Java四种引用方式

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最近写了个例子，说明了一下强引用、软引用、弱引用、虚引用的区别。

1、NString.java

package com.neohope.reference;

/**
 * Created by Hansen
 */
public class NString {
    private String name;
    private String value;

    public NString(String name, String value)
    {
        this.name = name;
        this.value=value;
    }

    public NString(String name, StringBuilder builder)
    {
        this.name = name;
        this.value=builder.toString();
    }

    public String Name()
    {
        return name;
    }

    public String Value()
    {
        return value;
    }

    @Override
    protected void finalize()
    {
        System.out.println(">> "+Name()+" finalize called");
        try {
            super.finalize();
        } catch (Throwable throwable) {
            throwable.printStackTrace();
        }
    }
}

2、TestReferenceA.java

package com.neohope.reference;

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

/**
 * Created by Hansen
 * JVM参数：
 * -XX:+PrintGCDetails
 * -Xms6m -Xmx12m
 */
public class TestReferenceA {
    public static void main(String[] args) {
        //申请内存
        Runtime rt=Runtime.getRuntime();
        System.out.println("Total memory is "+rt.totalMemory());
        int MemSize = 1024*1024*1;
        StringBuilder builder = new StringBuilder(MemSize);
        for (int i=0;i<MemSize/8;i++) {
            builder.append("ABCDEFGH");
        }
        System.out.println("Total memory is "+rt.totalMemory());

        //用s00与s01两个大字符串演示强引用与软引用的区别
        NString s00 = new NString("s00",builder);
        NString s01 = new NString("s01",builder);
        //s02与s03演示WeakReference的两种用法
        NString s02 = new NString("s02","s03");
        String key03 = new String("k03");
        NString s03 = new NString("s03","s03");
        //s04演示PhantomReference
        NString s04 = new NString("s04","s04");
        System.out.println("Total memory is "+rt.totalMemory());

        //第1类、强引用的对象，也最常用到的类型，内存不足时JVM将抛出异常也不要释放这些内存
        NString strongRef = s00;
        //第2类、只有软引用的数据，内存不足时JVM会回收这些内存
        SoftReference<NString> softRef = new SoftReference<NString>(s01);
        //第3类、只有弱引用的数据，无论内存是否充足JVM会回收这些内存，也就是弱引用不会影响对象的生命周期
        WeakReference<NString> weakRef = new WeakReference<NString>(s02);
        //WeakHashMap很有用，当key022为null时，s022也就会被GC咯
        WeakHashMap weakHashMap = new WeakHashMap();
        weakHashMap.put(key03,s03);
        //第4类、只有虚引用的数据，一开始refQueue为空，被GC才会有值
        ReferenceQueue refQueue = new ReferenceQueue<String>();
        PhantomReference<NString> phantomRef = new PhantomReference<NString>(s04, refQueue);

        //有强引用时，输出初始化情况
        System.out.println("With strong ref strongRef is " + (strongRef==null?"null":"not null"));
        System.out.println("With strong ref softRef is " + (softRef.get()==null?"null":"not null"));
        System.out.println("With strong ref weakRef is " + (weakRef.get()==null?"null":"not null"));
        System.out.println("With strong ref weakHashMap is " + (weakHashMap.isEmpty() ?"empty":"not empty"));
        //这里为null，当对象被GC时，会得到虚引用对象
        System.out.println("With strong ref refQueue has " + refQueue.poll());
        System.out.println();

        //去掉初始的强引用
        s00 = null;
        s01 = null;
        s02 = null;
        s03 = null;
        s04 = null;

        //多数情况下，没有来得及GC
        System.out.println("Before gc strongRef is " + (strongRef==null?"null":"not null"));
        System.out.println("Before gc softRef is " + (softRef.get()==null?"null":"not null"));
        System.out.println("Before gc weakRef is " + (weakRef.get()==null?"null":"not null"));
        System.out.println("Before gc weakHashMap is " + (weakHashMap.isEmpty() ?"empty":"not empty"));
        //null，当对象被GC时，会得到虚引用对象
        System.out.println("Before gc refQueue has " + refQueue.poll());
        System.out.println();

        //强制GC
        System.gc();
        System.out.println("After first gc strongRef is " + (strongRef==null?"null":"not null"));
        System.out.println("After first gc softRef is " + (softRef.get()==null?"null":"not null"));
        //null，weakRef已经被释放了
        System.out.println("After first gc weakRef is " + (weakRef.get()==null?"null":"not null"));
        System.out.println("After first gc weakHashMap is " + (weakHashMap.isEmpty() ?"empty":"not empty"));
        //与运行环境及GC方法有关，会出现一次有值的情况，然后一直为null
        System.out.println("After first gc refQueue has " + refQueue.poll());
        System.out.println();

        //将key03设为null，强制GC
        key03 = null;
        System.gc();
        System.out.println("After second gc strongRef is " + (strongRef==null?"null":"not null"));
        System.out.println("After second gc softRef is " + (softRef.get()==null?"null":"not null"));
        //null
        System.out.println("After second gc weakRef is " + (weakRef.get()==null?"null":"not null"));
        //empty，由于key被设为空，则会释放掉弱引用对象了
        System.out.println("After second gc weakHashMap is " + (weakHashMap.isEmpty() ?"empty":"not empty"));
        //与运行环境及GC方法有关，会出现一次有值的情况，然后一直为null
        System.out.println("After second gc refQueue has " + refQueue.poll());
        System.out.println();

        //Finalization
        System.runFinalization();
        System.out.println("After Finalization strongRef is " + (strongRef==null?"null":"not null"));
        System.out.println("After Finalization softRef is " + (softRef.get()==null?"null":"not null"));
        //null
        System.out.println("After Finalization weakRef is " + (weakRef.get()==null?"null":"not null"));
        //empty
        System.out.println("After Finalization weakHashMap is " + (weakHashMap.isEmpty() ?"empty":"not empty"));
        //与运行环境及GC方法有关，会出现一次有值的情况，然后一直为null
        System.out.println("After Finalization refQueue has " + refQueue.poll());
        System.out.println();

        //申请内存，造成内存不足
        System.out.println("Total memory is "+rt.totalMemory());
        StringBuilder builder2 = new StringBuilder(MemSize);
        for (int i=0;i<MemSize/8;i++) {
            builder2.append("ABCDEFGH");
        }
        NString tmp0 = new NString("tmp0", builder2);
        System.out.println("Total memory is "+rt.totalMemory());

        System.out.println("When mem is low strongRef is " + (strongRef==null?"null":"not null"));
        //null，内存不足时软引用会被释放
        System.out.println("When mem is low softRef is " + (softRef.get()==null?"null":"not null"));
        //null
        System.out.println("When mem is low weakRef is " + (weakRef.get()==null?"null":"not null"));
        //empty
        System.out.println("When mem is low weakHashMap is " + (weakHashMap.isEmpty() ?"empty":"not empty"));
        //与运行环境及GC方法有关，会出现一次有值的情况，然后一直为null
        System.out.println("When mem is low refQueue has " + refQueue.poll());
        System.out.println();

        //再次申请内存，JMV会内存爆掉
        NString tmp1 = null;
        try
        {
            tmp1 = new NString("tmp1",builder2);
        }
        catch(OutOfMemoryError ex)
        {
            System.out.println("Not enough memory");
        }

        //null
        System.out.println("tmp1 is " + (tmp1==null?"null":"not null"));
        //就算是抛出OutOfMemoryError，仍然不会释放
        System.out.println("When not enough memory strongRef is " + (strongRef==null?"null":"not null"));
        //null
        System.out.println("When not enough memory softRef is " + (softRef.get()==null?"null":"not null"));
        //null
        System.out.println("When not enough memory weakRef is " + (weakRef.get()==null?"null":"not null"));
        //empty
        System.out.println("When not enough memory weakHashMap is " + (weakHashMap.isEmpty() ?"empty":"not empty"));
        //null
        System.out.println("When not enough memory refQueue has " + refQueue.poll());
        System.out.println();
    }
}

3、输出

Total memory is 6094848
Total memory is 6094848
Total memory is 11390976
With strong ref strongRef is not null
With strong ref softRef is not null
With strong ref weakRef is not null
With strong ref weakHashMap is not empty
With strong ref refQueue has null

Before gc strongRef is not null
Before gc softRef is not null
Before gc weakRef is not null
Before gc weakHashMap is not empty
Before gc refQueue has null

After first gc strongRef is not null
After first gc softRef is not null
After first gc weakRef is null
After first gc weakHashMap is not empty
>> s04 finalize called
After first gc refQueue has null
>> s02 finalize called

After second gc strongRef is not null
After second gc softRef is not null
After second gc weakRef is null
After second gc weakHashMap is not empty
After second gc refQueue has java.lang.ref.PhantomReference@15d63da

After Finalization strongRef is not null
After Finalization softRef is not null
After Finalization weakRef is null
After Finalization weakHashMap is not empty
After Finalization refQueue has null

Total memory is 14221312
Total memory is 14221312
When mem is low strongRef is not null
When mem is low softRef is not null
When mem is low weakRef is null
When mem is low weakHashMap is not empty
When mem is low refQueue has null

>> s01 finalize called
Not enough memory
tmp1 is null
When not enough memory strongRef is not null
When not enough memory softRef is null
When not enough memory weakRef is null
When not enough memory weakHashMap is empty
When not enough memory refQueue has null

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