Java WeakHashMap何时会清理空键? [英] When will Java WeakHashMap clean null key?

问题描述

在nameRef.get()和System.gc()之后的nameRef.get()下面的代码中为null.

In the code below nameRef.get() is null , after name = null and System.gc().

import java.lang.ref.WeakReference;

public class Main {

    public static void main(String[] args) {
        String name = new String("ltt");

        WeakReference<String> nameRef = new WeakReference<>(name);    
        System.out.println(nameRef.get()); // ltt

        name = null;
        System.gc();

        System.out.println(nameRef.get());  // null
    }    
}

WeakHashMap基于WeakReference.最后，我认为map.size()将为0.实际上是1.

WeakHashMap is based on WeakReference. At last, I think map.size() will be 0. In fact, it's 1.

import java.util.WeakHashMap;

public class Main2 {

    public static void main(String[] args) {
        String name = new String("ltt");
        WeakHashMap<String, Integer> map = new WeakHashMap<>();
        map.put(name, 18);
        System.out.println(map.size()); // 1

        name = null;
        System.gc();

        System.out.println(map.size());  // it's 1. why not 0 ?
    }    
}

Java WeakHashMap何时清理null键?

When will Java WeakHashMap clean the null key?

推荐答案

简单的答案:您不知道.

The simple answer: you don't know.

含义:当jvm启动并触发GC周期时，您无法控制.您实际上无法控制何时收集符合条件的对象.

Meaning: you have no control when the jvm kicks in and triggers a GC cycle. You have no control when eligible objects are actually collected.

因此，您无法知道该地图的状态"何时更改.

Thus there is no way for you to know when the "state" of that map changes.

是的，另一部分是调用System.gc()只是对jvm进行垃圾收集的建议.在 standard 设置中，如果jvm遵循该请求或将其忽略，则可以将控制权设为零.例如，您将需要使用非常特殊的JVM来进行更改.

And yes, the other part is that calling System.gc() is only a recommendation to the jvm to do garbage collection. In a standard setup you have zero control if jvm follows that request, or ignores it. You would need to use very special JVMs for example to change that.

通常，仅当jvm认为有必要时，才会运行gc.因此，只要不占用内存，您的地图就可以保持其大小为1.霍尔格(Holger)的 answer 很好地勾勒出了轮廓，即使GC运行也不会强制地图更新该部分.

Typically, gc runs only take place when the jvm considers them necessary. So your map might keep its size at 1 as long as there is no shortage of memory. And as the answer by Holger nicely outlines even the GC running doesn't force the map to update that part.

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