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javalog4j

Log4j Logging OutputStream chaining

I have a legacy Java app, It uses Log4j as logging tool. The app is lacking metrics, so I want to find a way to smartly retrieve some Java class based metrics.

The issue is, I want to intercept all Log4j logging before they are served.

The idea is inspired by the chaining of the System.out looking like this

TeeOutputStream myOut=new TeeOutputStream(System.out, mySpecificOut);
PrintStream ps = new PrintStream(myOut, true); //true - auto-flush after println
System.setOut(ps);

So is there a way to chain the Log4j output stream and fully control it on Java because I need to do some logic before logging.

Thanks

Solution

  • Yes it is, you can define your own Stream using

    LoggingOutputStream extends OutputStream {

    and chain in order to intercept the logs

    System.setOut(new PrintStream(new LoggingOutputStream(

    Have a look at this nice example based on innoq/log4j LoggingOutputStream.