java - System.gc() 的可用对象

Question

基本上，我尝试进行测试并遇到有关垃圾收集器的问题:

以下代码片段中有多少个可用于 System.gc() 的对象:

public class WorkGC {
    static class StaticNestedClass  {
        private String name;
        public StaticNestedClass(String name) {
        this.name = name;
        }
    }

    public static void main(String[] args) {
        StaticNestedClass n1 = new StaticNestedClass("n1");
        StaticNestedClass n2 = new StaticNestedClass("n2");
        Collection list = new ArrayList();
        list.add(n1);
        StaticNestedClass[] arr = new StaticNestedClass[2];
        arr[0] = n2;
        n2 = n1;
        clear(arr);
        n1 = null;
        n2 = null;
        System.gc();
       //the rest of the code
    }

    private static void clear(StaticNestedClass[] arr) {
        arr = null;
    }
}

我认为有 2 个对象可用:

1)arr – 清除(arr)后

2)n2 – n2=null 之后

我不确定 arr – 是一个对象，可能 arr 是一系列对象。另外，我不确定 StaticNestedClass 的实例，它是静态的并且位于 PermGen 中，而不是作为典型对象位于堆中。 GC 在永久代和堆中都有效吗？

Answer 1

我认为:`没有对象已准备好进行 GC。 我认为情况是这样的:

public static void main(String[] args) {
        StaticNestedClass n1 = new StaticNestedClass("n1"); //  1st object created 
        StaticNestedClass n2 = new StaticNestedClass("n2");  // 2nd object created 
        Collection list = new ArrayList(); // 3rd object
        list.add(n1); // n1 has a strong reference, so n1 is not ready for GC.
        StaticNestedClass[] arr = new StaticNestedClass[2];// 4th object created
        arr[0] = n2; //n2 has strong reference, so, even n2 is not ready.
        n2 = n1; // both n1 and n2 point to same object as that of n1. but arr[0] still references n2. 
        clear(arr); // you are passing references by value. SO, this has absolutely no effect on GC.
        n1 = null;// Object pointed to by n1 is still being referenced by n2
        n2 = null; //Object pointed to by n1 has a reference in the list.
        System.gc();
       //the rest of the code
    }


    private static void clear(StaticNestedClass[] arr) {
        arr = null;
    }

所以，基本上没有对象准备好进行 GC。