Log4J2 - assigning file appender filename at runtime

Question

I have a log4j2.xml config file in the class path. One of the appenders is a File appender, and I would like to set the target file name at run time in the Java application.

According to the docs I should be able to use a double "$" and a context prefix in the log4j2.xml file:

<appenders>
    <File name="MyFile" fileName="$${sys:logFilename}">
        <PatternLayout pattern="%-4r %d{${datestamp}} [%t] %-5level %logger{36} - %msg%n"/>
    </File>
</appenders>

where the "sys" prefix indicates that the Configurator will lookup the property "logFilename" in the system properties. So in the application, I call (rather early on):

System.setProperty("logFilename", filename);

I have also turned on auto-reconfiguration for log4j2 in the xml file:

<configuration status="debug" monitorInterval="5">>

Unfortunately, this has no effect whatsoever, and the log file is never created. Some of the log4j2 status output is below:

2013-02-13 15:36:37,574 DEBUG Calling createAppender on class org.apache.logging.log4j.core.appender.FileAppender for element File with params(fileName="${sys:logFilename}", append="null", locking="null", name="MyFile", immediateFlush="null", suppressExceptions="null", bufferedIO="null", PatternLayout(%-4r %d{yyyy-MM-dd/HH:mm:ss.SSS/zzz} [%t] %-5level %logger{36} - %msg%n), null)

2013-02-13 15:36:37,576 DEBUG Starting FileManager ${sys:logFilename}

How can I have the value of "fileName" in the File Appender be set at run time? Alternatively, how can I simply add a new File Appender to the root logger at run time? In Log4j 2.0 most of the API to change the configuration is hidden.

Answer 1

h/t rgoers The FileAppender doesn't support two dollar signs on the file name as the file is opened when the appender is started. What you are indicating with two dollar signs is that you want - potentially - a different file name for each event.

With a single $ (as in ${sys:logFilename}), the system will look for property "logFilename" in the system properties.

Thus, the log4j2.xml should have:

<appenders>
    <File name="MyFile" fileName="${sys:logFilename}">
        <PatternLayout pattern="%-4r %d{${datestamp}} [%t] %-5level %logger{36} - %msg%n"/>
    </File>
</appenders>

The Java application should set the system property:

System.setProperty("logFilename", filename);

and reconfigure the logger:

org.apache.logging.log4j.core.LoggerContext ctx =
    (org.apache.logging.log4j.core.LoggerContext) LogManager.getContext(false);
ctx.reconfigure();

This produces the desired behavior.