     * Diagnostic information is also logged.
    1112     * <p>
    1113     * <strong>Usage:</strong> to diagnose whether a class loader conflict is the cause
    1114     * of incompatibility. The test used is whether the class is assignable from
    1115     * the {@code LogFactory} class loaded by the class's class loader.
    1116     * @param logFactoryClass {@code Class} which may implement {@code LogFactory}
    1117     * @return true if the {@code logFactoryClass} does extend
    1118     * {@code LogFactory} when that class is loaded via the same
    1119     * class loader that loaded the {@code logFactoryClass}.
    1120     */
    1121    private static boolean implementsLogFactory(final Class<?> logFactoryClass) {
    1122        boolean implementsLogFactory = false;
    1123        if (logFactoryClass != null) {
    1124            try {
    1125                final ClassLoader logFactoryClassLoader = logFactoryClass.getClassLoader();
    1126                if (logFactoryClassLoader == null) {
    1127                    logDiagnostic("[CUSTOM LOG FACTORY] was loaded by the boot class loader");
    1128                } else {
    1129                    logHierarchy("[CUSTOM LOG FACTORY] ", logFactoryClassLoader);
    1130                    final Class<?> factoryFromCustomLoader = Class.forName("org.apache.commons.logging.LogFactory", false, logFactoryClassLoader);
    1131                    implementsLogFactory = factoryFromCustomLoader.isAssignableFrom(logFactoryClass);
    1132                    final String logFactoryClassName = logFactoryClass.getName();
    1133                    if (implementsLogFactory) {
    1134                        logDiagnostic("[CUSTOM LOG FACTORY] " + logFactoryClassName + " implements LogFactory but was loaded by an incompatible class loader.");
    1135                    } else {
    1136                        logDiagnostic("[CUSTOM LOG FACTORY] " + logFactoryClassName + " does not implement LogFactory.");
    1137                    }
    1138                }
    1139            } catch (final SecurityException e) {
    1140                //
    1141                // The application is running within a hostile security environment.
    1142                // This will make it very hard to diagnose issues with JCL.
    1143                // Consider running less securely whilst debugging this issue.
    1144                //
    1145                logDiagnostic("[CUSTOM LOG FACTORY] SecurityException caught trying to determine whether "
    1146                        + "the compatibility was caused by a class loader conflict: " + e.getMessage());
    1147            } catch (final LinkageError e) {
    1148                //
    1149                // This should be an unusual circumstance.
    1150                // LinkageError's usually indicate that a dependent class has incompatibly changed.
    1151                // Another possibility may be an exception thrown by an initializer.
    1152                // Time for a clean rebuild?
    1153                //
    1154                logDiagnostic("[CUSTOM LOG FACTORY] LinkageError caught trying to determine whether "
    1155                        + "the compatibility was caused by a class loader conflict: " + e.getMessage());
    1156            } catch (final ClassNotFoundException e) {
    1157                //
    1158                // LogFactory cannot be loaded by the class loader which loaded the custom factory implementation.
    1159                // The custom implementation is not viable until this is corrected.
    1160                // Ensure that the JCL jar and the custom class are available from the same class loader.
    1161                // Running with diagnostics on should give information about the class loaders used
    1162                // to load the custom factory.
    1163                //
    1164                logDiagnostic("[CUSTOM LOG FACTORY] LogFactory class cannot be loaded by the class loader which loaded "
    1165                        + "the custom LogFactory implementation. Is the custom factory in the right class loader?");
    1166            }
    1167        }
    1168        return implementsLogFactory;
    1169    }
    1170
    1171    /**
    1172     * Determines whether the user wants internal diagnostic output. If so,
    1173     * returns an appropriate writer object. Users can enable diagnostic
    1174     * output by setting the system property named {@link #DIAGNOSTICS_DEST_PROPERTY} to
    1175     * a file name, or the special values STDOUT or STDERR.
    1176     */
    1177    private static PrintStream initDiagnostics() {
    1178        String dest;
    1179        try {
    1180            dest = getSystemProperty(DIAGNOSTICS_DEST_PROPERTY, null);
    1181            if (dest == null) {
    1182                return null;
    1183            }
    1184        } catch (final SecurityException ex) {
    1185            // We must be running in some very secure environment.
    1186            // We just have to assume output is not wanted..
    1187            return null;
    1188        }
    1189
    1190        if (dest.equals("STDOUT")) {
    1191            return System.out;
    1192        }
    1193        if (dest.equals("STDERR")) {
    1194            return System.err;
    1195        }
    1196        try {
    1197            // open the file in append mode
    1198            final FileOutputStream fos = new FileOutputStream(dest, true);
    1199            return new PrintStream(fos, false, StandardCharsets.UTF_8.name());
    1200        } catch (final IOException ex) {
    1201            // We should report this to the user - but how?
    1202            return null;
    1203        }
    1204    }
    1205
    1206    private static boolean isClassAvailable(final String className, final ClassLoader classLoader) {
    1207        final ClassLoader loader = LogFactory.class.getClassLoader();
    1208        logDiagnostic("Checking if class '" + className + "' is available in class loader " + objectId(loader));
    1209        try {
    1210            Class.forName(className, true, classLoader);
    1211            return true;
    1212        } catch (final ClassNotFoundException | LinkageError e) {
    1213            logDiagnostic("Failed to load class '" + className + "' from class loader " + objectId(loader) + ": " + e.getMessage());
    1214        }
    1215        return false;
    1216    }
    1217
    1218    /**
    1219     * Indicates true if the user has enabled internal logging.
    1220     * <p>
    1221     * By the way, sorry for the incorrect grammar, but calling this method
    1222     * areDiagnosticsEnabled just isn't Java beans style.
    1223     *
    1224     * @return true if calls to logDiagnostic will have any effect.
    1225     * @since 1.1
    1226     */
    1227    protected static boolean isDiagnosticsEnabled() {
    1228        return DIAGNOSTICS_STREAM != null;
    1229    }
    1230
    1231    /**
    1232     * Generate useful diagnostics regarding the class loader tree for
    1233     * the specified class.
    1234     * <p>
    1235     * As an example, if the specified class was loaded via a webapp's
    1236     * class loader, then you may get the following output:
    1237     * <pre>
    1238     * Class com.acme.Foo was loaded via class loader 11111
    1239     * ClassLoader tree: 11111 -> 22222 (SYSTEM) -> 33333 -> BOOT
    1240     * </pre>
    1241     * <p>
    1242     * This method returns immediately if isDiagnosticsEnabled()
    1243     * returns false.
    1244     *
    1245     * @param clazz is the class whose class loader + tree are to be
    1246     * output.
    1247     */
    1248    private static void logClassLoaderEnvironment(final Class<?> clazz) {
    1249        if (!isDiagnosticsEnabled()) {
    1250            return;
    1251        }
    1252
    1253        try {
    1254            // Deliberately use System.getProperty here instead of getSystemProperty; if
    1255            // the overall security policy for the calling application forbids access to
    1256            // these variables then we do not want to output them to the diagnostic stream.
    1257            logDiagnostic("[ENV] Extension directories (java.ext.dir): " + System.getProperty("java.ext.dir"));
    1258            logDiagnostic("[ENV] Application classpath (java.class.path): " + System.getProperty("java.class.path"));
    1259        } catch (final SecurityException ex) {
    1260            logDiagnostic("[ENV] Security setting prevent interrogation of system classpaths.");
    1261        }
    1262
    1263        final String className = clazz.getName();
    1264        ClassLoader classLoader;
    1265
    1266        try {
    1267            classLoader = getClassLoader(clazz);
    1268        } catch (final SecurityException ex) {
    1269            // not much useful diagnostics we can print here!
    1270            logDiagnostic("[ENV] Security forbids determining the class loader for " + className);
    1271            return;
    1272        }
    1273
    1274        logDiagnostic("[ENV] Class " + className + " was loaded via class loader " + objectId(classLoader));
    1275        logHierarchy("[ENV] Ancestry of class loader which loaded " + className + " is ", classLoader);
    1276    }
    1277
    1278    /**
    1279     * Write the specified message to the internal logging destination.
    1280     * <p>
    1281     * Note that this method is private; concrete subclasses of this class
    1282     * should not call it because the diagnosticPrefix string this
    1283     * method puts in front of all its messages is LogFactory@....,
    1284     * while subclasses should put SomeSubClass@...
    1285     * <p>
    1286     * Subclasses should instead compute their own prefix, then call
    1287     * logRawDiagnostic. Note that calling isDiagnosticsEnabled is
    1288     * fine for subclasses.
    1289     * <p>
    1290     * Note that it is safe to call this method before initDiagnostics
    1291     * is called; any output will just be ignored (as isDiagnosticsEnabled
    1292     * will return false).
    1293     *
    1294     * @param msg is the diagnostic message to be output.
    1295     */
    1296    private static void logDiagnostic(final String msg) {
    1297        if (DIAGNOSTICS_STREAM != null) {
    1298            DIAGNOSTICS_STREAM.print(diagnosticPrefix);
    1299            DIAGNOSTICS_STREAM.println(msg);
    1300            DIAGNOSTICS_STREAM.flush();
    1301        }
    1302    }
    1303
    1304    /**
    1305     * Logs diagnostic messages about the given class loader
    1306     * and it's hierarchy. The prefix is prepended to the message
    1307     * and is intended to make it easier to understand the logs.
    1308     * @param prefix
    1309     * @param classLoader
    1310     */
    1311    private static void logHierarchy(final String prefix, ClassLoader classLoader) {
    1312        if (!isDiagnosticsEnabled()) {
    1313            return;
    1314        }
    1315        ClassLoader systemClassLoader;
    1316        if (classLoader != null) {
    1317            final String classLoaderString = classLoader.toString();
    1318            logDiagnostic(prefix + objectId(classLoader) + " == '" + classLoaderString + "'");
    1319        }
    1320
    1321        try {
    1322            systemClassLoader = ClassLoader.getSystemClassLoader();
    1323        } catch (final SecurityException ex) {
    1324            logDiagnostic(prefix + "Security forbids determining the system class loader.");
    1325            return;
    1326        }
    1327        if (classLoader != null) {
    1328            final StringBuilder buf = new StringBuilder(prefix + "ClassLoader tree:");
    1329            for(;;) {
    1330                buf.append(objectId(classLoader));
    1331                if (classLoader == systemClassLoader) {
    1332                    buf.append(" (SYSTEM) ");
    1333                }
    1334
    1335                try {
    1336                    classLoader = classLoader.getParent();
    1337                } catch (final SecurityException ex) {
    1338                    buf.append(" --> SECRET");
    1339                    break;
    1340                }
    1341
    1342                buf.append(" --> ");
    1343                if (classLoader == null) {
    1344                    buf.append("BOOT");
    1345                    break;
    1346                }
    1347            }
    1348            logDiagnostic(buf.toString());
    1349        }
    1350    }
    1351
    1352    /**
    1353     * Write the specified message to the internal logging destination.
    1354     *
    1355     * @param msg is the diagnostic message to be output.
    1356     * @since 1.1
    1357     */
    1358    protected static final void logRawDiagnostic(final String msg) {
    1359        if (DIAGNOSTICS_STREAM != null) {
    1360            DIAGNOSTICS_STREAM.println(msg);
    1361            DIAGNOSTICS_STREAM.flush();
    1362        }
    1363    }
    1364
    1365    /**
    1366     * Method provided for backwards compatibility; see newFactory version that
    1367     * takes 3 parameters.
    1368     * <p>
    1369     * This method would only ever be called in some rather odd situation.
    1370     * Note that this method is static, so overriding in a subclass doesn't
    1371     * have any effect unless this method is called from a method in that
    1372     * subclass. However this method only makes sense to use from the
    1373     * getFactory method, and as that is almost always invoked via
    1374     * LogFactory.getFactory, any custom definition in a subclass would be
    1375     * pointless. Only a class with a custom getFactory method, then invoked
    1376     * directly via CustomFactoryImpl.getFactory or similar would ever call
    1377     * this. Anyway, it's here just in case, though the "managed class loader"
    1378     * value output to the diagnostics will not report the correct value.
    1379     * </p>
    1380     *
    1381     * @param factoryClass factory class.
    1382     * @param classLoader class loader.
    1383     * @return a LogFactory.
    1384     */
    1385    protected static LogFactory newFactory(final String factoryClass,
    1386                                           final ClassLoader classLoader) {
    1387        return newFactory(factoryClass, classLoader, null);
    1388    }
    1389
    1390    /**
    1391     * Gets a new instance of the specified {@code LogFactory} implementation class, loaded by the specified class loader. If that fails, try the class loader
    1392     * used to load this (abstract) LogFactory.
    1393     * <p>
    1394     * <b>ClassLoader conflicts</b>
    1395     * </p>
    1396     * <p>
    1397     * Note that there can be problems if the specified ClassLoader is not the same as the class loader that loaded this class, that is, when loading a concrete
    1398     * LogFactory subclass via a context class loader.
    1399     * </p>
    1400     * <p>
    1401     * The problem is the same one that can occur when loading a concrete Log subclass via a context class loader.
    1402     * </p>
    1403     * <p>
    1404     * The problem occurs when code running in the context class loader calls class X which was loaded via a parent class loader, and class X then calls
    1405     * LogFactory.getFactory (either directly or via LogFactory.getLog). Because class X was loaded via the parent, it binds to LogFactory loaded via the
    1406     * parent. When the code in this method finds some LogFactoryYYYY class in the child (context) class loader, and there also happens to be a LogFactory class
    1407     * defined in the child class loader, then LogFactoryYYYY will be bound to LogFactory@childloader. It cannot be cast to LogFactory@parentloader, that is,
    1408     * this method cannot return the object as the desired type. Note that it doesn't matter if the LogFactory class in the child class loader is identical to
    1409     * the LogFactory class in the parent class loader, they are not compatible.
    1410     * </p>
    1411     * <p>
    1412     * The solution taken here is to simply print out an error message when this occurs then throw an exception. The deployer of the application must ensure
    1413     * they remove all occurrences of the LogFactory class from the child class loader in order to resolve the issue. Note that they do not have to move the
    1414     * custom LogFactory subclass; that is ok as long as the only LogFactory class it can find to bind to is in the parent class loader.
    1415     * </p>
    1416     *
    1417     * @param factoryClass       Fully qualified name of the {@code LogFactory} implementation class
    1418     * @param classLoader        ClassLoader from which to load this class
    1419     * @param contextClassLoader is the context that this new factory will manage logging for.
    1420     * @return a new instance of the specified {@code LogFactory}.
    1421     * @throws LogConfigurationException if a suitable instance cannot be created
    1422     * @since 1.1
    1423     */
    1424    protected static LogFactory newFactory(final String factoryClass,
    1425                                           final ClassLoader classLoader,
    1426                                           final ClassLoader contextClassLoader)
    1427            throws LogConfigurationException {
    1428        // Note that any unchecked exceptions thrown by the createFactory
    1429        // method will propagate out of this method; in particular a
    1430        // ClassCastException can be thrown.
    1431        final Object result = AccessController.doPrivileged((PrivilegedAction<?>) () -> createFactory(factoryClass, classLoader));
    1432
    1433        if (result instanceof LogConfigurationException) {
    1434            final LogConfigurationException ex = (LogConfigurationException) result;
    1435            if (isDiagnosticsEnabled()) {
    1436                logDiagnostic("An error occurred while loading the factory class:" + ex.getMessage());
    1437            }
    1438            throw ex;
    1439        }
    1440        if (isDiagnosticsEnabled()) {
    1441            logDiagnostic("Created object " + objectId(result) + " to manage class loader " + objectId(contextClassLoader));
    1442        }
    1443        return (LogFactory) result;
    1444    }
    1445
    1446    /**
    1447     * Returns a string that uniquely identifies the specified object, including
    1448     * its class.
    1449     * <p>
    1450     * The returned string is of form "className@hashCode", that is, is the same as
    1451     * the return value of the Object.toString() method, but works even when
    1452     * the specified object's class has overridden the toString method.
    1453     * </p>
    1454     *
    1455     * @param o may be null.
    1456     * @return a string of form className@hashCode, or "null" if param o is null.
    1457     * @since 1.1
    1458     */
    1459    public static String objectId(final Object o) {
    1460        if (o == null) {
    1461            return "null";
    1462        }
    1463        return o.getClass().getName() + "@" + System.identityHashCode(o);
    1464    }
    1465
    1466    /**
    1467     * Release any internal references to previously created {@link LogFactory}
    1468     * instances that have been associated with the specified class loader
    1469     * (if any), after calling the instance method {@code release()} on
    1470     * each of them.
    1471     *
    1472     * @param classLoader ClassLoader for which to release the LogFactory
    1473     */
    1474    public static void release(final ClassLoader classLoader) {
    1475        if (isDiagnosticsEnabled()) {
    1476            logDiagnostic("Releasing factory for class loader " + objectId(classLoader));
    1477        }
    1478        // factories is not final and could be replaced in this block.
    1479        final Hashtable<ClassLoader, LogFactory> factories = LogFactory.factories;
    1480        synchronized (factories) {
    1481            if (classLoader == null) {
    1482                if (nullClassLoaderFactory != null) {
    1483                    nullClassLoaderFactory.release();
    1484                    nullClassLoaderFactory = null;
    1485                }
    1486            } else {
    1487                final LogFactory factory = factories.get(classLoader);
    1488                if (factory != null) {
    1489                    factory.release();
    1490                    factories.remove(classLoader);
    1491                }
    1492            }
    1493        }
    1494    }
    1495
    1496    /**
    1497     * Release any internal references to previously created {@link LogFactory}
    1498     * instances, after calling the instance method {@code release()} on
    1499     * each of them.  This is useful in environments like servlet containers,
    1500     * which implement application reloading by throwing away a ClassLoader.
    1501     * Dangling references to objects in that class loader would prevent
    1502     * garbage collection.
    1503     */
    1504    public static void releaseAll() {
    1505        if (isDiagnosticsEnabled()) {
    1506            logDiagnostic("Releasing factory for all class loaders.");
    1507        }
    1508        // factories is not final and could be replaced in this block.
    1509        final Hashtable<ClassLoader, LogFactory> factories = LogFactory.factories;
    1510        synchronized (factories) {
    1511            factories.values().forEach(LogFactory::release);
    1512            factories.clear();
    1513
    1514            if (nullClassLoaderFactory != null) {
    1515                nullClassLoaderFactory.release();
    1516                nullClassLoaderFactory = null;
    1517            }
    1518        }
    1519    }
    1520
    1521    /** Utility method to safely trim a string. */
    1522    private static String trim(final String src) {
    1523        if (src == null) {
    1524            return null;
    1525        }
    1526        return src.trim();
    1527    }
    1528
    1529    /**
    1530     * Protected constructor that is not available for public use.
    1531     */
    1532    protected LogFactory() {
    1533    }
    1534
    1535    /**
    1536     * Gets the configuration attribute with the specified name (if any),
    1537     * or {@code null} if there is no such attribute.
    1538     *
    1539     * @param name Name of the attribute to return
    1540     * @return the configuration attribute with the specified name.
    1541     */
    1542    public abstract Object getAttribute(String name);
    1543
    1544    /**
    1545     * Gets an array containing the names of all currently defined configuration attributes. If there are no such attributes, a zero length array is returned.
    1546     *
    1547     * @return an array containing the names of all currently defined configuration attributes
    1548     */
    1549    public abstract String[] getAttributeNames();
    1550
    1551    /**
    1552     * Convenience method to derive a name from the specified class and call {@code getInstance(String)} with it.
    1553     *
    1554     * @param clazz Class for which a suitable Log name will be derived
    1555     * @return a name from the specified class.
    1556     * @throws LogConfigurationException if a suitable {@code Log} instance cannot be returned
    1557     */
    1558    public abstract Log getInstance(Class<?> clazz) throws LogConfigurationException;
    1559
    1560    /**
    1561     * Constructs (if necessary) and return a {@code Log} instance, using the factory's current set of configuration attributes.
    1562     * <p>
    1563     * <strong>NOTE</strong> - Depending upon the implementation of the {@code LogFactory} you are using, the {@code Log} instance you are returned may or may
    1564     * not be local to the current application, and may or may not be returned again on a subsequent call with the same name argument.
    1565     * </p>
    1566     *
    1567     * @param name Logical name of the {@code Log} instance to be returned (the meaning of this name is only known to the underlying logging implementation that
    1568     *             is being wrapped)
    1569     * @return a {@code Log} instance.
    1570     * @throws LogConfigurationException if a suitable {@code Log} instance cannot be returned
    1571     */
    1572    public abstract Log getInstance(String name)
    1573        throws LogConfigurationException;
    1574
    1575    /**
    1576     * Release any internal references to previously created {@link Log}
    1577     * instances returned by this factory.  This is useful in environments
    1578     * like servlet containers, which implement application reloading by
    1579     * throwing away a ClassLoader.  Dangling references to objects in that
    1580     * class loader would prevent garbage collection.
    1581     */
    1582    public abstract void release();
    1583
    1584    /**
    1585     * Remove any configuration attribute associated with the specified name.
    1586     * If there is no such attribute, no action is taken.
    1587     *
    1588     * @param name Name of the attribute to remove
    1589     */
    1590    public abstract void removeAttribute(String name);
    1591
    1592    //
    1593    // We can't do this in the class constructor, as there are many
    1594    // static methods on this class that can be called before any
    1595    // LogFactory instances are created, and they depend upon this
    1596    // stuff having been set up.
    1597    //
    1598    // Note that this block must come after any variable declarations used
    1599    // by any methods called from this block, as we want any static initializer
    1600    // associated with the variable to run first. If static initializers for
    1601    // variables run after this code, then (a) their value might be needed
    1602    // by methods called from here, and (b) they might *override* any value
    1603    // computed here!
    1604    //
    1605    // So the wisest thing to do is just to place this code at the very end
    1606    // of the class file.
    1607
    1608    /**
    1609     * Sets the configuration attribute with the specified name.  Calling
    1610     * this with a {@code null} value is equivalent to calling
    1611     * {@code removeAttribute(name)}.
    1612     *
    1613     * @param name Name of the attribute to set
    1614     * @param value Value of the attribute to set, or {@code null}
    1615     *  to remove any setting for this attribute
    1616     */
    1617    public abstract void setAttribute(String name, Object value);
    1618
    1619}