How to remove System.out.println's from codebase

Question

We have a huge (old legacy java) code-base, where many files (around 5k) have System.out.println's. We are planning to remove them for cleanup/performance reasons. How can we write a script that will replace them without introducing any issues in the code? The script cannot blindly delete them as following case can be an issue:

if ()
  some.code...
else
  System.out.println(...);
DB.close();

I'm thinking of replacing them with ';'. That will take care of above case. Do you see any other issues? Any other suggestions?

Answer 1

You could use a conditional compilation to have a debug build with the print statements and a release build without them.

Basically, the idea is to create a final static class with a final static boolean that you use as a switch at compile time.

public final class Debug {
   //set to false to allow compiler to identify and eliminate
   //unreachable code
   public static final boolean ON = true;
}

Then you can just replace all of your System.out.println statements with

if(Debug.ON)
{
    System.out.println...
}

Since the compiler will ignore any unreachable branches of code, you can just set ON = false when you do a release build and the print statements will be excluded from your bytecode.

Note: This doesn't deal with the case that Oscar pointed out, where a print statement may change the state of some object. You could use conditional compilation to print to a null object when in release mode, as he suggested, instead of removing the prints altogether.

Answer 2

Have you consider the silly case:

System.out.println(" Print " +  object.changeState() );

I don't think it happen but chances are the println executes a method that is actually performing some action on which the system depends on and may introduce subtle bugs ( believe me or not, but I have witnessed this )

Probably replacing with a logger and disabling the loggers may do.

Or creating a null object using the NullObject pattern:

public final class DevNull { 
    public final static PrintStream out = new PrintStream(new OutputStream() {
        public void close() {}
        public void flush() {}
        public void write(byte[] b) {}
        public void write(byte[] b, int off, int len) {}
        public void write(int b) {}

    } );
}

And replacing

 System.out.println();

With

 DevNull.out.println();