Redirect System.out.println to log

Question

In my project test suite there is big usage of

System.out.println 

I\'m trying to redirect these output to log file (through configuration

Answer 1

Given that it's better replace the System.out.println(), sometimes we have no choice. Anyway I've made a little utility for that:

SystemOutToSlf4j.enableForClass(MyClass.class)

Then all the println originated from MyClass will be redirected to the logger. See this post for more details...

public class SystemOutToSlf4j extends PrintStream {

  private static final PrintStream originalSystemOut = System.out;
  private static SystemOutToSlf4j systemOutToLogger;

  /**
   * Enable forwarding System.out.println calls to the logger if the stacktrace contains the class parameter
   * @param clazz
   */
  public static void enableForClass(Class clazz) {
    systemOutToLogger = new SystemOutToSlf4j(originalSystemOut, clazz.getName());
    System.setOut(systemOutToLogger);
  }


  /**
   * Enable forwarding System.out.println calls to the logger if the stacktrace contains the package parameter
   * @param packageToLog
   */
  public static void enableForPackage(String packageToLog) {
    systemOutToLogger = new SystemOutToSlf4j(originalSystemOut, packageToLog);
    System.setOut(systemOutToLogger);
  }

  /**
   * Disable forwarding to the logger resetting the standard output to the console
   */
  public static void disable() {
    System.setOut(originalSystemOut);
    systemOutToLogger = null;
  }

  private String packageOrClassToLog;

  private SystemOutToSlf4j(PrintStream original, String packageOrClassToLog) {
    super(original);
    this.packageOrClassToLog = packageOrClassToLog;
  }

  @Override
  public void println(String line) {
    StackTraceElement[] stack = Thread.currentThread().getStackTrace();
    StackTraceElement caller = findCallerToLog(stack);
    if (caller == null) {
      super.println(line);
      return;
    }

    org.slf4j.Logger log = org.slf4j.LoggerFactory.getLogger(caller.getClass());
    log.info(line);
  }

  public StackTraceElement findCallerToLog(StackTraceElement[] stack) {
    for (StackTraceElement element : stack) {
      if (element.getClassName().startsWith(packageOrClassToLog))
        return element;
    }

    return null;
  }

}

Answer 2

public class RecursiveLogging {
public static void main(String[] args) {
    System.setOut(new PrintStream(new CustomOutputStream()));

    TestMyException.testPrint();
}

}

class CustomOutputStream extends OutputStream {
private Logger logger = Logger.getLogger(this.getClass());

@Override
public final void write(int b) throws IOException {
    // the correct way of doing this would be using a buffer
    // to store characters until a newline is encountered,
    // this implementation is for illustration only
    logger.info((char) b);
}

@Override
public void write(byte[] b, int off, int len) throws IOException {
    if (b == null) {
        throw new NullPointerException();
    } else if ((off < 0) || (off > b.length) || (len < 0) ||
               ((off + len) > b.length) || ((off + len) < 0)) {
        throw new IndexOutOfBoundsException();
    } else if (len == 0) {
        return;
    }
    byte[] pb = new byte[len];
    for (int i = 0 ; i < len ; i++) {
        pb[i] = (b[off + i]);
    }
    String str = new String(pb);
    logger.info(str);
}
}

Answer 3

I think you can use System.setOut(PrintStream) to set your output to a file output stream. Then you can put this line in your BeforeClass method. I like to use a BaseTest class and put this line of code in the beforeclass method of that class. Then make all test cases extend this cclass.

Answer 4

Use shell redirection. Figure out the "java" invocation for your project, if you're on most vaguely UNIX-like systems, ps aux | grep java will help.

Then just run this command with > /path/to/logfile. Example:

java -jar myjar.jar -cp path/to/lib.jar:path/to/otherlib.jar com.bigcorp.myproject.Main > /var/log/myproject.log

Answer 5

My suggestion would be to refactor if possible. For a possible solution, check these similar questions

log4j redirect stdout to DailyRollingFileAppender

Redirect System.out.println to Log4J, while keeping class name information