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= Log4j 2 API

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== Overview

The Log4j 2 API provides the interface that applications should code to
and provides the adapter components required for implementers to create
a logging implementation. Although Log4j 2 is broken up between an API
and an implementation, the primary purpose of doing so was not to allow
multiple implementations, although that is certainly possible, but to
clearly define what classes and methods are safe to use in "normal"
application code.

=== Hello World!

No introduction would be complete without the customary Hello, World
example. Here is ours. First, a Logger with the name "HelloWorld" is
obtained from the
link:../log4j-api/apidocs/org/apache/logging/log4j/LogManager.html[`LogManager`].
Next, the logger is used to write the "Hello, World!" message, however
the message will be written only if the Logger is configured to allow
informational messages.

[source,java]
----
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class HelloWorld {
    private static final Logger logger = LogManager.getLogger("HelloWorld");
    public static void main(String[] args) {
        logger.info("Hello, World!");
    }
}
----

The output from the call to `logger.info()` will vary significantly
depending on the configuration used. See the
link:configuration.html[Configuration] section for more details.

=== Substituting Parameters

Frequently the purpose of logging is to provide information about what
is happening in the system, which requires including information about
the objects being manipulated. In Log4j 1.x this could be accomplished
by doing:

[source,java]
----
if (logger.isDebugEnabled()) {
    logger.debug("Logging in user " + user.getName() + " with birthday " + user.getBirthdayCalendar());
}
----

Doing this repeatedly has the effect of making the code feel like it is
more about logging than the actual task at hand. In addition, it results
in the logging level being checked twice; once on the call to
isDebugEnabled and once on the debug method. A better alternative would
be:

[source,java]
----
logger.debug("Logging in user {} with birthday {}", user.getName(), user.getBirthdayCalendar());
----

With the code above the logging level will only be checked once and the
String construction will only occur when debug logging is enabled.

=== Formatting Parameters

Formatter Loggers leave formatting up to you if `toString()` is not what
you want. To facilitate formatting, you can use the same format strings
as Java's
http://docs.oracle.com/javase/6/docs/api/java/util/Formatter.html#syntax[`Formatter`].
For example:

[source,java]
----
public static Logger logger = LogManager.getFormatterLogger("Foo");

logger.debug("Logging in user %s with birthday %s", user.getName(), user.getBirthdayCalendar());
logger.debug("Logging in user %1$s with birthday %2$tm %2$te,%2$tY", user.getName(), user.getBirthdayCalendar());
logger.debug("Integer.MAX_VALUE = %,d", Integer.MAX_VALUE);
logger.debug("Long.MAX_VALUE = %,d", Long.MAX_VALUE);
----

To use a formatter Logger, you must call one of the `LogManager`
link:../log4j-api/apidocs/org/apache/logging/log4j/LogManager.html#getFormatterLogger(java.lang.Class)[`getFormatterLogger`]
methods. The output for this example shows that `Calendar::toString` is
verbose compared to custom formatting:

[source,java]
----
2012-12-12 11:56:19,633 [main] DEBUG: User John Smith with birthday java.util.GregorianCalendar[time=?,areFieldsSet=false,areAllFieldsSet=false,lenient=true,zone=sun.util.calendar.ZoneInfo[id="America/New_York",offset=-18000000,dstSavings=3600000,useDaylight=true,transitions=235,lastRule=java.util.SimpleTimeZone[id=America/New_York,offset=-18000000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=8,startDayOfWeek=1,startTime=7200000,startTimeMode=0,endMode=3,endMonth=10,endDay=1,endDayOfWeek=1,endTime=7200000,endTimeMode=0]],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=?,YEAR=1995,MONTH=4,WEEK_OF_YEAR=?,WEEK_OF_MONTH=?,DAY_OF_MONTH=23,DAY_OF_YEAR=?,DAY_OF_WEEK=?,DAY_OF_WEEK_IN_MONTH=?,AM_PM=0,HOUR=0,HOUR_OF_DAY=0,MINUTE=0,SECOND=0,MILLISECOND=?,ZONE_OFFSET=?,DST_OFFSET=?]
2012-12-12 11:56:19,643 [main] DEBUG: User John Smith with birthday 05 23, 1995
2012-12-12 11:56:19,643 [main] DEBUG: Integer.MAX_VALUE = 2,147,483,647
2012-12-12 11:56:19,643 [main] DEBUG: Long.MAX_VALUE = 9,223,372,036,854,775,807
----

=== Mixing Loggers with Formatter Loggers

Formatter loggers give fine-grained control over the output format, but
have the drawback that the correct type must be specified (for example,
passing anything other than a decimal integer for a %d format parameter
gives an exception).

If your main usage is to use \{}-style parameters, but occasionally you
need fine-grained control over the output format, you can use the
`printf` method:

[source,java]
----
public static Logger logger = LogManager.getLogger("Foo");

logger.debug("Opening connection to {}...", someDataSource);
logger.printf(Level.INFO, "Logging in user %1$s with birthday %2$tm %2$te,%2$tY", user.getName(), user.getBirthdayCalendar());
----

[#LambdaSupport]
=== Java 8 lambda support for lazy logging

In release 2.4, the `Logger` interface added support for lambda
expressions. This allows client code to lazily log messages without
explicitly checking if the requested log level is enabled. For example,
previously you would write:

[source,java]
----
// pre-Java 8 style optimization: explicitly check the log level
// to make sure the expensiveOperation() method is only called if necessary
if (logger.isTraceEnabled()) {
    logger.trace("Some long-running operation returned {}", expensiveOperation());
}
----

With Java 8 you can achieve the same effect with a lambda expression.
You no longer need to explicitly check the log level:

[source,java]
----
// Java-8 style optimization: no need to explicitly check the log level:
// the lambda expression is not evaluated if the TRACE level is not enabled
logger.trace("Some long-running operation returned {}", () -> expensiveOperation());
----

=== Logger Names

Most logging implementations use a hierarchical scheme for matching
logger names with logging configuration. In this scheme, the logger name
hierarchy is represented by `'.'` characters in the logger name, in a
fashion very similar to the hierarchy used for Java package names. For
example, `org.apache.logging.appender` and `org.apache.logging.filter`
both have `org.apache.logging` as their parent. In most cases,
applications name their loggers by passing the current class's name to
`LogManager.getLogger(...)`. Because this usage is so common, Log4j 2
provides that as the default when the logger name parameter is either
omitted or is null. For example, in all examples below the Logger will
have a name of `"org.apache.test.MyTest"`.

[source,java]
----
package org.apache.test;

public class MyTest {
    private static final Logger logger = LogManager.getLogger(MyTest.class);
}
----

[source,java]
----
package org.apache.test;

public class MyTest {
    private static final Logger logger = LogManager.getLogger(MyTest.class.getName());
}
----

[source,java]
----
package org.apache.test;

public class MyTest {
    private static final Logger logger = LogManager.getLogger();
}
----
