java - Redirect System.out and System.err

Question

I have some legacy code (or rather some code we don't control but we have to use) that writes a lot of statements to system.out/err.

At the same time, we are using a framework that uses a custom logging system wrapped around log4j (again, unfortunately we don't control this).

So I'm trying to redirect the out and err stream to a custom PrintStream that will use the logging system. I was reading about the System.setLog() and System.setErr() methods but the problem is that I would need to write my own PrintStream class that wraps around the logging system in use. That would be a huge headache.

Is there a simple way to achieve this?

Answer 1

Just to add to Rick's and Mikhail's solutions, which are really the only option in this scenario, I wanted to give an example of how creating a custom OutputStream can potentially lead to not so easy to detect/fix problems. Here's some code:

import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintStream;
import org.apache.log4j.Logger;

public class RecursiveLogging {
  /**
   * log4j.properties file:
   * 
   * log4j.rootLogger=DEBUG, A1
   * log4j.appender.A1=org.apache.log4j.ConsoleAppender
   * log4j.appender.A1.layout=org.apache.log4j.PatternLayout
   * log4j.appender.A1.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n
   * 
   */
  public static void main(String[] args) {
    // Logger.getLogger(RecursiveLogging.class).info("This initializes log4j!");
    System.setOut(new PrintStream(new CustomOutputStream()));
    System.out.println("This message causes a stack overflow exception!");
  }
}

class CustomOutputStream extends OutputStream {
  @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.getLogger(CustomOutputStream.class).info((char) b);
  }
}

This example shows the pitfalls of using a custom output stream. For simplicity the write() function uses a log4j logger, but this can be replaced with any custom logging facility (such as the one in my scenario). The main function creates a PrintStream that wraps a CustomOutputStream and set the output stream to point to it. Then it executes a System.out.println() statement. This statement is redirected to the CustomOutputStream which redirects it to a logger. Unfortunately, since the logger is lazy initialized, it will acquire a copy of the console output stream (as per the log4j configuration file which defines a ConsoleAppender) too late, i.e., the output stream will point to the CustomOutputStream we just created causing a redirection loop and thus a StackOverflowError at runtime.

Now, with log4j this is easy to fix: we just need to initialize the log4j framework before we call System.setOut(), e.g., by uncommenting the first line of the main function. Luckily for me, the custom logging facility I have to deal with is just a wrapper around log4j and I know it will get initialized before it's too late. However, in the case of a totally custom logging facility that uses System.out/err under the cover, unless the source code is accessible, it's impossible to tell if and where direct calls to System.out/err are performed instead of calls to a PrintStream reference acquired during initialization. The only work around I can think of for this particular case would be to retrieve the function call stack and detect redirection loops, since the write() functions should not be recursive.