Creating a Custom Java Console

Here's where the string actually gets shown:

  // non-static method for actually displaying new text
  private void showString( Object o ) {
    ta.append( o.toString() );
  }

This non-static method is called on the unique

Console

object that we created above, and it simply adds the string to the end of the scrolling text window.

Voila! Console!

Hooking the System

OutputStreams

Here's our prize feature. If our runtime environment allows it, we'd like to redirect

System.out

and

System.err

to print directly to our console.

Since this is something we don't necessarily want to in every case, we've created a method you have to call to get it going, called

Console.hookStandards()

:

  // hook stdout and stderr, and redirect output to the console
  // keep the originals around, of course
  static public void hookStandards() {
    synchronized( hookLock ) {
      if (out!=null)
        return;

      out = System.out;
      err = System.err;
      PrintStream dwout =
        new PrintStream( new ConsoleOutputStream() );
      System.setOut( dwout );
      System.setErr( dwout );
    }
  }

Since we're going to want to provide the user with a way of restoring the original

System.out

and

System.err

(namely,

Console.unhookStandards()

), we keep them around in variables called

out

and

err

. And to make sure nothing goes awry in a multithreaded situation, we put a

synchronized()

block around it.

We replace both

System.out

and

System.err

with a new OutputStream object called a ConsoleOutputStream

. This is a class we've created that redirects all output back to our console. Take a look at the source

to see how we've done it — it's quite simple.

Trying It Out

We've provided two ways to try this out: an applet and an application. The applet demonstrates the use of

Console.println()

directly, while the application also shows the use of

Console.hookStandards()

and

Console.unhookStandards()

. You can try the applet out right here

. You can run the application by typing

java TestApp

at the command-line.

Bonus Feature

Another nice thing you can do in the console is add your own custom debugging utilities. Some browsers provide useful things like memory dumps — there's no reason we can't do that too.

If you press 'c' in the console window, you'll see that it clears the window. And if you press 'd', you'll see a nice stack dump of all running threads. Well, you might see it — not all runtime environments will allow it.

Further Ideas

There are a few tidbits you might want to consider if you create a class like this one.

Typing

Console.println()

(or

System.out.println()

, for that matter) is a lot of typing if you type it many, many times. If you don't mind a bit of obfuscation in your code, it might be good to rename

Console

to

C

, because

C.println()

is shorter.

I mentioned above that this system only has one

Console

, and all output goes to this same console. This might not be ideal for your situation — consider the possibility of allowing the creation of multiple consoles and allowing your code to send different messages to different consoles. It gets away from our original idea of trying to create a drop-in replacement for the system console, but that doesn't mean it's not a good idea.

Our console doesn't have a close button. It doesn't really need one, since the console is closed when the applet dies, but there's no reason not to allow the user to hide the console if they are sick of it. Remember, however, that you'll need to provide a way to get it back.

Source Code

• Console.java

• ConsoleOutputStream.java

• StateDump.java

• TestApp.java

• TestApplet.java

• everything

About the Author

---

Greg Travis is a freelance programmer in New York City. His interest in computers can probably be traced back to the episode of "The Bionic Woman" where Jamie runs around trying to escape a building whose lights and doors are controlled by an evil artificial intelligence, which mocks her through loudspeakers. He's a devout believer in the religious idea that when a computer program works, it's a complete coincidence. He can be reached at mito@panix.com.

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This article was originally published on December 14, 2000