Which part of throwing an Exception is expensive?

Question

In Java, using throw/catch as a part of logic when there\'s not actually an error is generally a bad idea (in part) because throwing and catching an exception is expensive,

Answer 1

The creation of the Exception with a null stack trace takes about as much time as the throw and try-catch block together. However, filling the stack trace takes on average 5x longer.

I created the following benchmark to demonstrate the impact on performance. I added the -Djava.compiler=NONE to the Run Configuration to disable compiler optimization. To measure the impact of building the stack trace, I extended the Exception class to take advantage of the stack-free constructor:

class NoStackException extends Exception{
    public NoStackException() {
        super("",null,false,false);
    }
}

The benchmark code is as follows:

public class ExceptionBenchmark {

    private static final int NUM_TRIES = 100000;

    public static void main(String[] args) {

        long throwCatchTime = 0, newExceptionTime = 0, newObjectTime = 0, noStackExceptionTime = 0;

        for (int i = 0; i < 30; i++) {
            throwCatchTime += throwCatchLoop();
            newExceptionTime += newExceptionLoop();
            newObjectTime += newObjectLoop();
            noStackExceptionTime += newNoStackExceptionLoop();
        }

        System.out.println("throwCatchTime = " + throwCatchTime / 30);
        System.out.println("newExceptionTime = " + newExceptionTime / 30);
        System.out.println("newStringTime = " + newObjectTime / 30);
        System.out.println("noStackExceptionTime = " + noStackExceptionTime / 30);

    }

    private static long throwCatchLoop() {
        Exception ex = new Exception(); //Instantiated here
        long start = System.currentTimeMillis();
        for (int i = 0; i < NUM_TRIES; i++) {
            try {
                throw ex; //repeatedly thrown
            } catch (Exception e) {

                // do nothing
            }
        }
        long stop = System.currentTimeMillis();
        return stop - start;
    }

    private static long newExceptionLoop() {
        long start = System.currentTimeMillis();
        for (int i = 0; i < NUM_TRIES; i++) {
            Exception e = new Exception();
        }
        long stop = System.currentTimeMillis();
        return stop - start;
    }

    private static long newObjectLoop() {
        long start = System.currentTimeMillis();
        for (int i = 0; i < NUM_TRIES; i++) {
            Object o = new Object();
        }
        long stop = System.currentTimeMillis();
        return stop - start;
    }

    private static long newNoStackExceptionLoop() {
        long start = System.currentTimeMillis();
        for (int i = 0; i < NUM_TRIES; i++) {
            NoStackException e = new NoStackException();
        }
        long stop = System.currentTimeMillis();
        return stop - start;
    }

}

Output:

throwCatchTime = 19
newExceptionTime = 77
newObjectTime = 3
noStackExceptionTime = 15

This implies that creating a NoStackException is approximately as expensive as repeatedly throwing the same Exception. It also shows that creating an Exception and filling its stack trace takes approximately 4x longer.