JVM调优总结（二）-一些概念

package com.TestRef;

import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.Map;
import java.util.WeakHashMap;

public class Ref {

    public Ref() {
    }

    /**
     * @param args
     */
    public static void main(String[] args) {

        try {
//            test1();
//            test2();
//            test3();
//            test4();
//            test5();
            test6();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    /** 强引用，JVM的默认实现 */  
    public static void test1() throws InterruptedException {  
        Object obj = new Object();  
        Object strong = obj;  
        obj = null;  
        System.gc();  
        Thread.sleep(1000);  
        System.out.println("strong="+strong);
    }  
    
    /** 
     * WeakReference 弱引用( 当所引用的对象在 JVM 内不再有强引用时, GC 后weak reference 将会被自动回收) 
     * */  
    public static void test2() throws InterruptedException {  
        Object obj = new Object();  
        WeakReference<Object> wr = new WeakReference<Object>(obj);  
        obj = null;  
        System.gc();  
        Thread.sleep(1000);  
        System.out.println("wr.get()="+wr.get());  
        System.out.println("wr="+wr);  
        wr.clear();
        System.out.println("w1111r="+wr.get());  
    }  
    
    /** 
     * SoftReference SoftReference 于 WeakReference 的特性基本一致， 最大的区别在于 
     * SoftReference 会尽可能长的保留引用直到 JVM 内存不足时才会被回收(虚拟机保证) 
     * */  
    public static void test3() throws InterruptedException {  
        Object obj = new Object();  
        SoftReference<Object> sr = new SoftReference<Object>(obj);  
        obj = null;  
        System.gc();  
        Thread.sleep(1000);  
        System.out.println("sr.get()="+sr.get());  
    }  
    
    /** 
     * PhantomReference Phantom Reference(幽灵引用) 与 WeakReference 和 SoftReference 
     * 有很大的不同, 因为它的 get() 方法永远返回 null 
     * */  
    public static void test4() throws InterruptedException {  
        Object obj = new Object();  
        ReferenceQueue<Object> rq = new ReferenceQueue<Object>();  
        PhantomReference<Object> pr = new PhantomReference<Object>(obj, rq);  
        System.out.println("pr.get()="+pr.get()); 
    }  
    
    /**
     * ReferenceQueue：
     * @throws InterruptedException
     */
    public static void test5() throws InterruptedException {  
        Object obj = new Object();  
        ReferenceQueue<Object> rq = new ReferenceQueue<Object>();  
        WeakReference<Object> pr = new WeakReference<Object>(obj, rq);  
        System.out.println("**pr.enqueue()="+pr.enqueue());  
        System.out.println("**pr.isEnqueued()="+pr.isEnqueued());      
        System.out.println("**pr="+pr);
        System.out.println("**rq.poll()="+rq.poll());  
        obj = null;  
        System.gc();  
//        System.out.println("pr.enqueue()="+pr.enqueue());  
//        System.out.println("**pr.isEnqueued()="+pr.isEnqueued());      
//        System.out.println("pr="+pr);
//        System.out.println("rq.poll()="+rq.poll());  
//        System.out.println("obj5="+obj);  
    }  
    
    /** 
     * 使用 WeakReference 作为 key， 一旦没有指向 key 的强引用,  
     * WeakHashMap 在 GC 后将自动删除相关的 
     * entry 
     */  
    public static void test6() throws InterruptedException {  
        Map<Object, Object> map = new WeakHashMap<Object, Object>();  
        Object key = new Object();  
        Object value = new Object();  
        map.put(key, value);  
        
        key = null;  
        
//        System.out.println("value="+value);  
//        System.out.println("key="+key);  
//        System.out.println("map.containsValue(value)="+map.containsValue(value)); 
//        System.out.println("map="+map);  
        
        System.gc();  
        Thread.sleep(1000);  
        
        System.out.println("value="+value);  
        System.out.println("key="+key);  
        System.out.println("map.containsValue(value)="+map.containsValue(value)); 
        System.out.println("map="+map);  
    }  
}