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javaalgorithmoptimizationobjectcreation

Creating objects makes the VM faster?

Look at this piece of code:

MessageParser parser = new MessageParser();
for (int i = 0; i < 10000; i++) {
    parser.parse(plainMessage, user);
}

For some reason, it runs SLOWER (by about 100ms) than 

for (int i = 0; i < 10000; i++) {
    MessageParser parser = new MessageParser();
    parser.parse(plainMessage, user);
}

Any ideas why? The tests were repeated a lot of times, so it wasn't just random. How could creating an object 10000 times be faster than creating it once?

Solution

  • Because Java has 'generational garbage collection' and can quickly identify (in the loop) that it doesn't re-use the same object/memory space, so the GC cost is virtually nil. On the other hand, your long-lived object will survive a generational pass over the nursery generation and have to be moved out to the main generation.

    In summary, you can't really assume about performance without doing tests to measure it in place.