关于程序员:查漏补缺强软弱虚引用

强援用

• 强援用：Object o = new Object()
• 软援用：new SoftReference(o);
• 弱援用：new WeakReference(o);
• 虚援用：new PhantomReference(o,Queue);

本次会用到命令-XX:+PrintGCDetails
会打印

[GC (Allocation Failure) [PSYoungGen: 29680K->4080K(29696K)] 73163K->72619K(98304K), 0.0245660 secs] [Times: user=0.15 sys=0.02, real=0.03 secs] 


[Full GC (Ergonomics) [PSYoungGen: 4080K->3557K(29696K)] [ParOldGen: 68539K->68498K(68608K)] 72619K->72055K(98304K), [Metaspace: 8977K->8977K(1056768K)], 0.1491246 secs] [Times: user=1.02 sys=0.01, real=0.15 secs] 

强援用

强援用就是咱们最罕用的创建对象的形式Object o = new Object()，只有援用还在，就不会被回收，堆内存占满就间接抛出OOM。然而当指向null后，在gc时就回回收对应内存

//        Thread.sleep(10000);
//        strongReference();
//        -XX:+PrintGCDetails -Xms100m -Xmx100m
    private static void strongReference() throws InterruptedException {
        int size=100000;
        Element[] arr= new Element[size];
        try{
            for (int i = 0; i < size; i++) {
                arr[i]=new Element(i);
                Thread.sleep(1);
                //arr[i]=null;
            }
        }catch (Throwable e){
            for (int i = 0; i < size; i++) {
                if(arr[i]==null){
                    System.out.println("null number:"+i);//null number:84318
                    break;
                }
            }
        }
    }

如果执显式设置为null后

软援用

软援用，应用SoftReference创立，然而，当内存不够的时候，回把对象回收，not null number:139479，整个程序执行，并无产生oom。

源代码正文中写了在抛出oom之前革除，通过PrintGCDetails也能够看到，根本都是在FullGC中解决，联合visualVM能够看到，并不是所有FullGC都会解决，一半都是堆内存占用快满了，才会触发

All soft references to softly-reachable objects are guaranteed to have
 * been cleared before the virtual machine throws an
 * <code>OutOfMemoryError</code>

    public static void main(String[] args) throws Exception {
        Thread.sleep(10000);
//        strongReference();
        softReference();
    }

    private static void softReference() throws InterruptedException {
        int size=200000;
        SoftReference<Element>[] list=new SoftReference[size];


        for (int i = 0; i < size; i++) {
            list[i]=new SoftReference<Element>(new Element(i));
            Thread.sleep(1);
        }
        for (int i = 0; i < size; i++) {
            if(list[i].get()!=null){
                System.out.println("not null number:"+i);
                break;
            }

        }
    }

弱援用

弱援用，调用形式和下面差不多，就是类换成了WeakReference，其余一律没有变，功过图能够看到它是在YoungGC的时候就革除了

    private static void weakReference() throws InterruptedException {
        int size=200000;
        WeakReference<Element>[] list=new WeakReference[size];


        for (int i = 0; i < size; i++) {
            list[i]=new WeakReference<Element>(new Element(i));
            Thread.sleep(1);
        }
        for (int i = 0; i < size; i++) {
            if(list[i].get()!=null){
                System.out.println("not null number:"+i);
                break;
            }

        }
    }

虚援用

PhantomReference和下面三个不大一样，我一开时认为也是不便清理，可是看了正文不大一样。

PhantomReference最罕用于以一种比Java finalization更灵便的形式来调度预剖析清理操作。

Phantom
 * references are most often used for scheduling pre-mortem cleanup actions in
 * a more flexible way than is possible with the Java finalization mechanism.

也就是说它设计的目标就是为了代替finalize

    private static void phantomReference() throws InterruptedException {
        Element e=new Element(1);
        ReferenceQueue referenceQueue=new ReferenceQueue();

        PhantomReference<Element> reference=new PhantomReference(e,referenceQueue){
            @Override
            public void clear() {
                System.out.println(1111111);
                super.clear();
            }
        };

        new Thread(){
            @Override
            public void run() {
                while (true){
                    try {
                        PhantomReference reference1= (PhantomReference) referenceQueue.remove();
                        reference1.clear();
                    } catch (InterruptedException interruptedException) {
                        interruptedException.printStackTrace();
                    }
                }
            }
        }.start();

        Thread.sleep(1000);
        e=null;
        System.out.println("element is null");
        System.gc();
        Thread.sleep(1000);


    }
    
    /*
    element is null
[GC (System.gc()) [PSYoungGen: 3083K->567K(29696K)] 3083K->567K(98304K), 0.0014892 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] 
[Full GC (System.gc()) [PSYoungGen: 567K->0K(29696K)] [ParOldGen: 0K->456K(68608K)] 567K->456K(98304K), [Metaspace: 2955K->2955K(1056768K)], 0.0046349 secs] [Times: user=0.02 sys=0.00, real=0.01 secs] 
1111111
    */

原本茶队升级曾经够快乐了，起初TA打李逵，带来了3场精彩对决，看的热血沸腾的，加油茶队

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