Saturday, March 26, 2011

How to redirect System.out ?

Redirect Console Output


The previous article talked about console input, it is time to write something about console output.

To print something to the console window, simply write :
// console output
System.out.println("Hello !");

System.out is a system variable in Java. Looking at the documentation of java.lang.System, you will find System.out is actually a PrintStream.

A PrintStream is capable of writing output to file or console window. To redirect System.out, we just need to define a new PrintStream targeting to a text file instead of the console window.

public class MySystemOut extends java.io.PrintStream
{
  private java.io.PrintStream oldOut=null;

  public MySystemOut() throws Exception
  {
      super(new java.io.FileOutputStream("out.txt"),true);
  }
}

We need to define two more methods in MySystemOut, namely startRedirect() and stopRedirect(). The method names are self explanatory.


// defined inside class MySystemOut
public void startRedirect()
{
  oldOut = System.out;
  System.setOut(this);
}

public void stopRedirect()
{
  System.setOut(oldOut);
}


You will need a testing program which contains a main() method. It just creates a MySystemOut object and starts the redirect process.

/******************************************************************************
* File : A.java
* Author : http://java.macteki.com/
* Description :
*   Main program for testing MySystemOut
* Tested with : JDK 1.6
******************************************************************************/

public class A
{
  public static void main(String[] args) throws Exception
  {
    MySystemOut out=new MySystemOut();
  
    System.out.println("Going to redirect System.out...");

    out.startRedirect();

    for (int i=0;i<10;i++)
      System.out.println("this should be in out.txt:"+i);


    out.stopRedirect();

    System.out.println("Console output restored !");

    for (int i=0;i<10;i++)  
      System.out.println("this should be on console screen:"+i);
  }

}

MySystemOut.java

/******************************************************************************
* File : MySystemOut.java
* Author : http://java.macteki.com/
* Description :
*   A class to redirect System.out
* Tested with : JDK 1.6
******************************************************************************/


public class MySystemOut extends java.io.PrintStream
{
  private java.io.PrintStream oldOut=null;

  public MySystemOut() throws Exception
  {
      super(new java.io.FileOutputStream("out.txt"),true);
  }

  public void startRedirect()
  {
    oldOut = System.out;
    System.setOut(this);
  }

  public void stopRedirect()
  {
    System.setOut(oldOut);
  }
}

Compilation and Testing

To compile, you need both A.java and MySystemOut.java in the same folder, issue :
javac *.java
To run, just type :
java A

An output file "out.txt" will be generated in the same folder.

Thanks for reading. Comments are welcome.

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