java中的四大引用（强，软，弱，虚）

package com.shi.ref;

import java.util.HashMap;
import java.util.WeakHashMap;

/**
 * 	弱引用HashMap的使用
 * @author shiye
 *
 *	结果：
	{1=HashMap}
	{1=HashMap}
	{1=HashMap} 1
	********************
	{2=WeakHashMap}
	{2=WeakHashMap}
	{} 0

 */
public class TestWeakHashMap {

	public static void main(String[] args) {
		myHashMap();
		System.out.println("********************");
		weakHashMap();
	}
	
	/**
	 * 强引用HashMap
	 */
	static void myHashMap() {
		HashMap<Integer,String> map = new HashMap<>();
		Integer key = new Integer(1);
		String value = "HashMap";
		map.put(key, value);
		
		System.out.println(map);
		
		key = null;
		System.out.println(map);
		
		System.gc();
		System.out.println(map + " " + map.size());
	}
	
	/**
	 * 弱引用HashMap  
	 */
	static void weakHashMap() {
		WeakHashMap<Integer,String> map = new WeakHashMap<>();
		Integer key = new Integer(2);
		String value = "WeakHashMap";
		map.put(key, value);
		
		System.out.println(map);
		
		key = null;
		System.out.println(map);
		
		System.gc();
		System.out.println(map + " " + map.size());
	}

}

package com.shi.ref;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;

/**
 * 	虚引用 结合 引用队列
 * @author shiye
 *	结果
		java.lang.Object@15db9742
		java.lang.Object@15db9742
		null
		************************
		null
		null
		java.lang.ref.WeakReference@6d06d69c
 */
public class TestReferenceQueue {

	public static void main(String[] args) throws InterruptedException {

		Object o1 = new Object();
		ReferenceQueue<Object> referenceQueue = new ReferenceQueue<>();
		WeakReference<Object> weakReference = new WeakReference<Object>(o1, referenceQueue);
		
		System.out.println(o1);
		System.out.println(weakReference.get());
		System.out.println(referenceQueue.poll());
		
		System.out.println("************************");
		
		o1 = null;
		System.gc();
		Thread.sleep(500);
		
		System.out.println(o1);//o1 = null;
		System.out.println(weakReference.get());//o1 = null   System.gc();
		System.out.println(referenceQueue.poll());//o1 = null  System.gc();
	}

}