究竟什么是可用于垃圾收集的对象？

Question

I'm battling out of memory issues with my application and am trying to get my head around garbage collection. If I have the following code:

public void someMethod() {
   MyObject myObject = new MyObject();
   myObject.doSomething();  //last use of myObject in this scope
   doAnotherThing();
   andEvenMoreThings();
}

So my question is, will myObject be available for garbage collection after myObject.doSomething() which is the last use of this object, or after the completion of someMethod() where it comes out of scope? Ie is the garbage collection smart enough to see that though a local variable is still in scope, it won't be used by the rest of the code?

Answer 1

"Where it comes out of scope"

public void someMethod() {
   MyObject myObject = new MyObject();
   myObject.doSomething();  //last use of myObject in this scope
   myObject = null; //Now available for gc
   doAnotherThing();
   andEvenMoreThings();
}

Answer 2

The best thing you can do is to take your code put it in a loop with a delay and hook up a profiler to it.

If you are using a later version of Java then JVisualVm comes as standard.

If you are on windows and have JAVA_HOME set

%JAVA_HOME%/bin/jvisualvm

This will launch a profiler and you can see what objects are being collected and what are not. In my opinion this is an essential part of being a programmer and its fun to find the memory leaks.

Hope this helps

Answer 3

顺便说一下，在后面的java 6中，有一种类型的转义分析 ，其中JVM可以发现你的MyObject实例没有离开方法，因此它甚至可以将它完全放在堆栈上，你根本不需要任何GC。

Answer 4

So my question is, will myObject be available for garbage collection after myObject.doSomething() which is the last use of this object, or after the completion of someMethod() where it comes out of scope?

The former.

Ie is the garbage collection smart enough to see that though a local variable is still in scope, it won't be used by the rest of the code?

Scope is not visible to the GC which sees only registers, stacks, global variables and references from heap blocks to other heap blocks. So scope is irrelevant.

Answer 5

After local scope is out. as object can be reused and can live in the local scope. ie. it is marked for gc. but not actually gc'ed .

Answer 6

代码优化可能会注意到myObject的最后一次使用的位置，并使其可用于那里的垃圾收集，但从技术上讲，它将直到变量不再引用它（通过分配给其他东西）或超出范围。