log4j redirect stdout to DailyRollingFileAppender

Question

I have a java app that uses log4j.

Config:

log4j.rootLogger=info, file

log4j.appender.file=org.apache.log4j.DailyRollingFileAppender
log4j.appender.file.File=${user.home}/logs/app.log
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d [%t] %c %p %m%n

So all the log statements are correctly appended to the file, but i am losing stdout and stderr. How do i redirect exception stack traces and sysouts to the daily rolled file ?

Answer 1

I presume you're logging stacktraces via e.printStackTrace() ? You can pass an exception object into the Log4j logging methods and those will appear in your log (see Logger.error(Object obj, Throwable t))

Note that you can change System.out and System.err to another PrintStream that redirects to Log4j. That would be a straightforward change and save you converting all your System.out.println() statements.

Answer 2

Standard output and error streams are managed from your container. For example Tomcat uses JULI to log output and error streams.

My recommendation is to leave these as it is. Avoid using System.out.print in your application. See here for stack traces.

Answer 3

If you are using an application server, servlet container or something similar, see kgiannakakis's answer.

For standalone apps see this. You can reassing stdin, stdout and stderr using java.lang.System class. Basically you create a new subclass of PrintStream and set that instance to System.out.

Something along these lines in start of your app (untested).

// PrintStream object that prints stuff to log4j logger
public class Log4jStream extends PrintStream {
      public void write(byte buf[], int off, int len) {
        try {
           // write stuff to Log4J logger
        } catch (Exception e) {
       }
    }
}

// reassing standard output to go to log4j
System.setOut(new Log4jStream());

Answer 4

// I set up a ConsoleAppender in Log4J to format Stdout/Stderr
log4j.rootLogger=DEBUG, CONSOLE
log4j.appender.CONSOLE=org.apache.log4j.ConsoleAppender
log4j.appender.CONSOLE.layout=org.apache.log4j.PatternLayout
log4j.appender.CONSOLE.layout.ConversionPattern=[%t] %-5p %c - %m%n


// And I call this StdOutErrLog.tieSystemOutAndErrToLog() on startup

public class StdOutErrLog {

    private static final Logger logger = Logger.getLogger(StdOutErrLog.class);

    public static void tieSystemOutAndErrToLog() {
        System.setOut(createLoggingProxy(System.out));
        System.setErr(createLoggingProxy(System.err));
    }

    public static PrintStream createLoggingProxy(final PrintStream realPrintStream) {
        return new PrintStream(realPrintStream) {
            public void print(final String string) {
                realPrintStream.print(string);
                logger.info(string);
            }
        };
    }
}

Answer 5

works like a charm :)

btw, is there any way to prevent prints to go to STDOUT? (while still writing to log4j)?