How to configure Log4J programmatically in two steps?

Steps 1: Programmatic Configuration

In this step, we create a configuration that extends an Xml Configuration (ie Log4j.xml)

• We add a logger called TEST_LOGGER
• that gets two programmatically created Appenders:
  • a file Appender
  • a console Appender

public class Log4JConfiguration extends XmlConfiguration {


  public static final String TEST_LOGGER = Log4JConfiguration.class.getName();

  public Log4JConfiguration(LoggerContext loggerContext, ConfigurationSource source) {
    super(loggerContext, source);
  }

  @Override
  protected void doConfigure() {
  
    super.doConfigure();


    PatternLayout defaultLayout = PatternLayout.createDefaultLayout();
    FileAppender fileAppender = FileAppender.newBuilder()
      .setName("TestLogFile")
      .setConfiguration(this)
      .withFileName("test.log")
      .setLayout(defaultLayout)
      .setImmediateFlush(true)
      .build();
    this.addAppender(fileAppender);
    
    ConsoleAppender console = ConsoleAppender.newBuilder()
      .setName("TestConsole") // mandatory
      .setConfiguration(this)
      .setTarget(ConsoleAppender.Target.SYSTEM_OUT)
      .build();
    testLogger.addAppender(console, null, null);
    
    LoggerConfig testLogger = LoggerConfig.newBuilder()
      .withLoggerName(TEST_LOGGER)
      .withAdditivity(false)
      .withConfig(this)
      .withLevel(Level.INFO)
      .build();
    
    // Add the appenders to the logger
    testLogger.addAppender(fileAppender, null, null);
    testLogger.addAppender(myFileAppender,null, null);
    
    // Add the logger to the configuration
    addLogger(testLogger.getName(), testLogger);

  }

}

Step 2: Advertise it with a ConfigurationFactory

The below code is a factory that:

• is registered via the @Plugin
• return the previous Log4JConfiguration

When a Log4j logger is asked for the first time, it invokes this factory and build the configuration.

@Plugin(name = "Log4jConfigurationFactoryPlugin", category = ConfigurationFactory.CATEGORY)
@Order(10)
public class Log4jConfigurationFactoryPlugin extends ConfigurationFactory {

  /**
   * Valid file extensions for XML files.
   */
  public static final String[] SUFFIXES = new String[]{".xml", "*"};

  /**
   * Return the Configuration.
   *
   * @param source The InputSource.
   * @return The Configuration.
   */
  @Override
  public Configuration getConfiguration(LoggerContext loggerContext, ConfigurationSource source) {
    return new Log4JConfiguration(loggerContext, source);
  }

  /**
   * Returns the file suffixes for XML files.
   *
   * @return An array of File extensions.
   */
  public String[] getSupportedTypes() {
    return SUFFIXES;
  }


}

Remark on Appenders

Note that Appenders created with the builder factory (and not with the Node class representing the file loaded) does not support out-of-the-box lookups to uses variables from the context.

You may then: * add them in your original configuration and * also create a new log4j configuration with new Appenders (See below)

How to load another log4j configuration

You can do it:

• with the xinclude syntax
• via the composite configuration where the log4j2.configurationFile property contains a list of files separated by comma.
• or programmatically

LoggerContext loggerContext = this.getLoggerContext();

// Get your configuration as string
String result = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<Configuration>....</Configuration>"; 
InputStream stream = new ByteArrayInputStream(result.getBytes(StandardCharsets.UTF_8));
ConfigurationSource configurationSource = new ConfigurationSource(stream);

Configuration currentReadConfiguration = ConfigurationFactory.getInstance().getConfiguration(loggerContext, configurationSource);
currentReadConfiguration.initialize();

// then you can access the appenders
Appender appender = currentReadConfiguration.getAppender("myAppenderName");