先来看个奇怪的demo

public class A {
    int i=0;
   // boolean flag =false;
    public synchronized void parse(){
        i++;
        JOLExample6.countDownLatch.countDown();
    }
}

睡眠5秒,测试

public class JOLExample3 {
   static A a;
    public static void main(String[] args) throws Exception {
        Thread.sleep(5000);
        a= new A();
        //a.hashCode();
        out.println("befor lock");
        out.println(ClassLayout.parseInstance(a).toPrintable());//无锁：偏向锁？
        synchronized (a){
            out.println("lock ing");
            out.println(ClassLayout.parseInstance(a).toPrintable());
        }

        out.println("after lock");
        out.println(ClassLayout.parseInstance(a).toPrintable());
    }
}

我注释的那行代码是什么锁?看下结果

 可以看出,没有线程持有锁的时候,是可偏向状态

 

 然后我们把睡眠的代码注释掉,再测试一下

//Thread.sleep(5000);

看下结果

 再看个两个线程的demo

首先是两个线程交替执行:

public class JOLExample10 {
   static A a;
    public static void main(String[] args) throws Exception {
        a= new A();
        Thread t1 = new Thread(){
            @Override
            public void run() {
                synchronized (a){
                    out.println("t1 lock ing");
                    out.println(ClassLayout.parseInstance(a).toPrintable());
                }
            }
        };
        t1.start();
       Thread.sleep(10000);//睡眠10秒,让main线程和t1线程交替执行
        synchronized (a){//a b c c+++
            out.println("main lock ing");
            out.println(ClassLayout.parseInstance(a).toPrintable());
        }
        out.println("after lock");
        out.println(ClassLayout.parseInstance(a).toPrintable());
    }
}

看下结果

 

 可以看出,交替执行时,是轻量锁

我们把睡眠的代码注释掉

//Thread.sleep(5000);//睡眠10秒,让main线程和t1线程交替执行

再次测试,

public class JOLExample10 {
   static A a;
    public static void main(String[] args) throws Exception {
        a= new A();
        Thread t1 = new Thread(){
            @Override
            public void run() {
                synchronized (a){
                    out.println("t1 lock ing");
                    out.println(ClassLayout.parseInstance(a).toPrintable());
                }
            }
        };
        t1.start();
       //Thread.sleep(5000);//睡眠10秒,让main线程和t1线程交替执行
        synchronized (a){//a b c c+++
            out.println("main lock ing");
            out.println(ClassLayout.parseInstance(a).toPrintable());
        }
        Thread.sleep(5000);//睡眠10秒
        out.println("after lock");
        out.println(ClassLayout.parseInstance(a).toPrintable());
    }
}

 

看下结果

 

 

自旋

自旋一段时间,可以理解为空转,时间很短,具体时间需要看jvm源码,如果在自旋时间内拿到了锁,就不再膨胀,如果还是拿不到锁,则膨胀为重量锁,如下

public static void main(String[] args) throws Exception {
        a= new A();
        Thread t1 = new Thread(){
            @Override
            public void run() {
                synchronized (a){
                    out.println("t1 lock ing");
                    out.println(ClassLayout.parseInstance(a).toPrintable());
                }
            }
        };
        t1.start();
       Thread.sleep(1670);//睡眠10秒,让main线程和t1线程交替执行
        synchronized (a){//自旋一段时间,可以理解为时间很短,具体时间需要看jvm源码,如果在自旋时间内拿到了锁,就不再膨胀,如果还是拿不到锁,则膨胀为重量锁
            out.println("main lock ing");
            out.println(ClassLayout.parseInstance(a).toPrintable());
        }

        Thread thread2 = new Thread(){
            @Override
            public void run() {
                synchronized (a){
                    out.println("t2 lock ing");
                    out.println(ClassLayout.parseInstance(a).toPrintable());
                }
            }
        };
        thread2.start();
        /*Thread.sleep(10);
        synchronized (a){
            out.println("main lock ing");
            out.println(ClassLayout.parseInstance(a).toPrintable());
        }*/
        //Thread.sleep(5000);//睡眠10秒,让main线程和t1线程交替执行
        out.println("after lock");
        out.println(ClassLayout.parseInstance(a).toPrintable());
    }

如果调用wait方法,则立即变成重量锁

看下demo

public class JOLExample11 {
    static A a;
    public static void main(String[] args) throws Exception {
        //Thread.sleep(5000);
        a = new A();
        out.println("befre lock");
        out.println(ClassLayout.parseInstance(a).toPrintable());

        Thread t1= new Thread(){
            public void run() {
                synchronized (a){
                    try {
                        synchronized (a) {
                            System.out.println("before wait");
                            out.println(ClassLayout.parseInstance(a).toPrintable());
                            a.wait();//如果调用wait方法,则立即变成重量锁
                            System.out.println(" after wait");
                            out.println(ClassLayout.parseInstance(a).toPrintable());
                        }
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        };
        t1.start();
        Thread.sleep(7000);
        synchronized (a) {
            a.notifyAll();
        }
    }
}

看下结果

 

 我们再看个synchronized膨胀的奇怪特性

让偏向锁无延迟启动

-XX:+UseBiasedLocking -XX:BiasedLockingStartupDelay=0

 

public class JOLExample12 {
    static List<A> list = new ArrayList<A>();
    public static void main(String[] args) throws Exception {
        Thread t1 = new Thread() {
            public void run() {
                for (int i=0;i<10;i++){
                    A a = new A();
                    synchronized (a){
                        System.out.println("111111");
                        list.add(a);
                    }
                }

            }

        };
        t1.start();
        t1.join();
        out.println("befre t2");
        //偏向
        out.println(ClassLayout.parseInstance(list.get(1)).toPrintable());
        Thread t2 = new Thread() {
            int k=0;
            public void run() {
                for(A a:list){
                   synchronized (a){
                       System.out.println("22222");
                       if (k==4){
                           out.println("t2 ing");
                           //轻量锁
                           out.println(ClassLayout.parseInstance(a).toPrintable());

                       }
                   }
                    k++;
                }

            }
        };
        t2.start();
    }
}

t1线程new10个对象,t2线程取第五个,看下结果

 

 我们把new对象的数量改一下,改成20个,再来试一下

public class JOLExample12 {
    static List<A> list = new ArrayList<A>();
    public static void main(String[] args) throws Exception {
        Thread t1 = new Thread() {
            public void run() {
                for (int i=0;i<20;i++){
                    A a = new A();
                    synchronized (a){
                        System.out.println("111111");
                        list.add(a);
                    }
                }

            }

        };
        t1.start();
        t1.join();
        out.println("befre t2");
        //偏向
        out.println(ClassLayout.parseInstance(list.get(1)).toPrintable());
        Thread t2 = new Thread() {
            int k=0;
            public void run() {
                for(A a:list){
                   synchronized (a){
                       System.out.println("22222");
                       if (k==19){
                           out.println("t2 ing");
                           //轻量锁
                           out.println(ClassLayout.parseInstance(a).toPrintable());

                       }
                   }
                    k++;
                }

            }
        };
        t2.start();
    }
}

这里我们取第20个对象,查看对象头信息

 

 我们可以看到,居然重偏向了,这里是jvm做的优化,20次以后就会冲偏向,小于20次时膨胀为轻量锁

这里我们称之为批量偏向,下面我们看下这个的原理

 

 最后总结一下:轻量锁释放的时候将mark word重置为无锁状态,附上网络上的图

 

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