How to add Log4J2 appenders at runtime programmatically?

There have been a number of requests to support better programmatic configuration for Log4j 2. Sorry it took so long. As of Log4j 2.4, API was added to log4j-core to facilitate programmatic configuration.

The new ConfigurationBuilder API allows users to construct component definitions. With this API, there is no need to work directly with actual configuration objects (like LoggerConfig and FileAppender) which require a lot of knowledge on how Log4j works under the hood. Component definitions are added to the ConfigurationBuilder, and once all the definitions have been collected all the actual configuration objects (like Loggers and Appenders) are constructed. It feels a bit like the XML configuration syntax, except that you are writing Java code.

Note that the new ConfigurationBuilder API allows user code to create a new configuration or completely replace the existing configuration. If your use case is different, and you want to programmatically modify (rather than replace) an existing configuration after Log4j was started, then you will need to work with actual configuration objects. In that case, please see the Programmatically Modifying the Current Configuration after Initialization section of the manual.

Edit: for the newest versions of log4j2, see https://stackoverflow.com/a/33472893/1899566 instead.

I get the impression they don't want you doing this, but this works for me:

if (arg[0].equals("log") ) {  org.apache.logging.log4j.Logger logger    = org.apache.logging.log4j.LogManager.getLogger("loggerNameFromXMLConfig");  org.apache.logging.log4j.core.Logger coreLogger    = (org.apache.logging.log4j.core.Logger)logger;  org.apache.logging.log4j.core.LoggerContext context    = (org.apache.logging.log4j.core.LoggerContext)coreLogger.getContext();  org.apache.logging.log4j.core.config.BaseConfiguration configuration    = (org.apache.logging.log4j.core.config.BaseConfiguration)context.getConfiguration();  coreLogger.addAppender(configuration.getAppender("appenderNameFromXMLConfig"));} else {  //...}

As I noted above, I couldn't get https://logging.apache.org/log4j/2.x/manual/customconfig.html#AddingToCurrent to work, at least, not the way I expected it to (my appender would never get messages routed to it). I did finally stumble across a pattern that works for me - allowing me to add an appender at runtime, and have that appender actually get log messages routed to it.

Edit I removed a bunch of confusing code from here that wasn't doing anything....

    LoggerContext lc = (LoggerContext) LogManager.getContext(false);    FileAppender fa = FileAppender.newBuilder().withName("mylogger").withAppend(false).withFileName(new File(outputDirectory, "ConsoleOutput.txt").toString())            .withLayout(PatternLayout.newBuilder().withPattern("%-5p %d  [%t] %C{2} (%F:%L) - %m%n").build())            .setConfiguration(lc.getConfiguration()).build();    fa.start();    lc.getConfiguration().addAppender(fa);    lc.getRootLogger().addAppender(lc.getConfiguration().getAppender(fa.getName()));    lc.updateLoggers();

A key point for me, was that calling addAppender and passing your appender directly doesn't work, but asking for your appender back by name seems to. Which doesn't make sense... but since working, and I'm tired of wasting time on something that should be so simple....