java_强、软、弱、虚四大引用

 

1、强引用（默认使用）

//强引用
public class StrongReferenceDemo {
    public static void main(String[] args) {
        Object obj1 = new Object();//这样定义的，默认就是强引用
        Object obj2 = obj1;
        obj1 = null;
        System.gc();//手动启动GC
        System.out.println(obj2);
    }
}

 

2、软引用

import java.lang.ref.SoftReference;

/**
 * 软引用：内存够用，不回收，内存不够就回收；
 */

public class SoftReferenceDemo {
    public static void main(String[] args) {
        //softRef_Memory_Enough();
        softRef_Memory_NotEnough();
    }

    //内存够用
    private static void softRef_Memory_Enough() {
        Object o1 = new Object();
        SoftReference<Object> soft = new SoftReference<>(o1);
        System.out.println(o1);
        System.out.println(soft.get());
        o1 = null;
        System.gc();//手动GC
        System.out.println(o1);
        System.out.println(soft.get());
    }

    /**
     * 内存不够用，自动GC，故意产生大对象并配置小内存，让它内存不够用导致OOM，
     * 看软引用的回收情况
     */
    private static void softRef_Memory_NotEnough() {
        Object o1 = new Object();
        SoftReference<Object> soft = new SoftReference<>(o1);
        System.out.println(o1);
        System.out.println(soft.get());
        o1 = null;
        try {
            byte[] bytes = new byte[30 * 1024 * 1024];
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            System.out.println(o1);
            System.out.println(soft.get());
        }
    }
}

 

3、弱引用

内存不管够不够，只要gc，就会回收！

import java.lang.ref.WeakReference;

/**
 * 弱引用 只要GC，就会回收
 */
public class WeakReferenceDemo {
    public static void main(String[] args) {
        Object o1 = new Object();
        WeakReference<Object> weak = new WeakReference<>(o1);
        System.out.println(o1);
        System.out.println(weak.get());
        o1 = null;
        System.gc();
        System.out.println(o1);
        System.out.println(weak.get());
    }
}

WeakHashMap

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

public class WeakHashMapDemo {
    public static void main(String[] args) {
        myHashMap();
        System.out.println("======");
        myWeakHashMap();
    }
    private 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);
    }
    private static void myWeakHashMap() {
        WeakHashMap<Integer, String> map = new WeakHashMap<>();
        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);
    }
}

 

4、虚引用

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

public class ReferenceQueueDemo {
    public static void main(String[] args)throws Exception {
        Object o1 = new Object();
        ReferenceQueue<Object> queue = new ReferenceQueue<>();
        WeakReference<Object> weak = new WeakReference<>(o1,queue);

        System.out.println(o1);
        System.out.println(weak.get());
        System.out.println(queue.poll());
        System.out.println("-=====");
        o1 = null;
        System.gc();
        Thread.sleep(500);
        System.out.println(o1);
        System.out.println(weak.get());
        System.out.println(queue.poll());
    }
}

 

虚引用

import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;

/**
 * 虚引用
 */
public class PhantomReferenceDemo {
    public static void main(String[] args)throws Exception {
        Object o1 = new Object();
        ReferenceQueue<Object> queue = new ReferenceQueue<>();
        PhantomReference<Object> phantom = new PhantomReference<>(o1,queue);
        System.out.println(o1);
        System.out.println(phantom.get());
        System.out.println(queue.poll());
        System.out.println("===");
        o1 = null;
        System.gc();
        Thread.sleep(500);
        System.out.println(o1);
        System.out.println(phantom.get());
        System.out.println(queue.poll());
    }
}