Configuration Architecture

In part because support for XML was added first, Log4j's configuration is reflected as a tree structure. In fact every configuration dialect, including the ConfigurationBuilder, generates a Node for every configuration element. A node is a fairly simple structure that contains a set of attributes, a set of child nodes and a PluginType. It is important to note that every Node must have a corresponding plugin, as the plugin is the component that actually performs the work represented by the node.

Every document type supported by Log4j has a ConfigurationFactory. The factory itself is a Log4j plugin that declares what file extensions it supports and what its priority is. Properties have the highest precedence with a value of 8, followed by yaml, json and xml. When autoconfiguration is performed Log4j will call each of these factories in order to determine which, if any, support the specified configuration file format. If one is found that factory will create the corresponding Configuration object and pass the reference to the configuration data to it.

Every configuration implementation, such as XMLConfiguration, YamlConfiguration, JsonConfiguration, etc. has the primary task of converting the configuration text into the Node tree, typically by parsing the text with whatever tool is available for that document type. It should be noted that while most of the supported document types are inherently tree structured, the Java properties syntax is not. Because of the need to convert the syntax into a Node tree the Java properties syntax used by Log4j required all properties follow a naming pattern that made the tree structure clear. As a consequence, the Java Properties format tends to be more verbose than using a different document type.

Once the Node tree is created control is delegated to AbstractConfiguration, which converts the Nodes into their respective Java objects using Log4j's Plugin system and provides all the common functionality.

Arbiters

In some situations it is desirable to have a single logging configuration that can be used in any deployment environment. For example, it may be necessary to have a different default logging level in production then in development. Another case might be where one type of appender is used when running natively but another is used when deployed to a docker container. One way to handle that is to use a tool such as Spring Cloud Config Server that can be environment aware and serve a different file for each environment. Another option is to include Arbiters in the configuration.

An Arbiter is a Log4j plugin that has the job of determining whether other configured elements should be included in the generated configuration. While all other "Core" plugins are designed to execute as part of Log4j's runtime logic Arbiters execute after the Node tree has been constructed but before the tree is converted to a configuration. An Arbiter is a Node itself which is always removed from the Node tree before it the tree is processed. All an arbiter really does is provide a method that returns a boolean result that determines whether the child nodes of the arbiter should remain in the configuration or be pruned.

Arbiters may occur anywhere an element is allowed in the configuration. So an Arbiter could encapsulate something as simple as a single property declaration or a whole set of Appenders or Loggers. Arbiters may also be nested although Arbiters that are the descendant of another arbiter will only be evaluated if the ancestor returned true. The child elements of an Arbiter must be valid elements for whatever element is the parent of the Arbiter.

This example shows two Arbiters configured that will include either a Console Appender or a List Appender depending on whether the value of the env System Property is "dev" or "prod".

<Configuration name="ConfigTest" status="ERROR" monitorInterval="5">
  <Appenders>

    <SystemPropertyArbiter propertyName="env" propertyValue="dev">
      <Console name="Out">
        <PatternLayout pattern="%m%n"/>
      </Console>
    </SystemPropertyArbiter>
    <SystemPropertyArbiter propertyName="env" propertyValue="prod">
      <List name="Out">
      </List>
    </SystemPropertyArbiter>

  </Appenders>
  <Loggers>
    <Logger name="org.apache.test" level="trace" additivity="false">
      <AppenderRef ref="Out"/>
    </Logger>
    <Root level="error">
      <AppenderRef ref="Out"/>
    </Root>
  </Loggers>
</Configuration>

Normally Arbiters act in isolation from other Arbiters. That is, the outcome of one Arbiter will not impact any other Arbiters. This can be cumbersome when you simply want to use one of a set of choices. A special plugin named "Select" can be used in this case. Each element under the Select is required to be an Arbiter. The first Arbiter that returns a true value will be the one used while others are ignored. If no Arbiter returns true a DefaultArbiter may be configured with the default configuration elements. The DefaultArbiter is an Arbiter that always returns true, so using it outside of a Select would result in its configured elements always being included just as if it hadn't been present.

This example shows an Arbiter that uses Javascript residing in a separate file to determine whether to include the Console Appender. If the result is false then a List Appender will be included.

<Configuration name="ConfigTest" status="ERROR" monitorInterval="5">
  <Appenders>
    <Select>
      <ScriptArbiter>
        <ScriptFile language="JavaScript" path="src/test/resources/scripts/prodtest.js" charset="UTF-8" />
        <Console name="Out">
          <PatternLayout pattern="%m%n"/>
        </Console>
      </ScriptArbiter>
      <DefaultArbiter>
        <List name="Out">
        </List>
      </DefaultArbiter>
    </Select>
  </Appenders>
  <Loggers>
    <Logger name="org.apache.test" level="trace" additivity="false">
      <AppenderRef ref="Out"/>
    </Logger>
    <Root level="error">
      <AppenderRef ref="Out"/>
    </Root>
  </Loggers>
</Configuration>

Natively Log4j contains the SystemProperty Arbiter that can evaluate whether to include elements based on whether a SystemProperty is non-null or has a specific value, a ClassArbiter that makes its decision based on whether the specified class is present, and a ScriptArbiter that makes its decision based on the result of the script configured with it.

For Spring Boot users an Arbiter named SpringProfile has been provided. The specified profiles are evaluated by Spring's Environment.acceptsProfiles() method, so any expressions it supports may be used as the name attribute.

This example will use a Console Appender when the Spring profile is "dev" or "staging" and a List Appender when the active profile is "prod".

<Configuration name="ConfigTest" status="ERROR" monitorInterval="5">
  <Appenders>

    <SpringProfile name="dev | staging">
      <Console name="Out">
        <PatternLayout pattern="%m%n"/>
      </Console>
    </SpringProfile>
    <SpringProfile name="prod">
      <List name="Out">
      </List>
    </SpringProfile>

  </Appenders>
  <Loggers>
    <Logger name="org.apache.test" level="trace" additivity="false">
      <AppenderRef ref="Out"/>
    </Logger>
    <Root level="error">
      <AppenderRef ref="Out"/>
    </Root>
  </Loggers>
</Configuration>

Automatic Configuration

Log4j has the ability to automatically configure itself during initialization. When Log4j starts it will locate all the ConfigurationFactory plugins and arrange them in weighted order from highest to lowest. As delivered, Log4j contains four ConfigurationFactory implementations: one for JSON, one for YAML, one for properties, and one for XML.

1. Log4j will inspect the "log4j2.configurationFile" system property and, if set, will attempt to load the configuration using the ConfigurationFactory that matches the file extension. Note that this is not restricted to a location on the local file system and may contain a URL.
2. If no system property is set the properties ConfigurationFactory will look for log4j2-test.properties in the classpath.
3. If no such file is found the YAML ConfigurationFactory will look for log4j2-test.yaml or log4j2-test.yml in the classpath.
4. If no such file is found the JSON ConfigurationFactory will look for log4j2-test.json or log4j2-test.jsn in the classpath.
5. If no such file is found the XML ConfigurationFactory will look for log4j2-test.xml in the classpath.
6. If a test file cannot be located the properties ConfigurationFactory will look for log4j2.properties on the classpath.
7. If a properties file cannot be located the YAML ConfigurationFactory will look for log4j2.yaml or log4j2.yml on the classpath.
8. If a YAML file cannot be located the JSON ConfigurationFactory will look for log4j2.json or log4j2.jsn on the classpath.
9. If a JSON file cannot be located the XML ConfigurationFactory will try to locate log4j2.xml on the classpath.
10. If no configuration file could be located the DefaultConfiguration will be used. This will cause logging output to go to the console.

An example application named MyApp that uses log4j can be used to illustrate how this is done.

import com.foo.Bar;

// Import log4j classes.
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.LogManager;

public class MyApp {

    // Define a static logger variable so that it references the
    // Logger instance named "MyApp".
    private static final Logger logger = LogManager.getLogger(MyApp.class);

    public static void main(final String... args) {

        // Set up a simple configuration that logs on the console.

        logger.trace("Entering application.");
        Bar bar = new Bar();
        if (!bar.doIt()) {
            logger.error("Didn't do it.");
        }
        logger.trace("Exiting application.");
    }
}

MyApp begins by importing Log4j related classes. It then defines a static logger variable with the name MyApp which happens to be the fully qualified name of the class.

MyApp uses the Bar class defined in the packagecom.foo.

package com.foo;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.LogManager;

public class Bar {
  static final Logger logger = LogManager.getLogger(Bar.class.getName());

  public boolean doIt() {
    logger.entry();
    logger.error("Did it again!");
    return logger.exit(false);
  }
}

Log4j will provide a default configuration if it cannot locate a configuration file. The default configuration, provided in the DefaultConfiguration class, will set up:

• A ConsoleAppender attached to the root logger.
• A PatternLayout set to the pattern "%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n" attached to the ConsoleAppender

Note that by default Log4j assigns the root logger to Level.ERROR.

The output of MyApp would be similar to:

17:13:01.540 [main] ERROR com.foo.Bar - Did it again!
17:13:01.540 [main] ERROR MyApp - Didn't do it.

As was described previously, Log4j will first attempt to configure itself from configuration files. A configuration equivalent to the default would look like:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
  <Appenders>
    <Console name="Console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
    </Console>
  </Appenders>
  <Loggers>
    <Root level="error">
      <AppenderRef ref="Console"/>
    </Root>
  </Loggers>
</Configuration>

Once the file above is placed into the classpath as log4j2.xml you will get results identical to those listed above. Changing the root level to trace will result in results similar to:

17:13:01.540 [main] TRACE MyApp - Entering application.
17:13:01.540 [main] TRACE com.foo.Bar - entry
17:13:01.540 [main] ERROR com.foo.Bar - Did it again!
17:13:01.540 [main] TRACE com.foo.Bar - exit with (false)
17:13:01.540 [main] ERROR MyApp - Didn't do it.
17:13:01.540 [main] TRACE MyApp - Exiting application.

Note that status logging is disabled when the default configuration is used.

Configuration From a URI

When log4j2.configurationFile references a URL, Log4j will first determine if the URL reference a file using the file protocol. If it does Log4j will validate that the file URL is valid and continue processing as previously described. If it contains a protocol other than file then Log4j will inspect the value of the log4j2.Configuration.allowedProtocols system property. If the provided list contains the protocol specified then Log4j will use the URI to locate the specified configuration file. If not an exception will be thrown and an error message will be logged. If no value is provided for the system property it will default to "https, file, jar". Use of any protocol other than "file" can be prevented by setting the system property value to "_none". This value would be an invalid protocol so cannot conflict with any custom protocols that may be present.

Log4j supports access to remote URLs that require authentication. Log4j supports basic authentication out of the box. If the log4j2.Configuration.username and log4j2.Configuration.password are specified those values will be used to perform the authentication. If the password is encrypted a custom password decryptor may be supplied by specifying the fully qualified class name in the log4j2.Configuration.passwordDecryptor system property. A custom AuthenticationProvider may be used by setting the log4j2.Configuration.authenticationProvider system property to the fully qualified class name of the provider.

Additivity

Perhaps it is desired to eliminate all the TRACE output from everything except com.foo.Bar. Simply changing the log level would not accomplish the task. Instead, the solution is to add a new logger definition to the configuration:

<Logger name="com.foo.Bar" level="TRACE"/>
<Root level="ERROR">
  <AppenderRef ref="STDOUT">
</Root>

With this configuration all log events from com.foo.Bar will be recorded while only error events will be recorded from all other components.

In the previous example all the events from com.foo.Bar were still written to the Console. This is because the logger for com.foo.Bar did not have any appenders configured while its parent did. In fact, the following configuration

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
  <Appenders>
    <Console name="Console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
    </Console>
  </Appenders>
  <Loggers>
    <Logger name="com.foo.Bar" level="trace">
      <AppenderRef ref="Console"/>
    </Logger>
    <Root level="error">
      <AppenderRef ref="Console"/>
    </Root>
  </Loggers>
</Configuration>

would result in

17:13:01.540 [main] TRACE com.foo.Bar - entry
17:13:01.540 [main] TRACE com.foo.Bar - entry
17:13:01.540 [main] ERROR com.foo.Bar - Did it again!
17:13:01.540 [main] TRACE com.foo.Bar - exit (false)
17:13:01.540 [main] TRACE com.foo.Bar - exit (false)
17:13:01.540 [main] ERROR MyApp - Didn't do it.

Notice that the trace messages from com.foo.Bar appear twice. This is because the appender associated with logger com.foo.Bar is first used, which writes the first instance to the Console. Next, the parent of com.foo.Bar, which in this case is the root logger, is referenced. The event is then passed to its appender, which is also writes to the Console, resulting in the second instance. This is known as additivity. While additivity can be quite a convenient feature (as in the first previous example where no appender reference needed to be configured), in many cases this behavior is considered undesirable and so it is possible to disable it by setting the additivity attribute on the logger to false:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
  <Appenders>
    <Console name="Console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
    </Console>
  </Appenders>
  <Loggers>
    <Logger name="com.foo.Bar" level="trace" additivity="false">
      <AppenderRef ref="Console"/>
    </Logger>
    <Root level="error">
      <AppenderRef ref="Console"/>
    </Root>
  </Loggers>
</Configuration>

Once an event reaches a logger with its additivity set to false the event will not be passed to any of its parent loggers, regardless of their additivity setting.

Automatic Reconfiguration

When configured from a File, Log4j has the ability to automatically detect changes to the configuration file and reconfigure itself. If the monitorInterval attribute is specified on the configuration element and is set to a non-zero value then the file will be checked the next time a log event is evaluated and/or logged and the monitorInterval has elapsed since the last check. The example below shows how to configure the attribute so that the configuration file will be checked for changes only after at least 30 seconds have elapsed. The minimum interval is 5 seconds.

<?xml version="1.0" encoding="UTF-8"?>
<Configuration monitorInterval="30">
...
</Configuration>

Chainsaw can automatically process your log files (Advertising appender configurations)

Log4j provides the ability to 'advertise' appender configuration details for all file-based appenders as well as socket-based appenders. For example, for file-based appenders, the file location and the pattern layout in the file are included in the advertisement. Chainsaw and other external systems can discover these advertisements and use that information to intelligently process the log file.

The mechanism by which an advertisement is exposed, as well as the advertisement format, is specific to each Advertiser implementation. An external system which would like to work with a specific Advertiser implementation must understand how to locate the advertised configuration as well as the format of the advertisement. For example, a 'database' Advertiser may store configuration details in a database table. An external system can read that database table in order to discover the file location and the file format.

Log4j provides one Advertiser implementation, a 'multicastdns' Advertiser, which advertises appender configuration details via IP multicast using the JmDNS library.

Chainsaw automatically discovers log4j's multicastdns-generated advertisements and displays those discovered advertisements in Chainsaw's Zeroconf tab (if the jmdns library is in Chainsaw's classpath). To begin parsing and tailing a log file provided in an advertisement, just double-click the advertised entry in Chainsaw's Zeroconf tab. Currently, Chainsaw only supports FileAppender advertisements.

To advertise an appender configuration:

• Add the JmDNS library from JmDNS GitHub Releases to the application classpath
• Set the 'advertiser' attribute of the configuration element to 'multicastdns'
• Set the 'advertise' attribute on the appender element to 'true'
• If advertising a FileAppender-based configuration, set the 'advertiseURI' attribute on the appender element to an appropriate URI

FileAppender-based configurations require an additional 'advertiseURI' attribute to be specified on the appender. The 'advertiseURI' attribute provides Chainsaw with information on how the file can be accessed. For example, the file may be remotely accessible to Chainsaw via ssh/sftp by specifying a Commons VFS (https://commons.apache.org/proper/commons-vfs/) sftp:// URI, an http:// URI may be used if the file is accessible through a web server, or a file:// URI can be specified if accessing the file from a locally-running instance of Chainsaw.

Here is an example advertisement-enabled appender configuration which can be used by a locally-running Chainsaw to automatically tail the log file (notice the file:// advertiseURI):

Please note, you must add the JmDNS library mentioned above.

<?xml version="1.0" encoding="UTF-8"?>
<Configuration advertiser="multicastdns">
...
</Configuration>
<Appenders>
  <File name="File1" fileName="output.log" bufferedIO="false" advertiseURI="file://path/to/output.log" advertise="true">
  ...
  </File>
</Appenders>

Configuration Syntax

As of version 2.9, for security reasons, Log4j does not process DTD in XML files. If you want to split the configuration in multiple files, use XInclude or Composite Configuration.

As the previous examples have shown as well as those to follow, Log4j allows you to easily redefine logging behavior without needing to modify your application. It is possible to disable logging for certain parts of the application, log only when specific criteria are met such as the action being performed for a specific user, route output to Flume or a log reporting system, etc. Being able to do this requires understanding the syntax of the configuration files.

The configuration element in the XML file accepts several attributes:

Configuration with XML

Log4j can be configured using two XML flavors; concise and strict.

<PatternLayout pattern="%m%n"/>

<PatternLayout>
  <Pattern>%m%n</Pattern>
</PatternLayout>

<?xml version="1.0" encoding="UTF-8"?>;
<Configuration>
  <Properties>
    <Property name="name1">value</property>
    <Property name="name2" value="value2"/>
  </Properties>
  <filter  ... />
  <Appenders>
    <appender ... >
      <filter  ... />
    </appender>
    ...
  </Appenders>
  <Loggers>
    <Logger name="name1">
      <filter  ... />
    </Logger>
    ...
    <Root level="level">
      <AppenderRef ref="name"/>
    </Root>
  </Loggers>
</Configuration>

<?xml version="1.0" encoding="UTF-8"?>;
<Configuration>
  <Properties>
    <Property name="name1">value</property>
    <Property name="name2" value="value2"/>
  </Properties>
  <Filter type="type" ... />
  <Appenders>
    <Appender type="type" name="name">
      <Filter type="type" ... />
    </Appender>
    ...
  </Appenders>
  <Loggers>
    <Logger name="name1">
      <Filter type="type" ... />
    </Logger>
    ...
    <Root level="level">
      <AppenderRef ref="name"/>
    </Root>
  </Loggers>
</Configuration>

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="debug" strict="true" name="XMLConfigTest"
               packages="org.apache.logging.log4j.test">
  <Properties>
    <Property name="filename">target/test.log</Property>
  </Properties>
  <Filter type="ThresholdFilter" level="trace"/>

  <Appenders>
    <Appender type="Console" name="STDOUT">
      <Layout type="PatternLayout" pattern="%m MDC%X%n"/>
      <Filters>
        <Filter type="MarkerFilter" marker="FLOW" onMatch="DENY" onMismatch="NEUTRAL"/>
        <Filter type="MarkerFilter" marker="EXCEPTION" onMatch="DENY" onMismatch="ACCEPT"/>
      </Filters>
    </Appender>
    <Appender type="Console" name="FLOW">
      <Layout type="PatternLayout" pattern="%C{1}.%M %m %ex%n"/><!-- class and line number -->
      <Filters>
        <Filter type="MarkerFilter" marker="FLOW" onMatch="ACCEPT" onMismatch="NEUTRAL"/>
        <Filter type="MarkerFilter" marker="EXCEPTION" onMatch="ACCEPT" onMismatch="DENY"/>
      </Filters>
    </Appender>
    <Appender type="File" name="File" fileName="${filename}">
      <Layout type="PatternLayout">
        <Pattern>%d %p %C{1.} [%t] %m%n</Pattern>
      </Layout>
    </Appender>
  </Appenders>

  <Loggers>
    <Logger name="org.apache.logging.log4j.test1" level="debug" additivity="false">
      <Filter type="ThreadContextMapFilter">
        <KeyValuePair key="test" value="123"/>
      </Filter>
      <AppenderRef ref="STDOUT"/>
    </Logger>

    <Logger name="org.apache.logging.log4j.test2" level="debug" additivity="false">
      <AppenderRef ref="File"/>
    </Logger>

    <Root level="trace">
      <AppenderRef ref="STDOUT"/>
    </Root>
  </Loggers>

</Configuration>

{ "configuration": { "status": "error", "name": "RoutingTest",
                     "packages": "org.apache.logging.log4j.test",
      "properties": {
        "property": { "name": "filename",
                      "value" : "target/rolling1/rollingtest-$${sd:type}.log" }
      },
    "ThresholdFilter": { "level": "debug" },
    "appenders": {
      "Console": { "name": "STDOUT",
        "PatternLayout": { "pattern": "%m%n" },
        "ThresholdFilter": { "level": "debug" }
      },
      "Routing": { "name": "Routing",
        "Routes": { "pattern": "$${sd:type}",
          "Route": [
            {
              "RollingFile": {
                "name": "Rolling-${sd:type}", "fileName": "${filename}",
                "filePattern": "target/rolling1/test1-${sd:type}.%i.log.gz",
                "PatternLayout": {"pattern": "%d %p %c{1.} [%t] %m%n"},
                "SizeBasedTriggeringPolicy": { "size": "500" }
              }
            },
            { "AppenderRef": "STDOUT", "key": "Audit"}
          ]
        }
      }
    },
    "loggers": {
      "logger": { "name": "EventLogger", "level": "info", "additivity": "false",
                  "AppenderRef": { "ref": "Routing" }},
      "root": { "level": "error", "AppenderRef": { "ref": "STDOUT" }}
    }
  }
}

{ "configuration": { "status": "debug", "name": "RoutingTest",
                      "packages": "org.apache.logging.log4j.test",
      "properties": {
        "property": { "name": "filename",
                      "value" : "target/rolling1/rollingtest-$${sd:type}.log" }
      },
    "ThresholdFilter": { "level": "debug" },
    "appenders": {
      "appender": [
         { "type": "Console", "name": "STDOUT", "PatternLayout": { "pattern": "%m%n" }, "ThresholdFilter": { "level": "debug" }},
         { "type": "Routing",  "name": "Routing",
          "Routes": { "pattern": "$${sd:type}",
            "Route": [
              {
                "RollingFile": {
                  "name": "Rolling-${sd:type}", "fileName": "${filename}",
                  "filePattern": "target/rolling1/test1-${sd:type}.%i.log.gz",
                  "PatternLayout": {"pattern": "%d %p %c{1.} [%t] %m%n"},
                  "SizeBasedTriggeringPolicy": { "size": "500" }
                }
              },
              { "AppenderRef": "STDOUT", "key": "Audit"}
            ]
          }
        }
      ]
    },
    "loggers": {
      "logger": [
        { "name": "EventLogger", "level": "info", "additivity": "false",
          "AppenderRef": { "ref": "Routing" }},
        { "name": "com.foo.bar", "level": "error", "additivity": "false",
          "AppenderRef": { "ref": "STDOUT" }}
      ],
      "root": { "level": "error", "AppenderRef": { "ref": "STDOUT" }}
    }
  }
}

Configuration:
  status: warn
  name: YAMLConfigTest
  properties:
    property:
      name: filename
      value: target/test-yaml.log
  thresholdFilter:
    level: debug
  appenders:
    Console:
      name: STDOUT
      target: SYSTEM_OUT
      PatternLayout:
        Pattern: "%m%n"
    File:
      name: File
      fileName: ${filename}
      PatternLayout:
        Pattern: "%d %p %C{1.} [%t] %m%n"
      Filters:
        ThresholdFilter:
          level: error

  Loggers:
    logger:
      -
        name: org.apache.logging.log4j.test1
        level: debug
        additivity: false
        ThreadContextMapFilter:
          KeyValuePair:
            key: test
            value: 123
        AppenderRef:
          ref: STDOUT
      -
        name: org.apache.logging.log4j.test2
        level: debug
        additivity: false
        AppenderRef:
          ref: File
    Root:
      level: error
      AppenderRef:
        ref: STDOUT
          

appender.stdout.type = Console
# ... other appender properties
appender.file.type = File
# ... other appender properties
logger.app = INFO, stdout, file
logger.app.name = com.example.app

# is equivalent to:
# appender.stdout.type = Console
# appender.stdout.name = stdout
# ...
appender.file.type = File
appender.file.name = file
# ...
logger.app.name = com.example.app
logger.app.level = INFO
logger.app.appenderRef.$1.ref = stdout
logger.app.appenderRef.$2.ref = file
            

status = error
dest = err
name = PropertiesConfig

property.filename = target/rolling/rollingtest.log

filter.threshold.type = ThresholdFilter
filter.threshold.level = debug

appender.console.type = Console
appender.console.name = STDOUT
appender.console.layout.type = PatternLayout
appender.console.layout.pattern = %m%n
appender.console.filter.threshold.type = ThresholdFilter
appender.console.filter.threshold.level = error

appender.rolling.type = RollingFile
appender.rolling.name = RollingFile
appender.rolling.fileName = ${filename}
appender.rolling.filePattern = target/rolling2/test1-%d{MM-dd-yy-HH-mm-ss}-%i.log.gz
appender.rolling.layout.type = PatternLayout
appender.rolling.layout.pattern = %d %p %C{1.} [%t] %m%n
appender.rolling.policies.type = Policies
appender.rolling.policies.time.type = TimeBasedTriggeringPolicy
appender.rolling.policies.time.interval = 2
appender.rolling.policies.time.modulate = true
appender.rolling.policies.size.type = SizeBasedTriggeringPolicy
appender.rolling.policies.size.size=100MB
appender.rolling.strategy.type = DefaultRolloverStrategy
appender.rolling.strategy.max = 5

logger.rolling = debug, RollingFile
logger.rolling.name = com.example.my.app
logger.rolling.additivity = false

rootLogger = info, STDOUT

# or using a grouping element:
# rootLogger.level = info
# rootLogger.appenderRef.stdout.ref = STDOUT
          

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="debug" name="XMLConfigTest" packages="org.apache.logging.log4j.test">
  <Properties>
    <Property name="filename">target/test.log</Property>
  </Properties>
  <ThresholdFilter level="trace"/>

  <Appenders>
    <Console name="STDOUT">
      <PatternLayout pattern="%m MDC%X%n"/>
    </Console>
    <Console name="FLOW">
      <!-- this pattern outputs class name and line number -->
      <PatternLayout pattern="%C{1}.%M %m %ex%n"/>
      <filters>
        <MarkerFilter marker="FLOW" onMatch="ACCEPT" onMismatch="NEUTRAL"/>
        <MarkerFilter marker="EXCEPTION" onMatch="ACCEPT" onMismatch="DENY"/>
      </filters>
    </Console>
    <File name="File" fileName="${filename}">
      <PatternLayout>
        <pattern>%d %p %C{1.} [%t] %m%n</pattern>
      </PatternLayout>
    </File>
  </Appenders>

  <Loggers>
    <Logger name="org.apache.logging.log4j.test1" level="debug" additivity="false">
      <ThreadContextMapFilter>
        <KeyValuePair key="test" value="123"/>
      </ThreadContextMapFilter>
      <AppenderRef ref="STDOUT"/>
    </Logger>

    <Logger name="org.apache.logging.log4j.test2" level="debug" additivity="false">
      <Property name="user">${sys:user.name}</Property>
      <AppenderRef ref="File">
        <ThreadContextMapFilter>
          <KeyValuePair key="test" value="123"/>
        </ThreadContextMapFilter>
      </AppenderRef>
      <AppenderRef ref="STDOUT" level="error"/>
    </Logger>

    <Root level="trace">
      <AppenderRef ref="STDOUT"/>
    </Root>
  </Loggers>

</Configuration>

Property Substitution

Log4j 2 supports the ability to specify tokens in the configuration as references to properties defined elsewhere. Some of these properties will be resolved when the configuration file is interpreted while others may be passed to components where they will be evaluated at runtime. To accomplish this, Log4j uses variations of Apache Commons Lang's StrSubstitutor and StrLookup classes. In a manner similar to Ant or Maven, this allows variables declared as ${name} to be resolved using properties declared in the configuration itself. For example, the following example shows the filename for the rolling file appender being declared as a property.

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="debug" name="RoutingTest" packages="org.apache.logging.log4j.test">
  <Properties>
    <Property name="filename">target/rolling1/rollingtest-$${sd:type}.log</Property>
  </Properties>
  <ThresholdFilter level="debug"/>

  <Appenders>
    <Console name="STDOUT">
      <PatternLayout pattern="%m%n"/>
      <ThresholdFilter level="debug"/>
    </Console>
    <Routing name="Routing">
      <Routes pattern="$${sd:type}">
        <Route>
          <RollingFile name="Rolling-${sd:type}" fileName="${filename}"
                       filePattern="target/rolling1/test1-${sd:type}.%i.log.gz">
            <PatternLayout>
              <pattern>%d %p %c{1.} [%t] %m%n</pattern>
            </PatternLayout>
            <SizeBasedTriggeringPolicy size="500" />
          </RollingFile>
        </Route>
        <Route ref="STDOUT" key="Audit"/>
      </Routes>
    </Routing>
  </Appenders>

  <Loggers>
    <Logger name="EventLogger" level="info" additivity="false">
      <AppenderRef ref="Routing"/>
    </Logger>

    <Root level="error">
      <AppenderRef ref="STDOUT"/>
    </Root>
  </Loggers>

</Configuration>

While this is useful, there are many more places properties can originate from. To accommodate this, Log4j also supports the syntax ${prefix:name} where the prefix identifies tells Log4j that variable name should be evaluated in a specific context. See the Lookups manual page for more details. The contexts that are built in to Log4j are:

Default Properties

A default property map can be declared in the configuration file by placing a Properties element directly after the Configuration element and before any Loggers, Filters, Appenders, etc. are declared. If the value cannot be located in the specified lookup the value in the default property map will be used. The default map is pre-populated with a value for "hostName" that is the current system's host name or IP address and the "contextName" with is the value of the current logging context. See many places a Properties element is used in this section for examples.

Default properties may also be specified in the Lookup by using the syntax ${lookupName:key:-defaultValue}. In some cases the key might contain a leading '-'. When this is the case an escape character must be included, such as ${main:\--file:-app.properties}. This would use the MainMapLookup for a key named --file. If the key is not found then app.properties would be used as the default value.

Enabling Message Pattern Lookups

A message is processed (by default) without using lookups, for example if you defined <Property name="foo.bar">FOO_BAR </Property>, then logger.info("${foo.bar}") will output ${foo.bar} instead of FOO_BAR. You could enable message pattern lookups by defining message pattern using %m{lookups}.

Lookup Variables with Multiple Leading '$' Characters

An interesting feature of StrLookup processing is that when a variable reference is declared with multiple leading '$' characters each time the variable is resolved the leading '$' is simply removed. In the previous example the "Routes" element is capable of resolving the variable at runtime. To allow this the prefix value is specified as a variable with two leading '$' characters. When the configuration file is first processed the first '$' character is simply removed. Thus, when the Routes element is evaluated at runtime it is the variable declaration "${sd:type}" which causes the event to be inspected for a StructuredDataMessage and if one is present the value of its type attribute to be used as the routing key. Not all elements support resolving variables at runtime. Components that do will specifically call that out in their documentation.

If no value is found for the key in the Lookup associated with the prefix then the value associated with the key in the properties declaration in the configuration file will be used. If no value is found the variable declaration will be returned as the value. Default values may be declared in the configuration by doing:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
  <Properties>
    <Property name="type">Audit</property>
  </Properties>
  ...
</Configuration>

As a footnote, it is worth pointing out that the variables in the RollingFile appender declaration will also not be evaluated when the configuration is processed. This is simply because the resolution of the whole RollingFile element is deferred until a match occurs. See RoutingAppender for more information.

Scripts

Log4j provides support for JSR 223 scripting languages to be used in some of its components. Any language that provides support for the JSR 223 scripting engine may be used. A list of the languages and bindings for them can be found at the Scripting Engine web site. However, some of the languages listed there, such as JavaScript, Groovy and Beanshell, directly support the JSR 223 scripting framework and only require that the jars for that language be installed.

As of Log4j 2.17.2 the languages to be supported must be specified as a comma separated list in the log4j2.Script.enableLanguages system property.

The components that support using scripts do so by allowing a <script>, <scriptFile>, or <scriptRef> element to be configured on them. The script element contains a name for the script, the language of the script, and the script text. The scriptFile element contains the name of the script, its location, its language, its charset, and whether the file should be watched for changes. The scriptRef element contains the name of the script that is defined in the <scripts> configuration element. The name of the script is used to store the script, along with its ScriptEngine, so it can quickly be located each time the script needs to be run. While the name is not required, providing it will help in debugging problems when the script is running. The language must be provided on the script element and must specify one of the language names that appear in the Configuration status log as described in the next section. If the language is not specified on the scriptFile element the language will be determined by the file extension of the script path. If file monitoring is requested it will only be enabled if a non-zero monitorInterval is specified on the configuration element. That interval will be used to check for changes in the file.

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="debug" name="RoutingTest">
  <Scripts>
    <Script name="selector" language="javascript"><![CDATA[
            var result;
            if (logEvent.getLoggerName().equals("JavascriptNoLocation")) {
                result = "NoLocation";
            } else if (logEvent.getMarker() != null && logEvent.getMarker().isInstanceOf("FLOW")) {
                result = "Flow";
            }
            result;
            ]]></Script>
    <ScriptFile name="groovy.filter" path="scripts/filter.groovy"/>
  </Scripts>

  <Appenders>
    <Console name="STDOUT">
      <ScriptPatternSelector defaultPattern="%d %p %m%n">
        <ScriptRef ref="selector"/>
          <PatternMatch key="NoLocation" pattern="[%-5level] %c{1.} %msg%n"/>
          <PatternMatch key="Flow" pattern="[%-5level] %c{1.} ====== %C{1.}.%M:%L %msg ======%n"/>
      </ScriptPatternSelector>
      <PatternLayout pattern="%m%n"/>
    </Console>
  </Appenders>

  <Loggers>
    <Logger name="EventLogger" level="info" additivity="false">
        <ScriptFilter onMatch="ACCEPT" onMisMatch="DENY">
          <Script name="GroovyFilter" language="groovy"><![CDATA[
            if (logEvent.getMarker() != null && logEvent.getMarker().isInstanceOf("FLOW")) {
                return true;
            } else if (logEvent.getContextMap().containsKey("UserId")) {
                return true;
            }
            return false;
            ]]>
          </Script>
        </ScriptFilter>
      <AppenderRef ref="STDOUT"/>
    </Logger>

    <Root level="error">
      <ScriptFilter onMatch="ACCEPT" onMisMatch="DENY">
        <ScriptRef ref="groovy.filter"/>
      </ScriptFilter>
      <AppenderRef ref="STDOUT"/>
    </Root>
  </Loggers>

</Configuration>
          

If the status attribute on the Configuration element is set to DEBUG the list of script engines currently installed and their attributes will be listed. Although some engines may say they are not thread safe, Log4j takes steps to insure that the scripts will run in a thread-safe manner if the engine advertises that it is not thread safe.

2015-09-27 16:13:22,925 main DEBUG Installed script engines
2015-09-27 16:13:22,963 main DEBUG AppleScriptEngine Version: 1.1, Language: AppleScript, Threading: Not Thread Safe,
            Compile: false, Names: {AppleScriptEngine, AppleScript, OSA}
2015-09-27 16:13:22,983 main DEBUG Groovy Scripting Engine Version: 2.0, Language: Groovy, Threading: MULTITHREADED,
            Compile: true, Names: {groovy, Groovy}
2015-09-27 16:13:23,030 main DEBUG BeanShell Engine Version: 1.0, Language: BeanShell, Threading: MULTITHREADED,
            Compile: true, Names: {beanshell, bsh, java}
2015-09-27 16:13:23,039 main DEBUG Mozilla Rhino Version: 1.7 release 3 PRERELEASE, Language: ECMAScript, Threading: MULTITHREADED,
            Compile: true, Names: {js, rhino, JavaScript, javascript, ECMAScript, ecmascript}
          

When the scripts are executed they will be provided with a set of variables that should allow them to accomplish whatever task they are expected to perform. See the documentation for the individual components for the list of variables that are available to the script.

The components that support scripting expect a return value to be passed back to the calling Java code. This is not a problem for several of the scripting languages, but Javascript does not allow a return statement unless it is within a function. However, Javascript will return the value of the last statement executed in the script. As a consequence, code such as that shown below will result in the desired behavior.

            var result;
            if (logEvent.getLoggerName().equals("JavascriptNoLocation")) {
                result = "NoLocation";
            } else if (logEvent.getMarker() != null && logEvent.getMarker().isInstanceOf("FLOW")) {
                result = "Flow";
            }
            result;
          

2015-09-27 16:13:23,095 main DEBUG Script BeanShellSelector is compilable
2015-09-27 16:13:23,096 main WARN Error compiling script java.lang.Error: unimplemented
            at bsh.engine.BshScriptEngine.compile(BshScriptEngine.java:175)
            at bsh.engine.BshScriptEngine.compile(BshScriptEngine.java:154)
            at org.apache.logging.log4j.core.script.ScriptManager$MainScriptRunner.<init>(ScriptManager.java:125)
            at org.apache.logging.log4j.core.script.ScriptManager.addScript(ScriptManager.java:94)
          

XInclude

XML configuration files can include other files with XInclude. Here is an example log4j2.xml file that includes two other files:

<?xml version="1.0" encoding="UTF-8"?>
<configuration xmlns:xi="http://www.w3.org/2001/XInclude"
               status="warn" name="XIncludeDemo">
  <properties>
    <property name="filename">xinclude-demo.log</property>
  </properties>
  <ThresholdFilter level="debug"/>
  <xi:include href="log4j-xinclude-appenders.xml" />
  <xi:include href="log4j-xinclude-loggers.xml" />
</configuration>

log4j-xinclude-appenders.xml:

<?xml version="1.0" encoding="UTF-8"?>
<appenders>
  <Console name="STDOUT">
    <PatternLayout pattern="%m%n" />
  </Console>
  <File name="File" fileName="${filename}" bufferedIO="true" immediateFlush="true">
    <PatternLayout>
      <pattern>%d %p %C{1.} [%t] %m%n</pattern>
    </PatternLayout>
  </File>
</appenders>

log4j-xinclude-loggers.xml:

<?xml version="1.0" encoding="UTF-8"?>
<loggers>
  <logger name="org.apache.logging.log4j.test1" level="debug" additivity="false">
    <ThreadContextMapFilter>
      <KeyValuePair key="test" value="123" />
    </ThreadContextMapFilter>
    <AppenderRef ref="STDOUT" />
  </logger>

  <logger name="org.apache.logging.log4j.test2" level="debug" additivity="false">
    <AppenderRef ref="File" />
  </logger>

  <root level="error">
    <AppenderRef ref="STDOUT" />
  </root>
</loggers>

Composite Configuration

Log4j allows multiple configuration files to be used by specifying them as a list of comma separated file paths on log4j.configurationFile or, when using URLs, by adding secondary configuration locations as query parameters named "override". The merge logic can be controlled by specifying a class that implements the MergeStrategy interface on the log4j.mergeStrategy property. The default merge strategy will merge the files using the following rules:

1. The global configuration attributes are aggregated with those in later configurations replacing those in previous configurations, with the exceptions:
   • the highest status level will be used
   • the lowest monitorInterval greater than 0 will be used
   • packages are joined with comma
2. Properties from all configurations are aggregated. Duplicate properties replace those in previous configurations.
3. Filters are aggregated under a CompositeFilter if more than one Filter is defined. Since Filters are not named duplicates may be present.
4. Scripts and ScriptFile references are aggregated. Duplicate definitions replace those in previous configurations.
5. Appenders are aggregated. Appenders with the same name are replaced by those in later configurations, including all of the Appender's subcomponents.
6. Loggers are all aggregated. Logger attributes are individually merged with duplicates being replaced by those in later configurations. Appender references on a Logger are aggregated with duplicates being replaced by those in later configurations. Filters on a Logger are aggregated under a CompositeFilter if more than one Filter is defined. Since Filters are not named duplicates may be present. Filters under Appender references included or discarded depending on whether their parent Appender reference is kept or discarded.

Status Messages

Just as it is desirable to be able to diagnose problems in applications, it is frequently necessary to be able to diagnose problems in the logging configuration or in the configured components. Since logging has not been configured, "normal" logging cannot be used during initialization. In addition, normal logging within appenders could create infinite recursion which Log4j will detect and cause the recursive events to be ignored. To accommodate this need, the Log4j 2 API includes a StatusLogger. Components declare an instance of the StatusLogger similar to:

protected final static Logger logger = StatusLogger.getLogger();

Since StatusLogger implements the Log4j 2 API's Logger interface, all the normal Logger methods may be used.

When configuring Log4j it is sometimes necessary to view the generated status events. This can be accomplished by adding the status attribute to the configuration element or a default value can be provided by setting the "Log4jDefaultStatusLevel" system property. Valid values of the status attribute are "trace", "debug", "info", "warn", "error" and "fatal". The following configuration has the status attribute set to debug.

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="debug" name="RoutingTest">
  <Properties>
    <Property name="filename">target/rolling1/rollingtest-$${sd:type}.log</Property>
  </Properties>
  <ThresholdFilter level="debug"/>

  <Appenders>
    <Console name="STDOUT">
      <PatternLayout pattern="%m%n"/>
      <ThresholdFilter level="debug"/>
    </Console>
    <Routing name="Routing">
      <Routes pattern="$${sd:type}">
        <Route>
          <RollingFile name="Rolling-${sd:type}" fileName="${filename}"
                       filePattern="target/rolling1/test1-${sd:type}.%i.log.gz">
            <PatternLayout>
              <pattern>%d %p %c{1.} [%t] %m%n</pattern>
            </PatternLayout>
            <SizeBasedTriggeringPolicy size="500" />
          </RollingFile>
        </Route>
        <Route ref="STDOUT" key="Audit"/>
      </Routes>
    </Routing>
  </Appenders>

  <Loggers>
    <Logger name="EventLogger" level="info" additivity="false">
      <AppenderRef ref="Routing"/>
    </Logger>

    <Root level="error">
      <AppenderRef ref="STDOUT"/>
    </Root>
  </Loggers>

</Configuration>

During startup this configuration produces:

2011-11-23 17:08:00,769 DEBUG Generated plugins in 0.003374000 seconds
2011-11-23 17:08:00,789 DEBUG Calling createProperty on class org.apache.logging.log4j.core.config.Property for element property with params(name="filename", value="target/rolling1/rollingtest-${sd:type}.log")
2011-11-23 17:08:00,792 DEBUG Calling configureSubstitutor on class org.apache.logging.log4j.core.config.PropertiesPlugin for element properties with params(properties={filename=target/rolling1/rollingtest-${sd:type}.log})
2011-11-23 17:08:00,794 DEBUG Generated plugins in 0.001362000 seconds
2011-11-23 17:08:00,797 DEBUG Calling createFilter on class org.apache.logging.log4j.core.filter.ThresholdFilter for element ThresholdFilter with params(level="debug", onMatch="null", onMismatch="null")
2011-11-23 17:08:00,800 DEBUG Calling createLayout on class org.apache.logging.log4j.core.layout.PatternLayout for element PatternLayout with params(pattern="%m%n", Configuration(RoutingTest), null, charset="null")
2011-11-23 17:08:00,802 DEBUG Generated plugins in 0.001349000 seconds
2011-11-23 17:08:00,804 DEBUG Calling createAppender on class org.apache.logging.log4j.core.appender.ConsoleAppender for element Console with params(PatternLayout(%m%n), null, target="null", name="STDOUT", ignoreExceptions="null")
2011-11-23 17:08:00,804 DEBUG Calling createFilter on class org.apache.logging.log4j.core.filter.ThresholdFilter for element ThresholdFilter with params(level="debug", onMatch="null", onMismatch="null")
2011-11-23 17:08:00,813 DEBUG Calling createRoute on class org.apache.logging.log4j.core.appender.routing.Route for element Route with params(AppenderRef="null", key="null", Node=Route)
2011-11-23 17:08:00,823 DEBUG Calling createRoute on class org.apache.logging.log4j.core.appender.routing.Route for element Route with params(AppenderRef="STDOUT", key="Audit", Node=Route)
2011-11-23 17:08:00,825 DEBUG Calling createRoutes on class org.apache.logging.log4j.core.appender.routing.Routes for element Routes with params(pattern="${sd:type}", routes={Route(type=dynamic default), Route(type=static Reference=STDOUT key='Audit')})
2011-11-23 17:08:00,827 DEBUG Calling createAppender on class org.apache.logging.log4j.core.appender.routing.RoutingAppender for element Routing with params(name="Routing", ignoreExceptions="null", Routes({Route(type=dynamic default),Route(type=static Reference=STDOUT key='Audit')}), Configuration(RoutingTest), null, null)
2011-11-23 17:08:00,827 DEBUG Calling createAppenders on class org.apache.logging.log4j.core.config.AppendersPlugin for element appenders with params(appenders={STDOUT, Routing})
2011-11-23 17:08:00,828 DEBUG Calling createAppenderRef on class org.apache.logging.log4j.core.config.plugins.AppenderRefPlugin for element AppenderRef with params(ref="Routing")
2011-11-23 17:08:00,829 DEBUG Calling createLogger on class org.apache.logging.log4j.core.config.LoggerConfig for element logger with params(additivity="false", level="info", name="EventLogger", AppenderRef={Routing}, null)
2011-11-23 17:08:00,830 DEBUG Calling createAppenderRef on class org.apache.logging.log4j.core.config.plugins.AppenderRefPlugin for element AppenderRef with params(ref="STDOUT")
2011-11-23 17:08:00,831 DEBUG Calling createLogger on class org.apache.logging.log4j.core.config.LoggerConfig$RootLogger for element root with params(additivity="null", level="error", AppenderRef={STDOUT}, null)
2011-11-23 17:08:00,833 DEBUG Calling createLoggers on class org.apache.logging.log4j.core.config.LoggersPlugin for element loggers with params(loggers={EventLogger, root})
2011-11-23 17:08:00,834 DEBUG Reconfiguration completed
2011-11-23 17:08:00,846 DEBUG Calling createLayout on class org.apache.logging.log4j.core.layout.PatternLayout for element PatternLayout with params(pattern="%d %p %c{1.} [%t] %m%n", Configuration(RoutingTest), null, charset="null")
2011-11-23 17:08:00,849 DEBUG Calling createPolicy on class org.apache.logging.log4j.core.appender.rolling.SizeBasedTriggeringPolicy for element SizeBasedTriggeringPolicy with params(size="500")
2011-11-23 17:08:00,851 DEBUG Calling createAppender on class org.apache.logging.log4j.core.appender.RollingFileAppender for element RollingFile with params(fileName="target/rolling1/rollingtest-Unknown.log", filePattern="target/rolling1/test1-Unknown.%i.log.gz", append="null", name="Rolling-Unknown", bufferedIO="null", immediateFlush="null", SizeBasedTriggeringPolicy(SizeBasedTriggeringPolicy(size=500)), null, PatternLayout(%d %p %c{1.} [%t] %m%n), null, ignoreExceptions="null")
2011-11-23 17:08:00,858 DEBUG Generated plugins in 0.002014000 seconds
2011-11-23 17:08:00,889 DEBUG Reconfiguration started for context sun.misc.Launcher$AppClassLoader@37b90b39
2011-11-23 17:08:00,890 DEBUG Generated plugins in 0.001355000 seconds
2011-11-23 17:08:00,959 DEBUG Generated plugins in 0.001239000 seconds
2011-11-23 17:08:00,961 DEBUG Generated plugins in 0.001197000 seconds
2011-11-23 17:08:00,965 WARN No Loggers were configured, using default
2011-11-23 17:08:00,976 DEBUG Reconfiguration completed

If the status attribute is set to error than only error messages will be written to the console. This makes troubleshooting configuration errors possible. As an example, if the configuration above is changed to have the status set to error and the logger declaration is:

<logger name="EventLogger" level="info" additivity="false">
  <AppenderRef ref="Routng"/>
</logger>

the following error message will be produced.

2011-11-24 23:21:25,517 ERROR Unable to locate appender Routng for logger EventLogger

Applications may wish to direct the status output to some other destination. This can be accomplished by setting the dest attribute to either "err" to send the output to stderr or to a file location or URL. This can also be done by insuring the configured status is set to OFF and then configuring the application programmatically such as:

StatusConsoleListener listener = new StatusConsoleListener(Level.ERROR);
StatusLogger.getLogger().registerListener(listener);

Testing in Maven

Maven can run unit and functional tests during the build cycle. By default, any files placed in src/test/resources are automatically copied to target/test-classes and are included in the classpath during execution of any tests. As such, placing a log4j2-test.xml into this directory will cause it to be used instead of a log4j2.xml or log4j2.json that might be present. Thus a different log configuration can be used during testing than what is used in production.

A second approach, which is extensively used by Log4j 2, is to set the log4j.configurationFile property in the method annotated with @BeforeClass in the junit test class. This will allow an arbitrarily named file to be used during the test.

A third approach, also used extensively by Log4j 2, is to use the LoggerContextRule JUnit test rule which provides additional convenience methods for testing. This requires adding the log4j-core test-jar dependency to your test scope dependencies. For example:

public class AwesomeTest {
    @Rule
    public LoggerContextRule init = new LoggerContextRule("MyTestConfig.xml");

    @Test
    public void testSomeAwesomeFeature() {
        final LoggerContext ctx = init.getLoggerContext();
        final Logger logger = init.getLogger("org.apache.logging.log4j.my.awesome.test.logger");
        final Configuration cfg = init.getConfiguration();
        final ListAppender app = init.getListAppender("List");
        logger.warn("Test message");
        final List<LogEvent> events = app.getEvents();
        // etc.
    }
}

System Properties

The Log4j documentation references a number of System Properties that can be used to control various aspects of Log4j 2 behavior. The table below lists these properties along with their default value and a description of what they control. Any spaces present in the property name are for visual flow and should be removed.

Note that beginning in Log4j 2.10, all system property names have been normalized to follow a consistent naming scheme. While the old property names are still supported for backwards compatibility, it is recommended to update configurations to use the new style. This system is extensible and is enabled through the PropertySource interface. Additional property source classes can be added through the standard ServiceLoader mechanism in Java SE.

Properties can be overridden by sources with a lower number priority (e.g.., -100 comes before 100). The following sources are all available by default:

The following is a list of available global configuration properties. Note that these can only be set once per JVM process unlike configuration settings available in configuration files. The Property Name column contains the name used in properties files and system properties; Environment Variable for the equivalent environment variable; and Legacy Property Name for the pre-2.10 name.