gc方法如何工作？

Question

public class GarbageC {
public void finalize()
{
System.out.println("Finalize method");  
}
public static void main(String[] args) {
    GarbageC c1=new GarbageC();
    GarbageC c2=new GarbageC();
    c1=null;
    c2=null;
    System.gc();
    System.out.println("abc");
}
}

While I'm debugging this code, the o/p is coming as

Finalize method
Finalize method
abc

and when I run, I get this

abc
Finalize method
Finalize method

Why am I getting different outputs in both the cases

Answer 1

Calling System.gc() is a request. The runtime doesn't have to do collect garbage in response. It may full well decide to ignore you. As such, you can't depend on it doing the same thing consistently.

Chances are, in debug mode it really does cause a garbage collection to be run. In non-debug mode it either just doesn't collect the garbage or waits until an opportune moment to do so.

Answer 2

If you run it often enough you could get any of the 3 possible outputs.

The garbage collection process possibly runs in a dedicated thread (at least in any recent VM it will), so the output depends on the speed of each thread (your main thread and the GC thread of the VM).

In debug mode, the main thread is significantly slower (you may pause it indefinetly when stepping through the code). Thus the GC overtakes your main thread. When you simply run the program; the main thread will most likely run through to completeion before the VM even attempts to garbage collect.

A call to System.gc() is only a request "hey please do a gc", even if the VM honors it (its not required to), your request will be passed to the GC thread; that involves at least a slight delay before the GC will actually begin work; and some more time until it actually discovers your garbage objects and then finally collects them.