Question

Trying to answer to this ticket : What is the difference between instanceof and Class.isAssignableFrom(…)?

I made a performance test :

class A{}
class B extends A{}

A b = new B();

void execute(){
  boolean test = A.class.isAssignableFrom(b.getClass());
  // boolean test = A.class.isInstance(b);
  // boolean test = b instanceof A;
}

@Test
public void testPerf() {
  // Warmup the code
  for (int i = 0; i < 100; ++i)
    execute();

  // Time it
  int count = 100000;
  final long start = System.nanoTime();
  for(int i=0; i<count; i++){
     execute();
  }
  final long elapsed = System.nanoTime() - start;
System.out.println(count+" iterations took " + TimeUnit.NANOSECONDS.toMillis(elapsed) + "ms.);
}

Which gave me :

• A.class.isAssignableFrom(b.getClass()) : 100000 iterations took 15ms
• A.class.isInstance(b) : 100000 iterations took 12ms
• b instanceof A : 100000 iterations took 6ms

But playing with the number of iterations, I can see the performance is constant. For Integer.MAX_VALUE :

• A.class.isAssignableFrom(b.getClass()) : 2147483647 iterations took 15ms
• A.class.isInstance(b) : 2147483647 iterations took 12ms
• b instanceof A : 2147483647 iterations took 6ms

Thinking it was a compiler optimization (I ran this test with JUnit), I changed it into this :

@Test
public void testPerf() {
    boolean test = false;

    // Warmup the code
    for (int i = 0; i < 100; ++i)
        test |= b instanceof A;

    // Time it
    int count = Integer.MAX_VALUE;
    final long start = System.nanoTime();
    for(int i=0; i<count; i++){
        test |= b instanceof A;
    }
    final long elapsed = System.nanoTime() - start;
    System.out.println(count+" iterations took " + TimeUnit.NANOSECONDS.toMillis(elapsed) + "ms. AVG= " + TimeUnit.NANOSECONDS.toMillis(elapsed/count));

    System.out.println(test);
}

But the performance is still “independent” of the number of iterations.
Could someone explain that behavior ?

Answer 1

The JIT compiler can eliminate loops which don’t anything. This can be triggered after 10,000 iterations.

What I suspect you are timing is how long it takes for the JIT to detect that the loop doesn’t do anything and remove it. This will be a little longer than it takes to do 10,000 iterations.