WeakHashMap 示例

java weakhashmap

2022-09-02 00:08:19

我创建了一个 WeakHashMap 作为

WeakHashMap<Employee,String> map = new WeakHashMap<Employee,String>();
map.put(emp,"hello");

其中 emp 是 Employee 对象。现在，如果我执行 emp = null 或说不再引用 emp 对象，那么该条目是否会从 WeakHashMap 中删除，即 Map 的大小会为零吗？
在HashMap的情况下，情况会反过来吗？
我对WeakHashMap的理解是否正确？

答案 1

一个非常简单的例子，来启发已经说过的话：

import java.util.WeakHashMap;

public class WeakHashMapDemo {

    public static void main(String[] args) {
        // -- Fill a weak hash map with one entry
        WeakHashMap<Data, String> map = new WeakHashMap<Data, String>();
        Data someDataObject = new Data("foo");
        map.put(someDataObject, someDataObject.value);
        System.out.println("map contains someDataObject ? " + map.containsKey(someDataObject));

        // -- now make someDataObject elligible for garbage collection...
        someDataObject = null;

        for (int i = 0; i < 10000; i++) {
            if (map.size() != 0) {
                System.out.println("At iteration " + i + " the map still holds the reference on someDataObject");
            } else {
                System.out.println("somDataObject has finally been garbage collected at iteration " + i + ", hence the map is now empty");
                break;
            }
        }
    }

    static class Data {
        String value;
        Data(String value) {
            this.value = value;
        }
    }
}

输出：

    map contains someDataObject ? true
    ...
    At iteration 6216 the map still holds the reference on someDataObject
    At iteration 6217 the map still holds the reference on someDataObject
    At iteration 6218 the map still holds the reference on someDataObject
    somDataObject has finally been garbage collected at iteration 6219, hence the map is now empty

答案 2

我运行了示例代码来了解和之间的区别HashMapWeakHashMap

            Map hashMap= new HashMap();
            Map weakHashMap = new WeakHashMap();

            String keyHashMap = new String("keyHashMap");
            String keyWeakHashMap = new String("keyWeakHashMap");

            hashMap.put(keyHashMap, "helloHash");
            weakHashMap.put(keyWeakHashMap, "helloWeakHash");
            System.out.println("Before: hash map value:"+hashMap.get("keyHashMap")+" and weak hash map value:"+weakHashMap.get("keyWeakHashMap"));

            keyHashMap = null;
            keyWeakHashMap = null;

            System.gc();  

            System.out.println("After: hash map value:"+hashMap.get("keyHashMap")+" and weak hash map value:"+weakHashMap.get("keyWeakHashMap"));

输出将为：

Before: hash map value:helloHash and weak hash map value:helloWeakHash
After: hash map value:helloHash and weak hash map value:null