    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 */
    017package org.apache.commons.lang3.concurrent;
    018
    019import java.util.Collections;
    020import java.util.HashMap;
    021import java.util.Map;
    022import java.util.NoSuchElementException;
    023import java.util.Set;
    024import java.util.concurrent.ExecutorService;
    025
    026import org.apache.commons.lang3.Validate;
    027
    028/**
    029 * <p>
    030 * A specialized {@link BackgroundInitializer} implementation that can deal with
    031 * multiple background initialization tasks.
    032 * </p>
    033 * <p>
    034 * This class has a similar purpose as {@link BackgroundInitializer}. However,
    035 * it is not limited to a single background initialization task. Rather it
    036 * manages an arbitrary number of {@code BackgroundInitializer} objects,
    037 * executes them, and waits until they are completely initialized. This is
    038 * useful for applications that have to perform multiple initialization tasks
    039 * that can run in parallel (i.e. that do not depend on each other). This class
    040 * takes care about the management of an {@code ExecutorService} and shares it
    041 * with the {@code BackgroundInitializer} objects it is responsible for; so the
    042 * using application need not bother with these details.
    043 * </p>
    044 * <p>
    045 * The typical usage scenario for {@code MultiBackgroundInitializer} is as
    046 * follows:
    047 * </p>
    048 * <ul>
    049 * <li>Create a new instance of the class. Optionally pass in a pre-configured
    050 * {@code ExecutorService}. Alternatively {@code MultiBackgroundInitializer} can
    051 * create a temporary {@code ExecutorService} and delete it after initialization
    052 * is complete.</li>
    053 * <li>Create specialized {@link BackgroundInitializer} objects for the
    054 * initialization tasks to be performed and add them to the {@code
    055 * MultiBackgroundInitializer} using the
    056 * {@link #addInitializer(String, BackgroundInitializer)} method.</li>
    057 * <li>After all initializers have been added, call the {@link #start()} method.
    058 * </li>
    059 * <li>When access to the result objects produced by the {@code
    060 * BackgroundInitializer} objects is needed call the {@link #get()} method. The
    061 * object returned here provides access to all result objects created during
    062 * initialization. It also stores information about exceptions that have
    063 * occurred.</li>
    064 * </ul>
    065 * <p>
    066 * {@code MultiBackgroundInitializer} starts a special controller task that
    067 * starts all {@code BackgroundInitializer} objects added to the instance.
    068 * Before the an initializer is started it is checked whether this initializer
    069 * already has an {@code ExecutorService} set. If this is the case, this {@code
    070 * ExecutorService} is used for running the background task. Otherwise the
    071 * current {@code ExecutorService} of this {@code MultiBackgroundInitializer} is
    072 * shared with the initializer.
    073 * </p>
    074 * <p>
    075 * The easiest way of using this class is to let it deal with the management of
    076 * an {@code ExecutorService} itself: If no external {@code ExecutorService} is
    077 * provided, the class creates a temporary {@code ExecutorService} (that is
    078 * capable of executing all background tasks in parallel) and destroys it at the
    079 * end of background processing.
    080 * </p>
    081 * <p>
    082 * Alternatively an external {@code ExecutorService} can be provided - either at
    083 * construction time or later by calling the
    084 * {@link #setExternalExecutor(ExecutorService)} method. In this case all
    085 * background tasks are scheduled at this external {@code ExecutorService}.
    086 * <strong>Important note:</strong> When using an external {@code
    087 * ExecutorService} be sure that the number of threads managed by the service is
    088 * large enough. Otherwise a deadlock can happen! This is the case in the
    089 * following scenario: {@code MultiBackgroundInitializer} starts a task that
    090 * starts all registered {@code BackgroundInitializer} objects and waits for
    091 * their completion. If for instance a single threaded {@code ExecutorService}
    092 * is used, none of the background tasks can be executed, and the task created
    093 * by {@code MultiBackgroundInitializer} waits forever.
    094 * </p>
    095 *
    096 * @since 3.0
    097 */
    098public class MultiBackgroundInitializer
    099        extends
    100        BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults> {
    101    /** A map with the child initializers. */
    102    private final Map<String, BackgroundInitializer<?>> childInitializers =
    103        new HashMap<>();
    104
    105    /**
    106     * Creates a new instance of {@code MultiBackgroundInitializer}.
    107     */
    108    public MultiBackgroundInitializer() {
    109        super();
    110    }
    111
    112    /**
    113     * Creates a new instance of {@code MultiBackgroundInitializer} and
    114     * initializes it with the given external {@code ExecutorService}.
    115     *
    116     * @param exec the {@code ExecutorService} for executing the background
    117     * tasks
    118     */
    119    public MultiBackgroundInitializer(final ExecutorService exec) {
    120        super(exec);
    121    }
    122
    123    /**
    124     * Adds a new {@code BackgroundInitializer} to this object. When this
    125     * {@code MultiBackgroundInitializer} is started, the given initializer will
    126     * be processed. This method must not be called after {@link #start()} has
    127     * been invoked.
    128     *
    129     * @param name the name of the initializer (must not be <b>null</b>)
    130     * @param init the {@code BackgroundInitializer} to add (must not be
    131     * <b>null</b>)
    132     * @throws IllegalArgumentException if a required parameter is missing
    133     * @throws IllegalStateException if {@code start()} has already been called
    134     */
    135    public void addInitializer(final String name, final BackgroundInitializer<?> init) {
    136        Validate.notNull(name, "Name of child initializer must not be null!");
    137        Validate.notNull(init, "Child initializer must not be null!");
    138
    139        synchronized (this) {
    140            if (isStarted()) {
    141                throw new IllegalStateException(
    142                        "addInitializer() must not be called after start()!");
    143            }
    144            childInitializers.put(name, init);
    145        }
    146    }
    147
    148    /**
    149     * Returns the number of tasks needed for executing all child {@code
    150     * BackgroundInitializer} objects in parallel. This implementation sums up
    151     * the required tasks for all child initializers (which is necessary if one
    152     * of the child initializers is itself a {@code MultiBackgroundInitializer}
    153     * ). Then it adds 1 for the control task that waits for the completion of
    154     * the children.
    155     *
    156     * @return the number of tasks required for background processing
    157     */
    158    @Override
    159    protected int getTaskCount() {
    160        int result = 1;
    161
    162        for (final BackgroundInitializer<?> bi : childInitializers.values()) {
    163            result += bi.getTaskCount();
    164        }
    165
    166        return result;
    167    }
    168
    169    /**
    170     * Creates the results object. This implementation starts all child {@code
    171     * BackgroundInitializer} objects. Then it collects their results and
    172     * creates a {@code MultiBackgroundInitializerResults} object with this
    173     * data. If a child initializer throws a checked exceptions, it is added to
    174     * the results object. Unchecked exceptions are propagated.
    175     *
    176     * @return the results object
    177     * @throws Exception if an error occurs
    178     */
    179    @Override
    180    protected MultiBackgroundInitializerResults initialize() throws Exception {
    181        Map<String, BackgroundInitializer<?>> inits;
    182        synchronized (this) {
    183            // create a snapshot to operate on
    184            inits = new HashMap<>(
    185                    childInitializers);
    186        }
    187
    188        // start the child initializers
    189        final ExecutorService exec = getActiveExecutor();
    190        for (final BackgroundInitializer<?> bi : inits.values()) {
    191            if (bi.getExternalExecutor() == null) {
    192                // share the executor service if necessary
    193                bi.setExternalExecutor(exec);
    194            }
    195            bi.start();
    196        }
    197
    198        // collect the results
    199        final Map<String, Object> results = new HashMap<>();
    200        final Map<String, ConcurrentException> excepts = new HashMap<>();
    201        for (final Map.Entry<String, BackgroundInitializer<?>> e : inits.entrySet()) {
    202            try {
    203                results.put(e.getKey(), e.getValue().get());
    204            } catch (final ConcurrentException cex) {
    205                excepts.put(e.getKey(), cex);
    206            }
    207        }
    208
    209        return new MultiBackgroundInitializerResults(inits, results, excepts);
    210    }
    211
    212    /**
    213     * A data class for storing the results of the background initialization
    214     * performed by {@code MultiBackgroundInitializer}. Objects of this inner
    215     * class are returned by {@link MultiBackgroundInitializer#initialize()}.
    216     * They allow access to all result objects produced by the
    217     * {@link BackgroundInitializer} objects managed by the owning instance. It
    218     * is also possible to retrieve status information about single
    219     * {@link BackgroundInitializer}s, i.e. whether they completed normally or
    220     * caused an exception.
    221     */
    222    public static class MultiBackgroundInitializerResults {
    223        /** A map with the child initializers. */
    224        private final Map<String, BackgroundInitializer<?>> initializers;
    225
    226        /** A map with the result objects. */
    227        private final Map<String, Object> resultObjects;
    228
    229        /** A map with the exceptions. */
    230        private final Map<String, ConcurrentException> exceptions;
    231
    232        /**
    233         * Creates a new instance of {@code MultiBackgroundInitializerResults}
    234         * and initializes it with maps for the {@code BackgroundInitializer}
    235         * objects, their result objects and the exceptions thrown by them.
    236         *
    237         * @param inits the {@code BackgroundInitializer} objects
    238         * @param results the result objects
    239         * @param excepts the exceptions
    240         */
    241        private MultiBackgroundInitializerResults(
    242                final Map<String, BackgroundInitializer<?>> inits,
    243                final Map<String, Object> results,
    244                final Map<String, ConcurrentException> excepts) {
    245            initializers = inits;
    246            resultObjects = results;
    247            exceptions = excepts;
    248        }
    249
    250        /**
    251         * Returns the {@code BackgroundInitializer} with the given name. If the
    252         * name cannot be resolved, an exception is thrown.
    253         *
    254         * @param name the name of the {@code BackgroundInitializer}
    255         * @return the {@code BackgroundInitializer} with this name
    256         * @throws NoSuchElementException if the name cannot be resolved
    257         */
    258        public BackgroundInitializer<?> getInitializer(final String name) {
    259            return checkName(name);
    260        }
    261
    262        /**
    263         * Returns the result object produced by the {@code
    264         * BackgroundInitializer} with the given name. This is the object
    265         * returned by the initializer's {@code initialize()} method. If this
    266         * {@code BackgroundInitializer} caused an exception, <b>null</b> is
    267         * returned. If the name cannot be resolved, an exception is thrown.
    268         *
    269         * @param name the name of the {@code BackgroundInitializer}
    270         * @return the result object produced by this {@code
    271         * BackgroundInitializer}
    272         * @throws NoSuchElementException if the name cannot be resolved
    273         */
    274        public Object getResultObject(final String name) {
    275            checkName(name);
    276            return resultObjects.get(name);
    277        }
    278
    279        /**
    280         * Returns a flag whether the {@code BackgroundInitializer} with the
    281         * given name caused an exception.
    282         *
    283         * @param name the name of the {@code BackgroundInitializer}
    284         * @return a flag whether this initializer caused an exception
    285         * @throws NoSuchElementException if the name cannot be resolved
    286         */
    287        public boolean isException(final String name) {
    288            checkName(name);
    289            return exceptions.containsKey(name);
    290        }
    291
    292        /**
    293         * Returns the {@code ConcurrentException} object that was thrown by the
    294         * {@code BackgroundInitializer} with the given name. If this
    295         * initializer did not throw an exception, the return value is
    296         * <b>null</b>. If the name cannot be resolved, an exception is thrown.
    297         *
    298         * @param name the name of the {@code BackgroundInitializer}
    299         * @return the exception thrown by this initializer
    300         * @throws NoSuchElementException if the name cannot be resolved
    301         */
    302        public ConcurrentException getException(final String name) {
    303            checkName(name);
    304            return exceptions.get(name);
    305        }
    306
    307        /**
    308         * Returns a set with the names of all {@code BackgroundInitializer}
    309         * objects managed by the {@code MultiBackgroundInitializer}.
    310         *
    311         * @return an (unmodifiable) set with the names of the managed {@code
    312         * BackgroundInitializer} objects
    313         */
    314        public Set<String> initializerNames() {
    315            return Collections.unmodifiableSet(initializers.keySet());
    316        }
    317
    318        /**
    319         * Returns a flag whether the whole initialization was successful. This
    320         * is the case if no child initializer has thrown an exception.
    321         *
    322         * @return a flag whether the initialization was successful
    323         */
    324        public boolean isSuccessful() {
    325            return exceptions.isEmpty();
    326        }
    327
    328        /**
    329         * Checks whether an initializer with the given name exists. If not,
    330         * throws an exception. If it exists, the associated child initializer
    331         * is returned.
    332         *
    333         * @param name the name to check
    334         * @return the initializer with this name
    335         * @throws NoSuchElementException if the name is unknown
    336         */
    337        private BackgroundInitializer<?> checkName(final String name) {
    338            final BackgroundInitializer<?> init = initializers.get(name);
    339            if (init == null) {
    340                throw new NoSuchElementException(
    341                        "No child initializer with name " + name);
    342            }
    343
    344            return init;
    345        }
    346    }
    347}