目录

Java中，synchronized关键字有2种用法：

1. 作为关键字修饰方法
2. 修饰一个代码块

线程争用

为了探究synchronized的具体用法，可以用一个简单的程序来说明:

package fc.learn.java.synchronize;import java.util.Random;public class LearningSynchronized {    public enum SyncTypeTag {        OBJ_METHOD, CLASS_METHOD, KEYWARD_METHOD, NORMAL_METHOD    }    static class MyRunnable implements Runnable{        private void counting(){            for (int i = 0; i< 5; i++){                System.out.println(String.format("Thread[%s] count for [%s]", Thread.currentThread().getName(), i));                int sleepSeconds = new Random().nextInt();                sleepSeconds = sleepSeconds % 10;                sleepSeconds = sleepSeconds * sleepSeconds;                sleepSeconds = sleepSeconds % 5;                System.out.println(String.format("Thread[%s] sleep for 0.%s seconds", Thread.currentThread().getName(), sleepSeconds));                try {                    Thread.sleep(sleepSeconds * 100);                } catch (InterruptedException e) {                    e.printStackTrace();                }            }        }        private void synchronizeObjectMethod() {            synchronized (this){                counting();            }        }        private void synchronizeClasstMethod(){            synchronized (MyRunnable.class){                counting();            }        }        private synchronized void synchronizedMethod(){            counting();        }        private void normalMethod(){            counting();        }        private SyncTypeTag synType;        public MyRunnable(SyncTypeTag type){            this.synType = type;        }        @Override        public void run() {            if (this.synType == SyncTypeTag.OBJ_METHOD) {                synchronizeObjectMethod();            } else if (this.synType == SyncTypeTag.CLASS_METHOD) {                synchronizeClasstMethod();            } else if (this.synType == SyncTypeTag.KEYWARD_METHOD) {                synchronizedMethod();            } else if (this.synType == SyncTypeTag.NORMAL_METHOD) {                normalMethod();            }        }    }    public static void main(String[] args) {        Runnable r1 = new MyRunnable(SyncTypeTag.NORMAL_METHOD);        Runnable r2 = new MyRunnable(SyncTypeTag.NORMAL_METHOD);        Thread t1 = new Thread(r1);        Thread t2 = new Thread(r2);        t1.start();        t2.start();    }}

运行代码，可以获得结果如下：

Thread[Thread-1] count for [0]Thread[Thread-0] count for [0]Thread[Thread-1] sleep for 0.4 secondsThread[Thread-0] sleep for 0.1 secondsThread[Thread-0] count for [1]Thread[Thread-0] sleep for 0.4 secondsThread[Thread-1] count for [1]Thread[Thread-1] sleep for 0.0 secondsThread[Thread-1] count for [2]Thread[Thread-1] sleep for 0.1 secondsThread[Thread-1] count for [3]Thread[Thread-0] count for [2]Thread[Thread-1] sleep for 0.4 secondsThread[Thread-0] sleep for 0.1 secondsThread[Thread-0] count for [3]Thread[Thread-0] sleep for 0.4 secondsThread[Thread-1] count for [4]Thread[Thread-1] sleep for 0.4 secondsThread[Thread-0] count for [4]Thread[Thread-0] sleep for 0.1 seconds

也就是说，普通情况下，counting()方法会被线程争用

类锁 synchronized(class)

那么我们加锁试试呢？首先试试对class加锁：

// 之前代码太多不再赘述，此处只展示main方法public static void main(String[] args) {    Runnable r1 = new MyRunnable(SyncTypeTag.CLASS_METHOD);    Runnable r2 = new MyRunnable(SyncTypeTag.CLASS_METHOD);    Thread t1 = new Thread(r1);    Thread t2 = new Thread(r2);    t1.start();    t2.start();}

可以得到如下结果：

Thread[Thread-0] count for [0]Thread[Thread-0] sleep for 0.1 secondsThread[Thread-0] count for [1]Thread[Thread-0] sleep for 0.0 secondsThread[Thread-0] count for [2]Thread[Thread-0] sleep for 0.1 secondsThread[Thread-0] count for [3]Thread[Thread-0] sleep for 0.1 secondsThread[Thread-0] count for [4]Thread[Thread-0] sleep for 0.4 secondsThread[Thread-1] count for [0]Thread[Thread-1] sleep for 0.0 secondsThread[Thread-1] count for [1]Thread[Thread-1] sleep for 0.0 secondsThread[Thread-1] count for [2]Thread[Thread-1] sleep for 0.1 secondsThread[Thread-1] count for [3]Thread[Thread-1] sleep for 0.1 secondsThread[Thread-1] count for [4]Thread[Thread-1] sleep for 0.1 seconds

此时，线程不再争用，目的达到。

对象锁 synchronized(object)

那么试试对象锁:

// 仔细观察，这里用的 this 也就是说，每个线程用的不是同一把锁private void synchronizeObjectMethod() {    synchronized (this){        counting();    }}public static void main(String[] args) {    Runnable r1 = new MyRunnable(SyncTypeTag.CLASS_METHOD);    Runnable r2 = new MyRunnable(SyncTypeTag.CLASS_METHOD);    Thread t1 = new Thread(r1);    Thread t2 = new Thread(r2);    t1.start();    t2.start();}

运行代码：

Thread[Thread-1] count for [0]Thread[Thread-0] count for [0]Thread[Thread-1] sleep for 0.0 secondsThread[Thread-0] sleep for 0.4 secondsThread[Thread-1] count for [1]Thread[Thread-1] sleep for 0.1 secondsThread[Thread-1] count for [2]Thread[Thread-1] sleep for 0.4 secondsThread[Thread-0] count for [1]Thread[Thread-0] sleep for 0.4 secondsThread[Thread-1] count for [3]Thread[Thread-1] sleep for 0.4 secondsThread[Thread-0] count for [2]Thread[Thread-0] sleep for 0.1 secondsThread[Thread-0] count for [3]Thread[Thread-0] sleep for 0.0 secondsThread[Thread-0] count for [4]Thread[Thread-0] sleep for 0.1 secondsThread[Thread-1] count for [4]Thread[Thread-1] sleep for 0.0 seconds

可以发现，锁并不起作用。这个原因也很容易理解，因为synchronizeObjectMethod方法中用的synchronized(this)进行加锁，而我们有2个进程对象在对counting()方法进行操作，所以会发生争用。如果代码修改为这样：

// synchronizedObjectMethod修改成这样:private void synchronizeObjectMethod(Object globalLock) {    synchronized (globalLock){        counting();    }}// 构造方法和成员变量:private SyncTypeTag synType;private Object globalLock;public MyRunnable(SyncTypeTag type, Object lock){    this.synType = type;    this.globalLock = lock;}@Overridepublic void run() {    if (this.synType == SyncTypeTag.OBJ_METHOD) {        synchronizeObjectMethod(this.globalLock);    } else if (this.synType == SyncTypeTag.CLASS_METHOD) {        synchronizeClasstMethod();    } else if (this.synType == SyncTypeTag.KEYWARD_METHOD) {        synchronizedMethod();    } else if (this.synType == SyncTypeTag.NORMAL_METHOD) {        normalMethod();    }}// main 方法:public static void main(String[] args) {    Object globalLock = new Object();    Runnable r1 = new MyRunnable(SyncTypeTag.OBJ_METHOD, globalLock);    Runnable r2 = new MyRunnable(SyncTypeTag.OBJ_METHOD, globalLock);    Thread t1 = new Thread(r1);    Thread t2 = new Thread(r2);    t1.start();    t2.start();}

运行代码:

Thread[Thread-0] count for [0]Thread[Thread-0] sleep for 0.1 secondsThread[Thread-0] count for [1]Thread[Thread-0] sleep for 0.1 secondsThread[Thread-0] count for [2]Thread[Thread-0] sleep for 0.1 secondsThread[Thread-0] count for [3]Thread[Thread-0] sleep for 0.0 secondsThread[Thread-0] count for [4]Thread[Thread-0] sleep for 0.0 secondsThread[Thread-1] count for [0]Thread[Thread-1] sleep for 0.0 secondsThread[Thread-1] count for [1]Thread[Thread-1] sleep for 0.4 secondsThread[Thread-1] count for [2]Thread[Thread-1] sleep for 0.1 secondsThread[Thread-1] count for [3]Thread[Thread-1] sleep for 0.1 secondsThread[Thread-1] count for [4]Thread[Thread-1] sleep for 0.4 seconds

争用消失。

关键字 synchronized method()

接下来再试试synchronized关键字直接修饰方法:

public static void main(String[] args) {    Object globalLock = new Object();    Runnable r1 = new MyRunnable(SyncTypeTag.KEYWARD_METHOD, globalLock);    Runnable r2 = new MyRunnable(SyncTypeTag.KEYWARD_METHOD, globalLock);    Thread t1 = new Thread(r1);    Thread t2 = new Thread(r2);    t1.start();    t2.start();}

运行代码:

Thread[Thread-0] count for [0]Thread[Thread-1] count for [0]Thread[Thread-0] sleep for 0.0 secondsThread[Thread-1] sleep for 0.1 secondsThread[Thread-0] count for [1]Thread[Thread-0] sleep for 0.1 secondsThread[Thread-1] count for [1]Thread[Thread-0] count for [2]Thread[Thread-1] sleep for 0.1 secondsThread[Thread-0] sleep for 0.1 secondsThread[Thread-0] count for [3]Thread[Thread-0] sleep for 0.1 secondsThread[Thread-1] count for [2]Thread[Thread-1] sleep for 0.4 secondsThread[Thread-0] count for [4]Thread[Thread-0] sleep for 0.4 secondsThread[Thread-1] count for [3]Thread[Thread-1] sleep for 0.4 secondsThread[Thread-1] count for [4]Thread[Thread-1] sleep for 0.1 seconds

依旧会起争用，说明synchronized修饰方法，等于 synchronized(this)。

References: