Java編程WeakHashMap實例解析

簡述：

《Thinking in Java》第4版P519 頁WeakHashMap一章讀書筆記

WeakHashMap 用來保存WeakReference，這一結構雲遜垃圾回收器自動清理鍵和值

在添加鍵和值的操作時，映射會自動使用WeakReference包裝它們，

見jdk源代碼，

 public V put(K key, V value) {Object k = maskNull(key);int h = hash(k);Entry<K,V>[] tab = getTable();int i = indexFor(h, tab.length);for (Entry<K,V> e = tab[i]; e != null; e = e.next) {if (h == e.hash && eq(k, e.get())) {V oldValue = e.value;if (value != oldValue) e.value = value;return oldValue;}}modCount++;Entry<K,V> e = tab[i];tab[i] = new Entry<>(k, value, queue, h, e);if (++size >= threshold) resize(tab.length * 2);return null;}

其中new Entry<>(k, value, queue, h, e)一行使用了ReferenceQueue

 /** * Reference queue for cleared WeakEntries */ private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

點入new Entry的構造函數，進入super頂層可以看到，

 /** * Creates a new weak reference that refers to the given object and is * registered with the given queue. * * @param referent object the new weak reference will refer to * @param q the queue with which the reference is to be registered, * or <tt>null</tt> if registration is not required */ public WeakReference(T referent, ReferenceQueue<? super T> q) { super(referent, q); }

這裡new Entry同時也構造出來了一個WeakRefence對象

測試:

 package com.anialy.test.data_structure.map;import java.util.Iterator;import java.util.WeakHashMap;public class WeakHashMapTest {public static void main(String[] args) {WeakHashMap wmap = new WeakHashMap<String, Object>();final int SIZE = 10;String[] str = new String[SIZE];for (int i=0; i<SIZE; i++){String key = Integer.toString(i);String value = Integer.toString(i);// 每隔3個保留一個引用if(i % 3 == 0) str[i] = key;wmap.put(key, value);}System.gc();Iterator iter = wmap.keySet().iterator();while(iter.hasNext()){System.out.println(wmap.get(iter.next()));}}}

可以預料到，部分由於String[] 保留了弱引用，所以輸出都是間隔3的

總結

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