src/site/asciidoc/faq.adoc

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= Frequently Asked Questions
:toc:

'''

[#missing_core]
== I'm seeing this error "Unable to locate a logging implementation, using SimpleLogger". What is wrong?

You have the log4j-api-2.x jar file in your classpath but you still need
to add the log4j-core-2.x jar to the classpath. (Also, it looks like you
are using an old version of Log4j 2. You may want to upgrade.)

[#which_jars]
== Which JAR files do I need?

You need at least the log4j-api-2.x and the log4j-core-2.x jar files.

The other jars are necessary if your application calls the API of
another logging framework and you want to route logging calls to the
Log4j 2 implementation.

image:images/whichjar-2.x.png[Diagram showing which JARs correspond to
which systems]

You can use the log4j-to-slf4j adapter jar when your application calls
the Log4j 2 API and you want to route logging calls to a SLF4J
implementation.

image:images/whichjar-slf4j-2.x.png[Diagram showing the dependency flow
to use Log4j 2 API with SLF4J]

Some of the Log4j components have features with optional dependencies.
The component page will have more detail. For example, the
link:log4j-core/index.html[log4j-core component page] has an outline of
which log4j-core features have external dependencies.

[#exclusions]
== How do I exclude conflicting dependencies?

There are several scenarios where you may end up with conflicting
dependencies, especially transitively included ones. The following table
shows for each Log4j dependency on the left (implicit groupId of
`org.apache.logging.log4j`), the following dependencies on the right can
be safely excluded (given in the format `groupId:artifactId`).

[cols="4*m",options="header"]
|===
|Log4j Dependency
3+|Dependencies to Exclude

|log4j-1.2-api
|log4j:log4j
2+|org.slf4j:log4j-over-slf4j

|log4j-core
|log4j:log4j
|ch.qos.logback:logback-core
|org.apache.logging.log4j:log4j-to-slf4j

|log4j-jcl
3+|org.slf4j:jcl-over-slf4j

|log4j-jul
3+|org.slf4j:jul-to-slf4j

|log4j-slf4j-impl
|org.apache.logging.log4j:log4j-to-slf4j
2+|ch.qos.logback:logback-core
|===

Using Apache Maven, dependencies can be globally excluded in your
project like so:

[source,xml]
----
<dependencies>
  <dependency>
    <groupId>log4j</groupId>
    <artifactId>log4j</artifactId>
    <version>1.2.17</version>
    <scope>provided</scope>
  </dependency>
</dependencies>
----

Dependencies can be explicitly excluded for specific dependencies as
well. For example, to use a project with Log4j 2 instead of Log4j 1.x:

[source,xml,subs="attributes,specialchars"]
----
<dependencies>
  <dependency>
    <groupId>com.example</groupId>
    <artifactId>example-project</artifactId>
    <version>1.0</version>
    <exclusions>
      <exclusion>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
      </exclusion>
      <exclusion>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
      </exclusion>
    </exclusions>
  </dependency>
  <dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core</artifactId>
    <version>{Log4jReleaseVersion}</version>
  </dependency>
  <dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-slf4j-impl</artifactId>
    <version>{Log4jReleaseVersion}</version>
  </dependency>
  <dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-1.2-api</artifactId>
    <version>{Log4jReleaseVersion}</version>
  </dependency>
</dependencies>
----

Dependencies can be globally excluded in Gradle like so:

[source,gradle]
----
configurations {
  all*.exclude group: 'log4j', module: 'log4j'
}
----

The equivalent Gradle config for the above Maven exclusion would look
like:

[source,gradle,subs=attributes]
----
dependencies {
  compile('com.example:example-project:1.0') {
    exclude group: 'log4j', module: 'log4j'
    exclude group: 'org.slf4j', module: 'slf4j-log4j12'
  }
  compile('org.apache.logging.log4j:log4j-core:{Log4jReleaseVersion}')
  compile('org.apache.logging.log4j:log4j-slf4j-impl:{Log4jReleaseVersion}')
  compile('org.apache.logging.log4j:log4j-1.2-api:{Log4jReleaseVersion}')
}
----

[#config_location]
== How do I specify the configuration file location?

By default, Log4j looks for a configuration file named *log4j2.xml* (not
log4j.xml) in the classpath.

You can also specify the full path of the configuration file with this
system property: `-Dlog4j.configurationFile=path/to/log4j2.xml`

That property can also be included in a classpath resource file named
`log4j2.component.properties`.

Web applications can specify the Log4j configuration file location with
a servlet context parameter. See
http://logging.apache.org/log4j/2.x/manual/webapp.html#ContextParams[this
section] of the Using Log4j 2 in Web Applications manual page.

[#config_from_code]
== How do I configure log4j2 in code without a configuration file?

Starting with version 2.4, Log4j 2 provides an
link:manual/customconfig.html[API for programmatic configuration] The
new
link:log4j-core/apidocs/org/apache/logging/log4j/core/config/builder/api/ConfigurationBuilder.html[`ConfigurationBuilder`
API] allows you to create Configurations in code by constructing
component definitions without requiring you to know about the internals
of actual configuration objects like Loggers and Appenders.

[#reconfig_from_code]
== How do I reconfigure log4j2 in code with a specific configuration file?

See the below example. Be aware that this LoggerContext class is not
part of the public API so your code may break with any minor release.

[source,java]
----
// import org.apache.logging.log4j.core.LoggerContext;

LoggerContext context = (org.apache.logging.log4j.core.LoggerContext) LogManager.getContext(false);
File file = new File("path/to/a/different/log4j2.xml");

// this will force a reconfiguration
context.setConfigLocation(file.toURI());
----

[#shutdown]
== How do I shut down log4j2 in code?

Normally there is no need to do this manually. Each `LoggerContext`
registers a shutdown hook that takes care of releasing resources when
the JVM exits (unless system property `log4j.shutdownHookEnabled` is set
to `false`). Web applications should include the log4j-web module in
their classpath which disables the shutdown hook but instead cleans up
log4j resources when the web application is stopped.

However, if you need to manually shut down Log4j, you can do so as in
the below example. Note that there is an optional parameter for
specifying which `LoggerContext` to shut down.

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

// ...

LogManager.shutdown();
----

[#config_sep_appender_level]
== How do I send log messages with different levels to different
appenders? You don’t need to declare separate loggers to achieve this.
You can set the logging level on the `AppenderRef` element.

[source,xml]
----
<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
  <Appenders>
    <File name="file" fileName="app.log">
      <PatternLayout>
        <Pattern>%d %p %c{1.} [%t] %m %ex%n</Pattern>
      </PatternLayout>
    </File>
    <Console name="STDOUT" target="SYSTEM_OUT">
      <PatternLayout pattern="%m%n"/>
    </Console>
  </Appenders>
  <Loggers>
    <Root level="trace">
      <AppenderRef ref="file" level="DEBUG"/>
      <AppenderRef ref="STDOUT" level="INFO"/>
    </Root>
  </Loggers>
</Configuration>
----

[#troubleshooting]
== How do I debug my configuration?

First, make sure you have link:#which_jars[the right jar files] on your
classpath. You need at least log4j-api and log4j-core.

Next, check the name of your configuration file. By default, log4j2 will
look for a configuration file named `log4j2.xml` on the classpath. Note
the ``2'' in the file name! (See the
link:manual/configuration.html#AutomaticConfiguration[configuration
manual page] for more details.)

From log4j-2.9 onward::
From log4j-2.9 onward, log4j2 will print all internal logging to the
console if system property `log4j2.debug` is defined (with any or no
value).

Prior to log4j-2.9::
Prior to log4j-2.9, there are two places where internal logging can be
controlled:
+
If the configuration file is found correctly, log4j2 internal status
logging can be controlled by setting `<Configuration status="trace">` in
the configuration file. This will display detailed log4j2-internal log
statements on the console about what happens during the configuration
process. This may be useful to trouble-shoot configuration issues. By
default the status logger level is WARN, so you only see notifications
when there is a problem.
+
If the configuration file is not found correctly, you can still enable
log4j2 internal status logging by setting system property
`-Dorg.apache.logging.log4j.simplelog.StatusLogger.level=TRACE`.

[#separate_log_files]
== How do I dynamically write to separate log files?

Look at the
http://logging.apache.org/log4j/2.x/manual/appenders.html#RoutingAppender[RoutingAppender].
You can define multiple routes in the configuration, and put values in
the `ThreadContext` map that determine which log file subsequent events
in this thread get logged to.

You can use the `ThreadContext` map value to determine the log file
name.

[source,xml]
----
<Routing name="Routing">
  <Routes pattern="$${ctx:ROUTINGKEY}">

    <!-- This route is chosen if ThreadContext has value 'special' for key ROUTINGKEY. -->
    <Route key="special">
      <RollingFile name="Rolling-${ctx:ROUTINGKEY}" fileName="logs/special-${ctx:ROUTINGKEY}.log"
    filePattern="./logs/${date:yyyy-MM}/${ctx:ROUTINGKEY}-special-%d{yyyy-MM-dd}-%i.log.gz">
    <PatternLayout>
      <pattern>%d{ISO8601} [%t] %p %c{3} - %m%n</pattern>
    </PatternLayout>
    <Policies>
      <TimeBasedTriggeringPolicy interval="6" modulate="true" />
          <SizeBasedTriggeringPolicy size="10 MB" />
    </Policies>
      </RollingFile>
    </Route>

    <!-- This route is chosen if ThreadContext has no value for key ROUTINGKEY. -->
    <Route key="$${ctx:ROUTINGKEY}">
      <RollingFile name="Rolling-default" fileName="logs/default.log"
    filePattern="./logs/${date:yyyy-MM}/default-%d{yyyy-MM-dd}-%i.log.gz">
        <PatternLayout>
      <pattern>%d{ISO8601} [%t] %p %c{3} - %m%n</pattern>
        </PatternLayout>
        <Policies>
          <TimeBasedTriggeringPolicy interval="6" modulate="true" />
          <SizeBasedTriggeringPolicy size="10 MB" />
        </Policies>
      </RollingFile>
    </Route>

    <!-- This route is chosen if ThreadContext has a value for ROUTINGKEY
         (other than the value 'special' which had its own route above).
         The value dynamically determines the name of the log file. -->
    <Route>
      <RollingFile name="Rolling-${ctx:ROUTINGKEY}" fileName="logs/other-${ctx:ROUTINGKEY}.log"
    filePattern="./logs/${date:yyyy-MM}/${ctx:ROUTINGKEY}-other-%d{yyyy-MM-dd}-%i.log.gz">
    <PatternLayout>
      <pattern>%d{ISO8601} [%t] %p %c{3} - %m%n</pattern>
    </PatternLayout>
    <Policies>
      <TimeBasedTriggeringPolicy interval="6" modulate="true" />
      <SizeBasedTriggeringPolicy size="10 MB" />
    </Policies>
      </RollingFile>
    </Route>
  </Routes>
</Routing>
----

[#reconfig_level_from_code]
== How do I set a logger’s level programmatically?

You can set a logger’s level with the class
link:log4j-core/apidocs/org/apache/logging/log4j/core/config/Configurator.html[`Configurator`]
from Log4j Core. Be aware that the `Configurator` class is not part of
the public API.

[source,java]
----
import org.apache.logging.log4j.core.config.Configurator;

// ...

Configurator.setLevel("com.example.Foo", Level.DEBUG);

// You can also set the root logger:
Configurator.setRootLevel(Level.DEBUG);
----

[#retention]
== How do I set my log archive retention policy? How do I delete old log archives?

The `DefaultRolloverStrategy` of the Rolling File appender (and Rolling
Random Access File appender) supports a
link:manual/appenders.html#CustomDeleteOnRollover[Delete] element.

Starting at a specified base directory, you can delete all files for
which some condition holds true, for example all files that match a
given file name pattern and are older than some number of days. More
complex conditions are possible, and if the built-in conditions are not
sufficient, users can provide custom conditions by creating
link:manual/appenders.html#DeletePathCondition[plugin conditions] or by
writing a link:manual/appenders.html#ScriptCondition[script condition].

[#api-tradeoffs]
== What are the trade-offs of using the Log4j 2 API versus the SLF4J API?

The Log4j 2 API and SLF4J have a lot in common. They both share the
objective of cleanly separating the logging API from the implementation.
We believe that the Log4j 2 API can help make your application more
performant while offering more functionality and more flexibility.

There may be a concern that using the Log4j 2 API will tightly couple
your application to Log4j 2. This is not the case: applications coded to
the Log4j 2 API always have the option to use any SLF4J-compliant
library as their logging implementation with the log4j-to-slf4j adapter.
See the link:#which_jars_log4j-to-slf4j[which jars] FAQ entry for
details.

There are several advantages to using the Log4j 2 API:

* SLF4J forces your application to log Strings. The Log4j 2 API supports
logging any CharSequence if you want to log text, but also supports
logging any Object as is. It is the responsibility of the logging
implementation to handle this object, and we consider it a design
mistake to limit applications to logging Strings.
* The Log4j 2 API offers support for logging
link:manual/messages.html[Message objects]. Messages allow support for
interesting and complex constructs to be passed through the logging
system and be efficiently manipulated. Users are free to create their
own Message types and write custom Layouts, Filters and Lookups to
manipulate them.
* The Log4j 2 API has support for Java 8
link:manual/api.html#LambdaSupport[lambda expressions].
* The Log4j 2 API has better support for
link:manual/garbagefree.html[garbage-free logging]: it avoids creating
vararg arrays and avoids creating Strings when logging CharSequence
objects.

[#gc-free-slf4j]
== Is Log4j 2 still garbage-free when I use the SLF4J API?

Yes, the log4j-slf4j-impl binding (together with log4j-core) implements
the `org.slf4j.Logger` methods to be GC-free. However, bear in mind that
there are some limitations:

The SLF4J API only offers up to two parameters for a parameterized
message. More than that uses varargs which creates a temporary object
for the parameter array. The Log4j 2.6 API has methods for up to ten
unrolled parameters.

Another consideration is that the SLF4J API forces your application to
log Strings. Log4j 2 API lets you log any java.lang.CharSequence, and
even any Objects. Log4j can log any Object that implements
`java.lang.CharSequence` or
`org.apache.logging.log4j.util.StringBuilderFormattable` without
creating garbage.

The
http://www.slf4j.org/api/org/slf4j/spi/LocationAwareLogger.html#log(org.slf4j.Marker,%20java.lang.String,%20int,%20java.lang.String,%20java.lang.Object%5B%5D,%20java.lang.Throwable)[`org.slf4j.spi.LocationAwareLogger::log`]
method is not yet implemented in a garbage-free manner in the
log4j-slf4j-impl binding. It creates a new message object for each call.

[#gc-free-domain-object]
== How do I log my domain object without creating garbage?

One option is to let the domain object implement java.lang.CharSequence.
However, for many domain objects it may not be trivial to implement this
without allocating temporary objects.

An alternative is to implement the
`org.apache.logging.log4j.util.StringBuilderFormattable` interface. If
an object is logged that implements this interface, its `formatTo`
method is called instead of `toString()`.

[source,java]
----
package org.apache.logging.log4j.util;
public interface StringBuilderFormattable {
    /**
     * Writes a text representation of this object into the specified {@code StringBuilder},
     * ideally without allocating temporary objects.
     *
     * @param buffer the StringBuilder to write into
     */
     void formatTo(StringBuilder buffer);
}
----

[#logger-wrapper]
== How do I create a custom logger wrapper that shows the correct class, method and line number?

Log4j remembers the fully qualified class name (FQCN) of the logger and
uses this to walk the stack trace for every log event when configured to
print location. (Be aware that logging with location is slow and may
impact the performance of your application.)

The problem with custom logger wrappers is that they have a different
FQCN than the actual logger, so Log4j can’t find the place where your
custom logger was called.

The solution is to provide the correct FQCN. The easiest way to do this
is to let Log4j generate the logger wrapper for you. Log4j comes with a
Logger wrapper generator tool. This tool was originally meant to support
custom log levels and is documented
https://logging.apache.org/log4j/2.x/manual/customloglevels.html#CustomLoggers[here].

The generated logger code will take care of the FQCN.