Tim Mattison

Hardcore tech

Mini Hack: Find Java Code Abusing Stdout

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I found myself recently trying to determine where a particular piece of code was that was polluting my stdout stream with hundreds of unwanted messages. This seemed like an impossible task since I could only guess that there was a line of code somewhere that was calling System.out.println(clazz.toString()) or something similar. I searched the codebase high and low and found nothing.

After some Googling I came up with an alternate approach. I disabled all of the code I could find that printed to stdout (actually this was easy since I do it all with one function so I just disabled that) and verified that I my application was still spewing out noise. Sure enough it was and then I added a tiny chunk of code to my application’s initialization. That code takes over stdout and forcibly causes a NullPointerException. It’s unconventional and I probably could’ve just put in a breakpoint but I wanted to make sure it crashed and spat out something I could work with immediately.

Here is what I did:

System.setOut(new PrintStream(new OutputStream() {
  public void write(int arg0) throws IOException {
    String forcedCrash = null;
    forcedCrash = forcedCrash + " ";

Let me know in the comments if you get a chance to use this to debug a project. Since I came up with this technique I find myself putting it in action a lot more often than I thought I would so hopefully it’ll be useful for you too.