Java引用

强引用

public static void main(String[] args) {
    Object obj1=new Object();
    Object obj2=obj1;
    obj1=null;
    System.gc();
    System.out.println(obj2);
}

软引用

public class SoftReferenceDemo {

    /**
     * 内存够用就不回收，内存不够就回收
     */
    public static void softRefMemoryEnough(){
        Object o1=new Object();
        SoftReference<Object> softReference=new SoftReference<Object>(o1);
        System.out.println(o1);
        System.out.println(softReference.get());
        o1=null;
        System.gc();
        System.out.println(o1);
        System.out.println(softReference.get());

    }

    /**
     * 故意产生oom，new一个大内存数组，设置堆内存大小小一点
     * -Xms5m -Xmx5m
     */
    public static void softRefMemoryNotEnough(){
        Object o1=new Object();
        SoftReference<Object> softReference=new SoftReference<Object>(o1);
        System.out.println(o1);
        System.out.println(softReference.get());
        o1=null;

        try {
            byte[] bytes=new byte[30*1024*1024];
        }catch (Exception e) {
            e.printStackTrace();
        }finally
         {
            System.out.println(o1);
            System.out.println(softReference.get());
        }

    }

    public static void main(String[] args) {
        //softRefMemoryEnough();
        softRefMemoryNotEnough();
    }
}

弱引用

public class WeekReferenceDemo {

    public static void main(String[] args) {
        Object o1=new Object();
        WeakReference<Object> weakReference=new WeakReference<>(o1);
        System.out.println(o1);
        System.out.println(weakReference.get());
        System.out.println("==================");
        o1=null;
        System.gc();
        System.out.println(o1);
        System.out.println(weakReference.get());
    }
}

软引用和弱引用英语场景

WeakHashMap

public class WeakHashMapDemo {

    public static void main(String[] args) {
        myHashMap();
        System.out.println("==============================");
        myWeakHashMap();
    }



    private static void myHashMap() {
        Map<Integer,String> hashMap=new HashMap<>();
        Integer a=new Integer(1);
        hashMap.put(a,"HashMap");
        System.out.println(hashMap);

        a=null;
        System.gc();
        System.out.println(hashMap);
        System.out.println(hashMap);

    }

    private static void myWeakHashMap() {
        Map<Integer,String> hashMap=new WeakHashMap<>();
        Integer a=new Integer(1);
        hashMap.put(a,"HashMap");
        System.out.println(hashMap);

        a=null;
        System.out.println(hashMap);
        System.gc();
        System.out.println(hashMap);

    }
}

虚引用

public class PhantomReferenceDemo {

    public static void main(String[] args) throws InterruptedException {
        Object o1=new Object();
        ReferenceQueue<Object> referenceQueue=new ReferenceQueue<>();
        PhantomReference<Object> phantomReference=new PhantomReference<>(o1,referenceQueue);

        System.out.println(o1);
        System.out.println(phantomReference.get());
        System.out.println(referenceQueue.poll());

        System.out.println("====================");
        o1=null;
        System.gc();
        Thread.sleep(500);

        System.out.println(o1);
        System.out.println(phantomReference.get());
        System.out.println(referenceQueue.poll());
    }
}

引用队列

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

        System.out.println(o1);
        System.out.println(weakReference.get());
        System.out.println(referenceQueue.poll());

        System.out.println("====================");
        o1=null;
        System.gc();
        Thread.sleep(500);

        System.out.println(o1);
        System.out.println(weakReference.get());
        System.out.println(referenceQueue.poll());

    }
}

总结