src/site/antora/modules/ROOT/pages/log4j-jul.adoc

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= Log4j JDK Logging Adapter

The JDK Logging Adapter is a custom implementation of http://docs.oracle.com/javase/6/docs/api/java/util/logging/LogManager.html[`java.util.logging.LogManager`] that uses link:javadoc/log4j-core/index.html[Log4j].
This adapter can be used with either the Log4j API or Log4j Core.
When used with the API, there are a couple features of JUL that aren't supported.
However, this does allow any other Log4j Provider besides the Core provider to be used with JUL.

== Requirements

The JDK Logging Adapter is dependent on the Log4j API and optionally Log4j Core.
For more information, see xref:runtime-dependencies.adoc[Runtime Dependencies].

== Usage

To use the JDK Logging Adapter, you must set the system property `java.util.logging.manager` to `org.apache.logging.log4j.jul.LogManager`.

This must be done either through the command line (i.e., using the `-Djava.util.logging.manager=org.apache.logging.log4j.jul.LogManager` argument) or by using `System.setProperty()` before any calls are made to `LogManager` or `Logger`.

== Compatibility

The use of a
https://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Filter.html[java.util.logging.Filter]
is supported on a
https://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Logger.html[per-Logger]
basis.
However, it is recommended to use the standard xref:manual/filters.adoc[Filters] feature in Log4j instead.

The use of
https://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Handler.html[java.util.logging.Handler]
classes is
_NOT_ supported.
Custom Handlers should instead use an appropriate
xref:manual/appenders.adoc[Appender]
or code their own
link:../javadoc/log4j-core/org/apache/logging/log4j/core/Appender.html[Appender]
plugin.

Java logging levels are translated into Log4j logging levels dynamically.
The following table lists the conversions between a Java logging level and its equivalent Log4j level.
Custom levels should be implemented as an implementation of `LevelConverter`, and the Log4j property
xref:manual/systemproperties.adoc#log4j2.julLevelConverter[log4j2.julLevelConverter]
must be set to your custom class name.
Using the default `LevelConverter` implementation, custom logging levels are mapped to whatever the current level of the `Logger` being logged to is using.

[#default-level-conversions]
=== Default Level Conversions

.JUL to Log4j level conversion
|===
| Java Level | Log4j Level

| http://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Level.html#OFF[OFF]
| link:../javadoc/log4j-api/org/apache/logging/log4j/Level#OFF[OFF]

| http://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Level.html#SEVERE[SEVERE]
| link:../javadoc/log4j-api/org/apache/logging/log4j/Level#ERROR[ERROR]

| http://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Level.html#WARNING[WARNING]
| link:../javadoc/log4j-api/org/apache/logging/log4j/Level#WARN[WARN]

| http://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Level.html#OFF[OFF]
| link:../javadoc/log4j-api/org/apache/logging/log4j/Level#INFO[INFO]

| http://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Level.html#CONFIG[CONFIG]
| custom `CONFIG` level

| http://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Level.html#FINE[FINE]
| link:../javadoc/log4j-api/org/apache/logging/log4j/Level#DEBUG[DEBUG]

| http://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Level.html#FINER[FINER]
| link:../javadoc/log4j-api/org/apache/logging/log4j/Level#TRACE[TRACE]

| http://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Level.html#FINEST[FINEST]
| custom `FINEST` level

| http://docs.oracle.com/javase/{java-target-version}/docs/api/java/util/logging/Level.html#ALL[ALL]
| link:../javadoc/log4j-api/org/apache/logging/log4j/Level#ALL[ALL]
|===

== Log4j JDK Logging Bridge Handler

The LogManager is not always useable because you have to set a JVM wide effective system property - e.g.
in web servers this is not possible if you are not the administrator.

The `Log4jBridgeHandler` is an alternative that can be declaratively used via `logging.properties`.

It is less performant than the LogManager but still okay to use: the LogManager replaces the JDK implementation, so your logging code (using JDK syntax) effectively directly uses log4j.
When using the BridgeHandler the original JDK implementation along with its configuration (e.g.
log levels) is still fully working but the log events are "written" via this handler to log4j as if you would have called log4j.Logger.debug() etc.;
it is like a FileHandler but instead of writing to a file, it "writes" to log4j Loggers - thus there is some overhead compared to using LogManager.

== Usage

The JUL configuration file `logging.properties` needs the line

`handlers = org.apache.logging.log4j.jul.Log4jBridgeHandler`

and JUL logs go to log4j2.
Additionally, you typically want to use to following:

`org.apache.logging.log4j.jul.Log4jBridgeHandler.propagateLevels = true`

In a webapp on Tomcat (and maybe other servers, too), you may simply create a `WEB-INF/classes/logging.properties` file with above content.
The bridge and the log levels defined in this file are only valid for your webapp and do _not_ have any impact on the other webapps on the same Tomcat instance.

Alternatively you may call `Log4jBridgeHandler.install()` inside your webapp's initialization code, e.g.
inside `ServletContextListener` or a `ServletFilter` static-class-init.
or `contextInitialized()`.

IMPORTANT: Log levels of JDK should match the ones of log4j.
You may do this manually or use the automatic level propagation via `Log4jBridgeHandler.propagateLevels = true`.