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Java多线程（三）：synchronized

多线程安全

脏读：多个线程对同一个对象的实例变量进行修改后访问，导致读到的数据是被修改过的。

实例

ThreadDomain16类

public class ThreadDomain16 {
    private int num = 0;

    public void addNum(String userName)
    {
        try
        {
            if ("a".equals(userName))
            {
                num = 100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            }
            else
            {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(userName + " num = " + num);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyThread16_0类

public class MyThread16_0 extends Thread{
    private ThreadDomain16 td;

    public MyThread16_0(ThreadDomain16 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.addNum("a");
    }
}

MyThread16_1类
启动MyThread16_0 和MyThread16_1 线程

public class MyThread16_1 extends Thread {
    private ThreadDomain16 td;

    public MyThread16_1(ThreadDomain16 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.addNum("b");
    }

}

main方法

public class MyThread16_main_0 {
    public static void main(String[] args)
    {
        ThreadDomain16 td = new ThreadDomain16();
        MyThread16_0 mt0 = new MyThread16_0(td);
        MyThread16_1 mt1 = new MyThread16_1(td);
        mt0.start();
        mt1.start();
    }
}

输出结果如下

a set over!
b set over!
b num = 200
a num = 200

理论上应该输出a num = 100,b num = 200，结果a num = 200。
我们来分析下运行流程，MyThread16_0线程修改num为100，此时sleep两秒，在MyThread16_0 sleep的过程中，MyThread16_1线程修改num为200。导致MyThread16_0的num也变成200。最后先输出b num = 200，再输出a num = 200。出现了脏读。
解决该问题很简单，我们在addNum()前面加上synchronized关键字

public synchronized void addNum(String userName)
    {
        try
        {
            if ("a".equals(userName))
            {
                num = 100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            }
            else
            {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(userName + " num = " + num);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }

输出结果如下

a set over!
a num = 100
b set over!
b num = 200

按序打印，synchronized修饰的方法形成了锁，哪个线程先执行了synchronized修饰的方法哪个线程就先获得了锁，其他线程必须等待，直到该线程释放锁，等待的线程获得锁方可执行。

一个对象一个锁

同步情况下，使用如下main方法

public class MyThread16_main_1 {
    public static void main(String[] args)
    {
        ThreadDomain16 td0 = new ThreadDomain16();
        ThreadDomain16 td1 = new ThreadDomain16();
        MyThread16_0 mt0 = new MyThread16_0(td0);
        MyThread16_1 mt1 = new MyThread16_1(td1);
        mt0.start();
        mt1.start();
    }
}

运行结果

a set over!
b set over!
b num = 200
a num = 100

出现了交叉打印，b没有等待a执行结束，说明synchronized是对象锁，而不是方法锁。这里声明了两个ThreadDomain16对象，所以有两个对象锁，不受“等待锁的释放”的制约。
那么我们把addNum方法前面的synchronized关键字去掉，异步情况下
运行结果如下

a set over!
b set over!
b num = 200
a num = 100

可以看到，没有synchronized的帮助，依然避免了脏读现象。实例变量num为每个对象独自占有。但是，这种解决方式需要声明多个对象，不推荐。

synchronized方法与锁对象

现在看一个例子，一个方法有synchronized修饰，另一个方法无synchronized修饰
ThreadDomain17类

public class ThreadDomain17 {
    public synchronized void methodA()
    {
        try
        {
            System.out.println("开始 methodA, 线程名称 = " +
                    Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("结束 methodA, 线程名称 = " +
                    Thread.currentThread().getName());
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }

    public void methodB()
    {
        try
        {
            System.out.println("开始 methodB, 线程名称 = " +
                    Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("结束 methodB, 线程名称 = " +
                    Thread.currentThread().getName());
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyThread17_0类

public class MyThread17_0 extends Thread{
    private ThreadDomain17 td;

    public MyThread17_0(ThreadDomain17 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.methodA();
    }
}

MyThread17_1类

public class MyThread17_1 extends Thread{
    private ThreadDomain17 td;

    public MyThread17_1(ThreadDomain17 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.methodB();
    }
}

main方法

public class MyThread17_main {
    public static void main(String[] args) {
        ThreadDomain17 td = new ThreadDomain17();
        MyThread17_0 mt0 = new MyThread17_0(td);
        mt0.setName("A");
        MyThread17_1 mt1 = new MyThread17_1(td);
        mt1.setName("B");
        mt0.start();
        mt1.start();
    }
}

运行结果如下

开始 methodB, 线程名称 = B
开始 methodA, 线程名称 = A
结束 methodA, 线程名称 = A
结束 methodB, 线程名称 = B

可以看到，非synchronized的methodB方法以异步方式被调用。
把MyThread17_1类的run方法改成td.methodA();
输出结果如下

开始 methodA, 线程名称 = A
结束 methodA, 线程名称 = A
开始 methodA, 线程名称 = B
结束 methodA, 线程名称 = B

待执行的线程在等待synchronized方法同步再执行。
A线程持有ThreadDomain17对象锁，B线程可以异步方式调用ThreadDomain17对象的非synchronized方法；
A线程持有ThreadDomain17对象锁，B线程调用ThreadDomain17对象的synchronized方法需要等待；
把MyThread17_1类的run方法改回td.methodB();把ThreadDomain17类的methodB方法加上synchronized关键字，输出结果如下

开始 methodA, 线程名称 = A
结束 methodA, 线程名称 = A
开始 methodB, 线程名称 = B
结束 methodB, 线程名称 = B

这个很好理解，先执行的线程获得同步锁，等该线程必须执行完毕，等待的线程才能执行，所有的操作都是同步的。

synchronized锁重入

synchronized是可重入锁，当一个线程得到对象锁之后，可以再次申请对象锁并获得对象锁。
ThreadDomain18类

public class ThreadDomain18 {
    public synchronized void print1()
    {
        System.out.println("ThreadDomain18.print1()");
        print2();
    }

    public synchronized void print2()
    {
        System.out.println("ThreadDomain18.print2()");
        print3();
    }

    public synchronized void print3()
    {
        System.out.println("ThreadDomain18.print3()");
    }
}

MyThread18类

public class MyThread18 extends Thread{
    public void run()
    {
        ThreadDomain18 td = new ThreadDomain18();
        td.print1();
    }

    public static void main(String[] args)
    {
        MyThread18 mt = new MyThread18();
        mt.start();
    }
    
}

结果如下

ThreadDomain18.print1()
ThreadDomain18.print2()
ThreadDomain18.print3()

结果说明，MyThread18在有锁的情况下多次获取到对象锁

异常释放锁

ThreadDomain19类

public class ThreadDomain19 {
    public synchronized void testMethod()
    {
        try
        {
            System.out.println("进入 ThreadDomain19.testMethod(), currentThread = " +
                    Thread.currentThread().getName());
            long l = 5;
            while (true)
            {
                long lo = 2 / l;
                Thread.sleep(1000);
                l--;
            }
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }
}

MyThread19类

public class MyThread19 extends Thread {
    private ThreadDomain19 td;

    public MyThread19(ThreadDomain19 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.testMethod();
    }

    public static void main(String[] args)
    {
        ThreadDomain19 td = new ThreadDomain19();
        MyThread19 mt0 = new MyThread19(td);
        MyThread19 mt1 = new MyThread19(td);
        mt0.start();
        mt1.start();
    }
}

输出结果如下

进入 ThreadDomain19.testMethod(), currentThread = Thread-1
java.lang.ArithmeticException: / by zero
进入 ThreadDomain19.testMethod(), currentThread = Thread-0
	at com.advance.MultiThread3.ThreadDomain19.testMethod(ThreadDomain19.java:18)
	at com.advance.MultiThread3.MyThread19.run(MyThread19.java:18)
java.lang.ArithmeticException: / by zero
	at com.advance.MultiThread3.ThreadDomain19.testMethod(ThreadDomain19.java:18)
	at com.advance.MultiThread3.MyThread19.run(MyThread19.java:18)

由上可见，一旦出现异常，锁自动释放。Thread-1刚抛完异常，此时Thread-1释放了锁，Thread-0就开始执行。

synchronized方法块

ThreadDomain20类

public class ThreadDomain20 extends Thread{
    public void doLongTimeTask() throws Exception
    {
        for (int i = 0; i < 100; i++)
        {
            System.out.println("nosynchronized threadName = " +
                    Thread.currentThread().getName() + ", i = " + (i + 1));
        }
        System.out.println();
        synchronized (this){
            for (int i = 0; i < 100; i++)
            {
                System.out.println("synchronized threadName = " +
                        Thread.currentThread().getName() + ", i = " + (i + 1));
            }
        }
    }
    
}

MyThread20类

public class MyThread20 extends Thread{
    private ThreadDomain20 td;

    public MyThread20(ThreadDomain20 td)
    {
        this.td = td;
    }

    public void run()
    {
        try
        {
            td.doLongTimeTask();
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }

    public static void main(String[] args)
    {
        ThreadDomain20 td = new ThreadDomain20();
        MyThread20 mt0 = new MyThread20(td);
        mt0.setName("A");
        MyThread20 mt1 = new MyThread20(td);
        mt1.setName("B");
        mt0.start();
        mt1.start();
    }

}

运行结果如下

nosynchronized threadName = A, i = 1
nosynchronized threadName = B, i = 1
nosynchronized threadName = A, i = 2
nosynchronized threadName = B, i = 2
nosynchronized threadName = A, i = 3
nosynchronized threadName = B, i = 3
nosynchronized threadName = A, i = 4
nosynchronized threadName = B, i = 4
nosynchronized threadName = A, i = 5
nosynchronized threadName = B, i = 5
nosynchronized threadName = A, i = 6
nosynchronized threadName = B, i = 6
nosynchronized threadName = A, i = 7
nosynchronized threadName = B, i = 7
nosynchronized threadName = A, i = 8
nosynchronized threadName = B, i = 8
nosynchronized threadName = A, i = 9
nosynchronized threadName = B, i = 9
nosynchronized threadName = A, i = 10
nosynchronized threadName = B, i = 10
nosynchronized threadName = A, i = 11
nosynchronized threadName = B, i = 11
nosynchronized threadName = A, i = 12
nosynchronized threadName = B, i = 12
nosynchronized threadName = A, i = 13
nosynchronized threadName = B, i = 13
nosynchronized threadName = A, i = 14
nosynchronized threadName = B, i = 14
nosynchronized threadName = A, i = 15
nosynchronized threadName = A, i = 16
nosynchronized threadName = A, i = 17
nosynchronized threadName = A, i = 18
nosynchronized threadName = B, i = 15
nosynchronized threadName = A, i = 19
nosynchronized threadName = B, i = 16
nosynchronized threadName = A, i = 20
nosynchronized threadName = B, i = 17
nosynchronized threadName = A, i = 21
nosynchronized threadName = B, i = 18
nosynchronized threadName = A, i = 22
nosynchronized threadName = B, i = 19
nosynchronized threadName = A, i = 23
nosynchronized threadName = A, i = 24
nosynchronized threadName = B, i = 20
nosynchronized threadName = A, i = 25
nosynchronized threadName = B, i = 21
nosynchronized threadName = A, i = 26
nosynchronized threadName = B, i = 22
nosynchronized threadName = A, i = 27
nosynchronized threadName = B, i = 23
nosynchronized threadName = A, i = 28
nosynchronized threadName = B, i = 24
nosynchronized threadName = A, i = 29
nosynchronized threadName = B, i = 25
nosynchronized threadName = A, i = 30
nosynchronized threadName = B, i = 26
nosynchronized threadName = A, i = 31
nosynchronized threadName = A, i = 32
nosynchronized threadName = B, i = 27
nosynchronized threadName = A, i = 33
nosynchronized threadName = B, i = 28
nosynchronized threadName = A, i = 34
nosynchronized threadName = B, i = 29
nosynchronized threadName = A, i = 35
nosynchronized threadName = B, i = 30
nosynchronized threadName = A, i = 36
nosynchronized threadName = B, i = 31
nosynchronized threadName = A, i = 37
nosynchronized threadName = B, i = 32
nosynchronized threadName = A, i = 38
nosynchronized threadName = A, i = 39
nosynchronized threadName = B, i = 33
nosynchronized threadName = A, i = 40
nosynchronized threadName = B, i = 34
nosynchronized threadName = A, i = 41
nosynchronized threadName = B, i = 35
nosynchronized threadName = A, i = 42
nosynchronized threadName = B, i = 36
nosynchronized threadName = A, i = 43
nosynchronized threadName = B, i = 37
nosynchronized threadName = A, i = 44
nosynchronized threadName = B, i = 38
nosynchronized threadName = A, i = 45
nosynchronized threadName = B, i = 39
nosynchronized threadName = A, i = 46
nosynchronized threadName = B, i = 40
nosynchronized threadName = A, i = 47
nosynchronized threadName = B, i = 41
nosynchronized threadName = A, i = 48
nosynchronized threadName = B, i = 42
nosynchronized threadName = A, i = 49
nosynchronized threadName = B, i = 43
nosynchronized threadName = A, i = 50
nosynchronized threadName = B, i = 44

nosynchronized threadName = B, i = 45
synchronized threadName = A, i = 1
nosynchronized threadName = B, i = 46
synchronized threadName = A, i = 2
nosynchronized threadName = B, i = 47
synchronized threadName = A, i = 3
nosynchronized threadName = B, i = 48
synchronized threadName = A, i = 4
nosynchronized threadName = B, i = 49
nosynchronized threadName = B, i = 50

synchronized threadName = A, i = 5
synchronized threadName = A, i = 6
synchronized threadName = A, i = 7
synchronized threadName = A, i = 8
synchronized threadName = A, i = 9
synchronized threadName = A, i = 10
synchronized threadName = A, i = 11
synchronized threadName = A, i = 12
synchronized threadName = A, i = 13
synchronized threadName = A, i = 14
synchronized threadName = A, i = 15
synchronized threadName = A, i = 16
synchronized threadName = A, i = 17
synchronized threadName = A, i = 18
synchronized threadName = A, i = 19
synchronized threadName = A, i = 20
synchronized threadName = A, i = 21
synchronized threadName = A, i = 22
synchronized threadName = A, i = 23
synchronized threadName = A, i = 24
synchronized threadName = A, i = 25
synchronized threadName = A, i = 26
synchronized threadName = A, i = 27
synchronized threadName = A, i = 28
synchronized threadName = A, i = 29
synchronized threadName = A, i = 30
synchronized threadName = A, i = 31
synchronized threadName = A, i = 32
synchronized threadName = A, i = 33
synchronized threadName = A, i = 34
synchronized threadName = A, i = 35
synchronized threadName = A, i = 36
synchronized threadName = A, i = 37
synchronized threadName = A, i = 38
synchronized threadName = A, i = 39
synchronized threadName = A, i = 40
synchronized threadName = A, i = 41
synchronized threadName = A, i = 42
synchronized threadName = A, i = 43
synchronized threadName = A, i = 44
synchronized threadName = A, i = 45
synchronized threadName = A, i = 46
synchronized threadName = A, i = 47
synchronized threadName = A, i = 48
synchronized threadName = A, i = 49
synchronized threadName = A, i = 50
synchronized threadName = B, i = 1
synchronized threadName = B, i = 2
synchronized threadName = B, i = 3
synchronized threadName = B, i = 4
synchronized threadName = B, i = 5
synchronized threadName = B, i = 6
synchronized threadName = B, i = 7
synchronized threadName = B, i = 8
synchronized threadName = B, i = 9
synchronized threadName = B, i = 10
synchronized threadName = B, i = 11
synchronized threadName = B, i = 12
synchronized threadName = B, i = 13
synchronized threadName = B, i = 14
synchronized threadName = B, i = 15
synchronized threadName = B, i = 16
synchronized threadName = B, i = 17
synchronized threadName = B, i = 18
synchronized threadName = B, i = 19
synchronized threadName = B, i = 20
synchronized threadName = B, i = 21
synchronized threadName = B, i = 22
synchronized threadName = B, i = 23
synchronized threadName = B, i = 24
synchronized threadName = B, i = 25
synchronized threadName = B, i = 26
synchronized threadName = B, i = 27
synchronized threadName = B, i = 28
synchronized threadName = B, i = 29
synchronized threadName = B, i = 30
synchronized threadName = B, i = 31
synchronized threadName = B, i = 32
synchronized threadName = B, i = 33
synchronized threadName = B, i = 34
synchronized threadName = B, i = 35
synchronized threadName = B, i = 36
synchronized threadName = B, i = 37
synchronized threadName = B, i = 38
synchronized threadName = B, i = 39
synchronized threadName = B, i = 40
synchronized threadName = B, i = 41
synchronized threadName = B, i = 42
synchronized threadName = B, i = 43
synchronized threadName = B, i = 44
synchronized threadName = B, i = 45
synchronized threadName = B, i = 46
synchronized threadName = B, i = 47
synchronized threadName = B, i = 48
synchronized threadName = B, i = 49
synchronized threadName = B, i = 50

结论：
A线程访问synchronized方法块的时候，B线程依然可以访问非synchronized方法块的内容；
A线程访问synchronized方法块的时候，B线程访问synchronized方法块会被阻塞；

synchronized方法块和synchronized方法

synchronized关键字修饰的方法和synchronized方法块修饰的方法效果一致。synchronized方法块可以提高代码执行效率，在需要同步的地方去同步。
ThreadDomain21类

public class ThreadDomain21 {
    public synchronized void otherMethod()
    {
        System.out.println("----------run--otherMethod");
    }

    public void doLongTask()
    {
        synchronized (this)
        {
            for (int i = 0; i < 100; i++)
            {
                System.out.println("synchronized threadName = " +
                        Thread.currentThread().getName() + ", i = " + (i + 1));
                try
                {
                    Thread.sleep(5);
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
            }
        }
    }
}

MyThread21_0类

public class MyThread21_0 extends Thread{
    private ThreadDomain21 td;

    public MyThread21_0(ThreadDomain21 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.doLongTask();
    }
}

MyThread21_1类

public class MyThread21_1 extends Thread{
    private ThreadDomain21 td;

    public MyThread21_1(ThreadDomain21 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.otherMethod();
    }
}

main方法，sleep一会，让mt0先执行。

public class MyThread21_main {
    public static void main(String[] args) throws Exception
    {
        ThreadDomain21 td = new ThreadDomain21();
        MyThread21_0 mt0 = new MyThread21_0(td);
        mt0.setName("A");
        MyThread21_1 mt1 = new MyThread21_1(td);
        mt1.setName("B");
        mt0.start();
        Thread.sleep(100);
        mt1.start();
    }
}

输出结果如下

synchronized threadName = A, i = 1
synchronized threadName = A, i = 2
synchronized threadName = A, i = 3
synchronized threadName = A, i = 4
synchronized threadName = A, i = 5
synchronized threadName = A, i = 6
synchronized threadName = A, i = 7
synchronized threadName = A, i = 8
synchronized threadName = A, i = 9
synchronized threadName = A, i = 10
synchronized threadName = A, i = 11
synchronized threadName = A, i = 12
synchronized threadName = A, i = 13
synchronized threadName = A, i = 14
synchronized threadName = A, i = 15
synchronized threadName = A, i = 16
synchronized threadName = A, i = 17
synchronized threadName = A, i = 18
synchronized threadName = A, i = 19
synchronized threadName = A, i = 20
synchronized threadName = A, i = 21
synchronized threadName = A, i = 22
synchronized threadName = A, i = 23
......
synchronized threadName = A, i = 89
synchronized threadName = A, i = 90
synchronized threadName = A, i = 91
synchronized threadName = A, i = 92
synchronized threadName = A, i = 93
synchronized threadName = A, i = 94
synchronized threadName = A, i = 95
synchronized threadName = A, i = 96
synchronized threadName = A, i = 97
synchronized threadName = A, i = 98
synchronized threadName = A, i = 99
synchronized threadName = A, i = 100
----------run--otherMethod

可以看到，otherMethod方法被doLongTask方法阻塞，说明synchronized锁定了整个对象，把otherMethod方法synchronized关键字去掉，输出结果如下

synchronized threadName = A, i = 1
synchronized threadName = A, i = 2
synchronized threadName = A, i = 3
synchronized threadName = A, i = 4
synchronized threadName = A, i = 5
synchronized threadName = A, i = 6
synchronized threadName = A, i = 7
synchronized threadName = A, i = 8
synchronized threadName = A, i = 9
synchronized threadName = A, i = 10
synchronized threadName = A, i = 11
synchronized threadName = A, i = 12
synchronized threadName = A, i = 13
synchronized threadName = A, i = 14
synchronized threadName = A, i = 15
synchronized threadName = A, i = 16
synchronized threadName = A, i = 17
synchronized threadName = A, i = 18
synchronized threadName = A, i = 19
synchronized threadName = A, i = 20
synchronized threadName = A, i = 21
----------run--otherMethod
......
synchronized threadName = A, i = 72
synchronized threadName = A, i = 73
synchronized threadName = A, i = 74
synchronized threadName = A, i = 75
synchronized threadName = A, i = 76
synchronized threadName = A, i = 77
synchronized threadName = A, i = 78
synchronized threadName = A, i = 79
synchronized threadName = A, i = 80
synchronized threadName = A, i = 81
synchronized threadName = A, i = 82
synchronized threadName = A, i = 83
synchronized threadName = A, i = 84
synchronized threadName = A, i = 85
synchronized threadName = A, i = 86
synchronized threadName = A, i = 87
synchronized threadName = A, i = 88
synchronized threadName = A, i = 89
synchronized threadName = A, i = 90
synchronized threadName = A, i = 91
synchronized threadName = A, i = 92
synchronized threadName = A, i = 93
synchronized threadName = A, i = 94
synchronized threadName = A, i = 95
synchronized threadName = A, i = 96
synchronized threadName = A, i = 97
synchronized threadName = A, i = 98
synchronized threadName = A, i = 99
synchronized threadName = A, i = 100

otherMethod方法异步执行了。
结论如下：
不同的方法被synchronized修饰会产生互斥，synchronized锁住的是对象；
synchronized方法块和synchronized方法效果一致；

synchronized(非this对象)

ThreadDomain22类

public class ThreadDomain22 {
    private String userNameParam;
    private String passwordParam;
    private String anyString = new String();

    public void setUserNamePassword(String userName, String password)
    {
        try
        {
            synchronized (anyString)
            {
                System.out.println("线程名称为：" + Thread.currentThread().getName() +
                        "在 " + new Date() + " 进入同步代码块");
                userNameParam = userName;
                Thread.sleep(3000);
                passwordParam = password;
                System.out.println("线程名称为：" + Thread.currentThread().getName() +
                        "在 " + new Date() + " 离开同步代码块");
            }
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyThread22_0类

public class MyThread22_0 extends Thread{
    private ThreadDomain22 td;

    public MyThread22_0(ThreadDomain22 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.setUserNamePassword("A", "AA");
    }
}

MyThread22_1类

public class MyThread22_1 extends Thread{
    private ThreadDomain22 td;

    public MyThread22_1(ThreadDomain22 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.setUserNamePassword("B", "B");
    }
}

main

public class MyThread22_main{
    public static void main(String[] args)
    {
        ThreadDomain22 td = new ThreadDomain22();
        MyThread22_0 mt0 = new MyThread22_0(td);
        MyThread22_1 mt1 = new MyThread22_1(td);
        mt0.start();
        mt1.start();
    }
}

输出结果

线程名称为：Thread-0在 Tue Jul 02 14:51:16 CST 2019 进入同步代码块
线程名称为：Thread-0在 Tue Jul 02 14:51:19 CST 2019 离开同步代码块
线程名称为：Thread-1在 Tue Jul 02 14:51:19 CST 2019 进入同步代码块
线程名称为：Thread-1在 Tue Jul 02 14:51:22 CST 2019 离开同步代码块

synchronized(anyString)和synchronized(this)效果一致，anyString作为全局变量是一个对象，如果放入try里面，就变成了多个对象，变成了异步执行。

再看一个例子
ThreadDomain23类

public class ThreadDomain23 {
    public void testMethod1(MyObject mo)
    {
        try
        {
            synchronized (mo)
            {
                System.out.println("testMethod1__getLock time = " +
                        new Date() + ", run ThreadName = " +
                        Thread.currentThread().getName());
                Thread.sleep(5000);
                System.out.println("testMethod1__releaseLock time = " +
                        new Date() + ", run ThreadName = " +
                        Thread.currentThread().getName());
            }
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyObject类

public class MyObject {
    public synchronized void speedPrintString()
    {
        System.out.println("speedPrintString__getLock time = " +
                new Date() + ", run ThreadName = " +
                Thread.currentThread().getName());
        System.out.println("speedPrintString__releaseLock time = " +
                new Date() + ", run ThreadName = " +
                Thread.currentThread().getName());
    }
}

MyThread23_0类

public class MyThread23_0 extends Thread{
    private ThreadDomain23 td;
    private MyObject mo;

    public MyThread23_0(ThreadDomain23 td, MyObject mo)
    {
        this.td = td;
        this.mo = mo;
    }

    public void run()
    {
        td.testMethod1(mo);
    }
}

MyThread23_1类

public class MyThread23_1 extends Thread {
    private MyObject mo;

    public MyThread23_1(MyObject mo)
    {
        this.mo = mo;
    }

    public void run()
    {
        mo.speedPrintString();
    }
}

main方法

public class MyThread23_main {
    public static void main(String[] args)
    {
        ThreadDomain23 td = new ThreadDomain23();
        MyObject mo = new MyObject();
        MyThread23_0 mt0 = new MyThread23_0(td, mo);
        MyThread23_1 mt1 = new MyThread23_1(mo);
        mt0.start();
        mt1.start();
    }
}

输出结果如下

testMethod1__getLock time = Tue Jul 02 15:23:05 CST 2019, run ThreadName = Thread-0
testMethod1__releaseLock time = Tue Jul 02 15:23:10 CST 2019, run ThreadName = Thread-0
speedPrintString__getLock time = Tue Jul 02 15:23:10 CST 2019, run ThreadName = Thread-1
speedPrintString__releaseLock time = Tue Jul 02 15:23:10 CST 2019, run ThreadName = Thread-1

speedPrintString方法总是在testMethod1方法之后执行，说明ThreadDomain23获得了锁，mo对象中的syncronized方法被阻塞，执行完testMethod1方法才会执行speedPrintString方法。
即当一个线程调用该方法时，其他线程阻塞。
结论：
1.当多个线程同时执行synchronized(非this对象){}同步代码块时呈同步效果
2.当其他线程执行非this对象中的synchronized同步方法时呈同步效果

synchronized静态方法

synchronized static可以对当前的类加锁，即类锁。
ThreadDomain24类

public class ThreadDomain24 {
    public synchronized static void printA()
    {
        try
        {
            System.out.println("线程名称为：" + Thread.currentThread().getName() +
                    "在" + System.currentTimeMillis() + "进入printA()方法");
            Thread.sleep(3000);
            System.out.println("线程名称为：" + Thread.currentThread().getName() +
                    "在" + System.currentTimeMillis() + "离开printA()方法");
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }

    public synchronized static void printB()
    {
        System.out.println("线程名称为：" + Thread.currentThread().getName() +
                "在" + System.currentTimeMillis() + "进入printB()方法");
        System.out.println("线程名称为：" + Thread.currentThread().getName() +
                "在" + System.currentTimeMillis() + "离开printB()方法");

    }

    public synchronized void printC()
    {
        System.out.println("线程名称为：" + Thread.currentThread().getName() +
                "在" + System.currentTimeMillis() + "进入printC()方法");
        System.out.println("线程名称为：" + Thread.currentThread().getName() +
                "在" + System.currentTimeMillis() + "离开printC()方法");
    }
}

MyThread24_0类，静态方法

public class MyThread24_0 extends Thread{
    public void run()
    {
        ThreadDomain24.printA();
    }
}

MyThread24_1类，静态方法

public class MyThread24_1 extends Thread {
    public void run()
    {
        ThreadDomain24.printB();
    }
}

MyThread24_2类，实例化调用方法

public class MyThread24_2 extends Thread{
    private ThreadDomain24 td;

    public MyThread24_2(ThreadDomain24 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.printC();
    }
}

main方法

public class MyThread24_main {
    public static void main(String[] args)
    {
        ThreadDomain24 td = new ThreadDomain24();
        MyThread24_0 mt0 = new MyThread24_0();
        MyThread24_1 mt1 = new MyThread24_1();
        MyThread24_2 mt2 = new MyThread24_2(td);
        mt0.start();
        mt1.start();
        mt2.start();
    }
}

输出结果如下

线程名称为：Thread-0在1562057924755进入printA()方法
线程名称为：Thread-2在1562057924755进入printC()方法
线程名称为：Thread-2在1562057924755离开printC()方法
线程名称为：Thread-0在1562057927755离开printA()方法
线程名称为：Thread-1在1562057927755进入printB()方法
线程名称为：Thread-1在1562057927755离开printB()方法

可以看到，printA方法（类锁）和printB方法（类锁）同步执行，printC方法（对象锁）异步执行，printA方法和printB方法互斥，printC不受类锁影响。
结论：
静态方法持有类锁，非静态方法持有对象锁，两者互不干扰。

查看全文

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原文地址：https://www.cnblogs.com/Java-Starter/p/11088977.html

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