Should I expect code not in a method in Mockito Tests to Run Faster because of Java Just In Time Compilation?

Question

I need help with a little experiment. I have two test files:

testFileA.java and testFileB.java

testFileA.java looks like

    @Test
    public void test1() throws Throwable {
        Instant start = Instant.now();
        mvc.perform(MockMvcRequestBuilders
                        .post('someEndpoint/')
                        .andDo(print())
                        .andExpect(status().isCreated())
        Instant end = Instant.now();
        System.out.println(">>>>>>>>>>>>>>>>>End" + Duration.between(start, end));
    }

    @Test
    public void test2() throws Throwable {
        //same contents as test 1
    }

    @Test
    public void test3() throws Throwable {
        //same contents as test 1
    }

testFileB.java looks like

    @Test
    public void test1() throws Throwable {
        Instant start = Instant.now();
        this.abstractedCode();
        Instant end = Instant.now();
        System.out.println(">>>>>>>>>>>>>>>>>End" + Duration.between(start, end));
    }

    @Test
    public void test2() throws Throwable {
           //same contents as test 1
    }

    @Test
    public void test3() throws Throwable {
        //same contents as test 1
    }

    public void abstractedCode(){
      mvc.perform(MockMvcRequestBuilders
                        .post('someEndpoint/')
                        .andDo(print())
                        .andExpect(status().isCreated())
    }

My expectation with testFileB.java is that the first test would take a little longer and then each subsequent test wouldn't take as long because the duplicated code is refactored into a method call; the Java JIT compiler makes it so we wouldn't have to recompile the same code again. My expectation was proven correct.

My expectation with testFileA.java is that each test would take as long as the previous one because the common code wasn't refactored into a function, but that ended up not being true. The behavior was the same, the first test took a little longer and then the next two were shorter.

I'm guessing that something is happening within the JVM that makes it so the mvc.perform() method that I'm calling doesn't have to get recompiled over and over again, the JIT compiler is helping my program out despite the fact that it's not in a function I've defined. Is that true?

Answer 1

A distinct non-answer: you are investing your time in the wrong place.

The gains from "JIT optimisations" only matter in a setup where an application runs for lengthy periods of time, invoking the same method millions of times.

The point of a JIT is NOT to optimize test methods that are invoked ONCE during the lifetime of a JVM.

Meaning: invest your time to train how to write testable production code, and clean test code. That is much more important here.

Of course: the execution time of your unit test suite still matters. But when that turns into an issue, then you are most likely facing poorly written tests, that for example use default timeouts and whatnot ... ending up with a test taking many seconds, if not minutes. That is something you care about. You absolutely do not care about JIT magic though for unit test environments.