Different behavior running and debugging the program Java, Eclipse

Question

I have this code (put aside its appropriateness for now):

    Class<?> cacheClass = Class.forName("java.lang.Integer$IntegerCache");
    Field cacheField = cacheClass.getDeclaredField("cache");
    cacheField.setAccessible(true);
    Field modifiersField = Field.class.getDeclaredField("modifiers");
    modifiersField.setAccessible(true);
    modifiersField.setInt(cacheField, cacheField.getModifiers() & ~Modifier.FINAL);

    Integer betterCache[] = new Integer[255];
    for (int i = 0; i < betterCache.length; i++) {
        betterCache[i] = 20;
    }
    cacheField.set(null,  betterCache);
    System.out.println(10);
    System.out.println((Integer) 10);

I expect the second println to print 20, as I replaced cached Integers with 20. When I debug program in Eclipse it does as I expect, it gets the value from the cache and prints 20, whereas it prints 10 in both cases when I just run it either from IDE or by invoking java. How can this behavior be explained?

UPD: It works this way if compiled with 1.8 javac. It prints 10 and 20 if compiled with 1.6 version.

Answer 1

This is definitely caused by Just in Time Compiler. You should add -XX:+PrintCompilation to JVM options, it is also more visible if you iterate

System.out.println((Integer) 10);

a lot of times. You will notice that compiling

java.lang.Integer::valueOf (32 bytes)

and

java.nio.ByteBuffer::arrayOffset (35 bytes) 

affects the result.

Answer 2

EDIT

I was completely wrong

Definitely you are playing with fire, in my point of view, this is for a race conditions (unsafe thread in java 8). If you check this:

    Class<?> cacheClass = Class.forName("java.lang.Integer$IntegerCache");
    Field cacheField = cacheClass.getDeclaredField("cache");
    cacheField.setAccessible(true);
    Field modifiersField = Field.class.getDeclaredField("modifiers");
    modifiersField.setAccessible(true);
    modifiersField.setInt(cacheField, cacheField.getModifiers() & ~Modifier.FINAL);
    Integer firstCache[] = (Integer[])cacheField.get(null);
    Integer betterCache[] = new Integer[255];
    for (int i = 0; i < betterCache.length; i++) {
        betterCache[i] = 20;
    }
    System.out.println(firstCache == betterCache);
    cacheField.set(null, betterCache);
    System.out.println(10);
    for (int i = 0; i < 1000000; i++) {
        System.out.println((Integer) 10);     
    }

You'll see the Java burn.