thinking in java 之Reference类的使用

Reference是java中的特殊引用类。描述的是特殊作用(主要是关于垃圾回收对象)的引用。

它有3个子类:

1.SoftReference;

2.WeakReference

3.PhantomReference

先看thinking in java 中的实例

package containers;

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.LinkedList;

//对引用的 描述与封装，Reference类

class VeryBig2{
    private static final int SIZE= 10000;
    private long[] la= new long[SIZE];
    private String ident;
    public VeryBig2(String id){
        ident= id;
    }
    @Override
    public String toString() {
        
        return ident;
    }
    protected void finalize(){
        System.out.println("Finalizing "+ ident);
    }
}

public class ReferencesTest {
    private static ReferenceQueue<VeryBig2> rq= 
            new ReferenceQueue<VeryBig2>();
    public static void checkQueue(){
        Reference<? extends VeryBig2> inq= rq.poll();
        if (inq != null) {
            System.out.println("In queue: "+ inq.get());
        }
    }
    
    public static void main(String[] args) {
        int size= 10;
        if (args.length> 0) {
            size= new Integer(args[0]);
        }
        LinkedList<SoftReference<VeryBig2>> sa= 
                new LinkedList<SoftReference<VeryBig2>>();
        for (int i = 0; i < size; i++) {
            sa.add(new SoftReference<VeryBig2>(new VeryBig2("Soft "+ i), rq));
            System.out.println("Just created: "+ sa.getLast());
            checkQueue();
        }
        
        LinkedList<WeakReference<VeryBig2>> wa= 
                new LinkedList<WeakReference<VeryBig2>>();
        for (int i = 0; i < size; i++) {
            wa.add(new WeakReference<VeryBig2>(new VeryBig2("Weak "+ i), rq));
            System.out.println("Just created: "+ wa.getLast());
            checkQueue();
        }
        
        SoftReference<VeryBig2> s= 
                new SoftReference<VeryBig2>(new VeryBig2("Soft"));
        WeakReference<VeryBig2> w= 
                new WeakReference<VeryBig2>(new VeryBig2("Weak"));
        System.gc();//对无引用的对象，执行垃圾回收操作。并且，调用回收对象的finalize方法
        
        LinkedList<PhantomReference<VeryBig2>> pa= 
                new LinkedList<PhantomReference<VeryBig2>>();
        for (int i = 0; i < size; i++) {
            pa.add(new PhantomReference<VeryBig2>(new VeryBig2("Phantom "+ i), rq));
            System.out.println("Just created: "+ pa.getLast());
            checkQueue();
        }
    }
}

output:

Just created: java.lang.ref.SoftReference@2a9931f5
Just created: java.lang.ref.SoftReference@2f9ee1ac
Just created: java.lang.ref.SoftReference@67f1fba0
Just created: java.lang.ref.SoftReference@3fbefab0
Just created: java.lang.ref.SoftReference@133c5982
Just created: java.lang.ref.SoftReference@5f186fab
Just created: java.lang.ref.SoftReference@3d4b7453
Just created: java.lang.ref.SoftReference@24c21495
Just created: java.lang.ref.SoftReference@41d5550d
Just created: java.lang.ref.SoftReference@1cc2ea3f
Just created: java.lang.ref.WeakReference@40a0dcd9
Just created: java.lang.ref.WeakReference@1034bb5
Just created: java.lang.ref.WeakReference@7f5f5897
Just created: java.lang.ref.WeakReference@4cb162d5
Just created: java.lang.ref.WeakReference@11cfb549
Just created: java.lang.ref.WeakReference@5b86d4c1
Just created: java.lang.ref.WeakReference@70f9f9d8
Just created: java.lang.ref.WeakReference@2b820dda
Just created: java.lang.ref.WeakReference@675b7986
Just created: java.lang.ref.WeakReference@2687816d
Just created: java.lang.ref.PhantomReference@7b11a3ac
In queue: null
Just created: java.lang.ref.PhantomReference@7ca83b8a
In queue: null
Finalizing Weak
Just created: java.lang.ref.PhantomReference@8dd20f6
In queue: null
Just created: java.lang.ref.PhantomReference@6d9efb05
In queue: null
Just created: java.lang.ref.PhantomReference@60723d7c
In queue: null
Just created: java.lang.ref.PhantomReference@6c22c95b
In queue: null
Just created: java.lang.ref.PhantomReference@5fd1acd3
In queue: null
Just created: java.lang.ref.PhantomReference@3ea981ca
In queue: null
Finalizing Weak 9
Just created: java.lang.ref.PhantomReference@6a8814e9
In queue: null
Finalizing Weak 8
Finalizing Weak 7
Just created: java.lang.ref.PhantomReference@c1503a3
In queue: null
Finalizing Weak 6
Finalizing Weak 5
Finalizing Weak 4
Finalizing Weak 3

从结果中，可以看出：

1.垃圾回收器回收优先级别:SoftReference< WeakReference < PhantomReference。

2.当运行垃圾回收方法System.gc时，会先调用目标对象的finalize方法。

3.当某个对象实例 ，只被Reference(包括上面三个派生类)引用的时候，在垃圾回收器回收的时候，被回收。

若这个实例，被Reference和普通引用 同时引用，那么 按照普通引用的垃圾回收规则来，不被回收。

普通引用的垃圾回收规则：

若某对象实例被普通引用 引用(即此对象是可获得的，参考thinking in java的解释)，那么不会被垃圾回收器回收。

若某对象实例不被任何普通引用 引用(即此对象是不可获得的，参考thinking in java的解释)，那么会被垃圾回收器回收。

参见下面的代码:(摘自thinking in java4ed)

package containers;

import java.util.WeakHashMap;


class Element{
    private String ident;
    public Element(String id){
        ident = id;
    }
    public String toString(){
        return ident;
    }
    public int hashCode(){
        return ident.hashCode();
    }
    public boolean equals(Object r){
        return r instanceof Element && ident.equals(((Element)r).ident);//此处应有掌声
    }
    
    protected void finalize(){
        System.out.println("Finalizing "+ getClass().getSimpleName()+ " "
                + ident);
    }
}

class Key extends Element{
    public Key(String id){
        super(id);
    }
}

class Value extends Element{
    public Value(String id){
        super(id);
    }
}

public class CanonicalMapping {
    public static void main(String[] args) {
        int size= 1000;
        if (args.length >0) {
            size= new Integer(args[0]);
        }
        Key[] keys= new Key[size];
        WeakHashMap<Key, Value> map= new WeakHashMap<Key, Value>();
        for (int i = 0; i < size; i++) {
            Key k= new Key(Integer.toString(i));//Key的实例被普通引用k引用
            Value v= new Value(Integer.toString(i));//Value的实例被普通引用v引用
            if (i % 3 ==0) {
                keys[i]= k;//Key的实例被普通引用keys[i]引用,这时，一个Key的实例既被k引用，也被keys[i]引用。
            }
            map.put(k, v);//k引用,v引用自动被WeakHashMap封装为WeakReference引用。
        }
        System.gc();//垃圾回收的时候，对于对象实例(Key的实例和Value的实例)只被Reference(WeakReference)引用的时候，被回收。
                    //如果这个实例既被Reference引用，也被普通引用引用，则按照普通引用垃圾回收规则来， 不被回收
    }
}