调用System.gc（）后，WeakReferenced对象不是垃圾回收(WeakReferenced object is not garbage collected after calling System.gc())

 我是Java的新学习者。 我现在正在学习WeakReference的概念。 我遇到了一个可能看起来很愚蠢的问题，但我只是想弄清楚原因。 问题是：根据Java doc，“弱引用对象，它们不会阻止它们的引用被最终化，最终化，然后被回收。” 
 
 所以我做了这个小测试： 
 
import java.lang.ref.WeakReference; 

public class A {
    public static void main(String[] args) {
        A a = new A();
        WeakReference<A> wr = new WeakReference<>(a);
        a = null;

        A a1 = wr.get();

        System.out.println(a);
        System.out.println(a1);

        try {
            System.gc();

            Thread.sleep(10000);

        } catch (Exception e) {
            e.printStackTrace();
        }

        System.out.println(a1);
    }

    @Override
    protected void finalize( ) {
        System.out.println(Thread.currentThread().getName() + ": See ya, nerds!");
    }
}
 
 但是，我注意到在GC运行之后， wr.get()仍然可以返回我期望为null的对象，并且没有调用方法finalize() 。 出了什么问题？ 感谢您的帮助！ :) 

I am a fresh new learner of Java. I'm now learning the concept of WeakReference. I came across a problem which probably looks stupid but I just wanna figure out the reason. The problem is: according to Java doc, "Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed." 
 
So I did this small test:
 
import java.lang.ref.WeakReference; 

public class A {
    public static void main(String[] args) {
        A a = new A();
        WeakReference<A> wr = new WeakReference<>(a);
        a = null;

        A a1 = wr.get();

        System.out.println(a);
        System.out.println(a1);

        try {
            System.gc();

            Thread.sleep(10000);

        } catch (Exception e) {
            e.printStackTrace();
        }

        System.out.println(a1);
    }

    @Override
    protected void finalize( ) {
        System.out.println(Thread.currentThread().getName() + ": See ya, nerds!");
    }
}
 
However, I noticed that after GC running, wr.get() could still return object which I expected null, and the method finalize() was not invoked. So what went wrong? Thanks for your kind help in advance! :)

原文：https://stackoverflow.com/questions/50718490