Thread issue in Android

Question

I'm making a simple 2d game for the android platform, which works perfectly from version 2.0 and above, but when testing it on a 1.6 device, it crashes immediately. On running the debugger, it seems that I'm getting a null pointer exception in the thread class. I was just wondering if anybody has any ideas as to where the problem might be.

Here's the code for the thread class:

package com.marcusortiz.burnination;

import android.graphics.Canvas;
import android.view.SurfaceHolder;

public class GameThread extends Thread
{
  private SurfaceHolder sHolder;
  private MainView panel;
  private boolean isRunning;

  public GameThread(SurfaceHolder sHolder, MainView panel)
  {
    this.sHolder = sHolder;
    this.panel = panel;
  }

  public void setRunning(boolean isRunning)
  {
    this.isRunning = isRunning;
  }

  public SurfaceHolder getSurfaceHolder()
  {
    return sHolder;
  }

  @Override
  public void run()
  {
    Canvas canvas;
    while(isRunning)
    {
      canvas = null;
      try
      {
        canvas = sHolder.lockCanvas(null);
        synchronized(sHolder)
        {
          for(Sprite s : panel.getSprites())
          {
            s.update();
          }
          panel.onDraw(canvas);
        }
      }
      finally
      {
        if(canvas != null)
        {
          sHolder.unlockCanvasAndPost(canvas);
        }
      }
    }
  }
}

Maybe there's some compatibility issues with this code that I'm not aware of--I'm very much a beginner when it comes to using threads.

EDIT: here's the requested stack trace

burnination [Android Application]   
  DalvikVM[localhost:8608]  
    Thread [<3> main] (Running) 
    Thread [<15> Binder Thread #3] (Running)    
    Thread [<13> Binder Thread #2] (Running)    
    Thread [<11> Binder Thread #1] (Running)    
    Thread [<17> Thread-9] (Suspended (exception NullPointerException)) 
      GameThread.run() line: 53 

Answer 1

That's not a stack trace, but it looks like the problem is on line 53, which is what I was trying to get at. It looks like the code you posted probably doesn't like up exactly with what you're running, because line 53 is a closing brace. There's probably a variable on that line that's null.

Answer 2

Hi, I give you some suggestions. 1. Find out which line the crash locate. 2. Check the difference of the APIs in the SDK(1.6 & 2.0) source code. If you can work on Android 2.0, then I don't think there is some problem on your code.

Answer 3

By default, most debuggers (including Eclipse) break only on "uncaught" exceptions. They do not break on "caught" exceptions, i.e. anything for which a "catch" block applies. Your "finally" block is acting as a catch-all here, since it includes code that must be executed even if an exception was thrown.

When a NullPointerException is thrown by a function in the "try" block, the debugger does not stop, because it's a "caught" exception that you might be trying to ignore. The code in the "finally" block executes, and then the exception is re-thrown. This time there's nobody to catch it, so Eclipse halts. You're left pointing at the closing brace of the "finally" block because that's where the re-throw happens.

If you configure a debug breakpoint for caught+uncaught NullPointerException, you will see the failure at the point of the throw. Also, if you just let it continue and let the thread die, you should see logcat output from the Android default uncaught exception handler indicating the point of the throw.