Poor Man's Java memory profiler

Here is an example of do-it-yourself memory profiling e.g. showing used heap memory over the life of a program run. I compared it with results from YourKit to convince myself it's ok.

The concept:

• Launch a separate thread that collects used memory stats
  
• add a shutdown hook to output the stats

Here's an example in a Junit test...

public class CacheRebuildReloadTest extends RollbackTestCase {
  private static int  j   = 0;
  private static final long t0 = System.currentTimeMillis();
  private static long t[] = new long[32767];
  private static long m[] = new long[32767];
  private static Runtime r = Runtime.getRuntime();
  private static boolean done = false;
  static {
    Runtime.getRuntime().addShutdownHook(new Thread() {
      public void run() {
        for (int i = 0; i < j; i++) {
          System.out.println(t[i] + "\t" + m[i]);
        }
      }
    });
    new Thread() {
      public void run() {
        while (!done) {
          t[j]   = System.currentTimeMillis() - t0;
          m[j++] = r.totalMemory() - r.freeMemory();
          try {Thread.sleep(500);} catch (Exception e) {}
          if (j > 32767) {done = true;}
        }
      }
    }.start();
  }
  public void testRebuildReload() {
    // stuff here
    done = true;
  }
}

Here is a comparison with a YourKit memory profile (and a plot of the 2 sampling frequencies and distributions - I used .5s and YourKit apparently 1s). Good enough for my purposes.