java的几种引用之二

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(){

}

public static void main(String[] args){
try {
// test1();
// test2();
// test3();
// test4();
// test5();
test6();
} catch (InterruptedException e) {
e.printStackTrace();
}

}

/**
* 强引用，JVM的默认实现
* @throws InterruptedException
*/
public static void test1()throws InterruptedException{
Object obj = new Object();
Object strong =obj;

obj =null;
//一个对象具有强引用垃圾机制不会回收它，当内存空 间不足，Java虚拟机宁愿抛出OutOfMemoryError错误，使程序异常终止，也不会靠随意回收具有强引用的对象来解决内存不足问题
System.gc();
Thread.sleep(1000);
System.out.println("strong="+strong);
}

/**
* WeakReference 弱引用 当所引用对象在JVM,内不再强引用是，GC后 weak reference 将会被自动回收
* @throws InterruptedException
*/
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("w111r="+wr.get());

}

/**
* SoftReference 软引用 SoftReference 于 weakReference 的特性基本一致，最大区别在于
* 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）
* @throws InterruptedException
*/
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 虚引用 Phantom Reference 与 WreakReference 和 SoftReference 有
* 很大的不同，因为get()方法永远返回null
* @throws InterruptedException
*/
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
* @throws InterruptedException
*/
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);

}
}转