问题描述

首先我想澄清我对 WeakReference 的理解，因为以下问题取决于相同的问题.

First of all I would like to clarify my understanding of the WeakReference as the following question depends on the same.

static void test() {
    Person p = new Person();
    WeakReference<Person> person = new WeakReference<>(p);
    p = null;
    System.gc();
    System.out.println(person.get());
    System.out.println(person);
}

static class Person {
    String name;
}

static class PersonMetadata {
    String someData;

    public PersonMetadata(String met) {
        someData = met;
    }
}

上面代码的输出是

空java.lang.ref.WeakReference@7852e922

这意味着虽然实际的 person 对象在 GC 运行后被垃圾收集，但内存中存在一个 WeakReference 类的对象，它不指向任何东西这一点.

Which means that although there is the actual person object is garbage collected once a GC runs, there is an object of WeakReference<Person> class is there in the memory which does not point to anything at this point.

现在考虑到上述理解是正确的，我对 WeakHashMap<K,V> 的工作原理感到困惑.在下面的代码中

Now considering the above understanding true, I am confused about how does WeakHashMap<K,V> works. In the below code

public static void main(String[] args) {
    Person p = new Person();
    p.name = "John";
    WeakHashMap<Person, PersonMetadata> map = new WeakHashMap<>();
    PersonMetadata meta = new PersonMetadata("Geek");
    map.put(p, meta);
    p = null;
    System.gc();
    if (map.values().contains(meta)) {
        System.out.println("Value present");
    } else {
        System.out.println("Value gone");
    }
}

static class Person {
    String name;
}

static class PersonMetadata {
    String someData;

    public PersonMetadata(String met) {
        someData = met;
    }
}

输出: 价值消失

现在的问题 据说 WeakHashMap 中的键是弱引用，这意味着在上面的代码中 p 变为 null 实际对象可以被垃圾回收，因为没有更多对该对象的强引用，但 和值如何这是 PersonMetadata 类的对象正在被垃圾收集，因为第一个代码证明 WeakReference 类的对象没有被垃圾收集，即使实际对象已被收集.

Now the the question is as it is said that the key in WeakHashMap<K,V> is an weak reference which means that in the above code when p becomes null the actual object can be garbage collected as there is no more strong reference to the object, but how does the and the value which is an object of PersonMetadata class is getting garbage collected as the first code proves that object of WeakReference class is not garbage collected even though the actual object is collected.

推荐答案

你误会了情况.当 map.values().contains(meta) 或短 map.containsValue(meta) 返回 false 时，并不意味着meta 已被垃圾回收.实际上，您在 meta 中持有对对象的引用，甚至将该引用传递给可能在其上调用 equals 的 contains 方法.那么这个对象怎么会被垃圾回收呢?

You are misunderstanding the situation. When map.values().contains(meta), or short map.containsValue(meta) returns false, it doesn’t imply that meta has been garbage collected. In fact, you are holding a reference to the object in meta and even passing that reference to the contains method which may invoke equals on it. So how could that object be garbage collected?

响应仅告诉您映射的一个键与该对象没有关联，并且由于唯一的键已被垃圾回收，因此这是正确的答案.或者，您可以只要求 map.isEmpty() 检查关联是否存在.

The response only tells you that there is no association from one of the map’s keys to that object and since the only key has been garbage collected, that’s the correct answer. Alternatively, you could just have asked map.isEmpty() to check for the presence of the association.

这就是 WeakHashMap 提供:

This is what the WeakHashMap provides:

Map 接口的基于哈希表的实现，带有弱键.WeakHashMap 中的条目在其键不再正常使用时将被自动删除.更准确地说，给定键的映射的存在不会阻止该键被垃圾收集器丢弃，也就是说，使其可终结，最终确定，然后回收.当一个键被丢弃时，它的条目实际上从映射中删除，所以这个类的行为与其他 Map 实现有些不同.

Hash table based implementation of the Map interface, with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently from other Map implementations.

条目的删除不是即时的.它依赖于将 WeakReference 排队到 ReferenceQueue 中，然后在您进行下一个查询时在内部对其进行轮询，例如 containsValue 甚至 >尺寸().例如.如果我将您的程序更改为:

Person p = new Person();
WeakHashMap<Person, PersonMetadata> map = new WeakHashMap<>();
PersonMetadata meta = new PersonMetadata("Geek");
map.put(p, meta);
WeakReference<?> ref = new WeakReference<>(p);
p = null;
while(ref.get() != null) System.gc();
System.out.println(map.containsValue(meta)? "Value present": "Value gone");

Person p = new Person();
WeakHashMap<Person, PersonMetadata> map = new WeakHashMap<>();
PersonMetadata meta = new PersonMetadata("Geek");
map.put(p, meta);
WeakReference<?> personRef = new WeakReference<>(p);
WeakReference<?> metaRef = new WeakReference<>(meta);
p = null;
meta = null;
while(personRef.get() != null) System.gc();
System.out.println("Person collected");
for(int i = 0; metaRef.get() != null && i < 10; i++) {
    System.out.println("PersonMetadata not collected");
    System.gc();
    Thread.sleep(1000);
}
System.out.println("calling a query method on map");
System.out.println("map.size() == "+map.size());
System.gc();
System.out.println("PersonMetadata "+(metaRef.get()==null? "collected": "not collected"));

Person collected
PersonMetadata not collected
PersonMetadata not collected
PersonMetadata not collected
PersonMetadata not collected
PersonMetadata not collected
PersonMetadata not collected
PersonMetadata not collected
PersonMetadata not collected
PersonMetadata not collected
PersonMetadata not collected
calling a query method on map
map.size() == 0
PersonMetadata collected