The Allocation Instrumenter is a Java agent written using the java.lang.instrument API and ASM. Each allocation in your Java program is instrumented; a user-defined callback is invoked on each allocation.

In order to write your own allocation tracking code, you have to implement the Sampler interface and pass an instance of that to AllocationRecorder.addSampler():

  AllocationRecorder.addSampler(new Sampler() {
    public void sampleAllocation(int count, String desc,
      Object newObj, long size) {
      System.out.println("I just allocated the object " + newObj + 
        " of type " + desc + " whose size is " + size);
      if (count != -1) { System.out.println("It's an array of size " + count); }
    }
  });

You can also use the allocation instrumenter to instrument constructors of particular classes. You do this by instantiating a ConstructorCallback and passing it to ConstructorInstrumenter.instrumentClass()

   try {
      ConstructorInstrumenter.instrumentClass(
          Thread.class, new ConstructorCallback<Thread>() {
            @Override public void sample(Thread t) {
              System.out.println("Instantiating a thread");
            }
          });
    } catch (UnmodifiableClassException e) {
      System.out.println("Class cannot be modified");
    }