synchronized (class)：class类的同步，同步的时候会同步整个class

与

synchronized (Object)：Object的同步，只对其中的对象同步

如下：对类B中的同步代码块的同步，对比之后放可明白

synchronized (MyThread.class)

例：

A：类UnsafeSequence

public class UnsafeSequence {

    private static int value;

    

    public int getValue()

    {

        synchronized (UnsafeSequence.class) {

            return value++;

        }

    }

    

    public int getValueA()

    {

        synchronized (this) {

            return value++;

        }

    }

}

B：类MyThread

public class MyThread implements Runnable {

    private UnsafeSequence unsafe;

    private static int value;

    

    @Override

    public void run() {

        unsafe = new UnsafeSequence();

        for(int i = 0; i < 60; i++)

        {

            synchronized (MyThread.class) {

                System.out.println(Thread.currentThread().getName() + "----in:");

                System.out.println(Thread.currentThread().getName() + " thread run i = " + i + ";value = " + unsafe.getValue());

                System.out.println(Thread.currentThread().getName() + "----out:" + value++);

            }

        }

    }

}

C：主函数

public class TestMain {

    public static void main(String[] args) {

        MyThread target_1 = new MyThread();

        MyThread target_2 = new MyThread();

        Thread thread_1 = new Thread(target_1, "A");

        Thread thread_2 = new Thread(target_2, "B");

        thread_1.start();

        thread_2.start();

    }

}

 

结果如下：

B----in:

B thread run i = 0;value = 0

B----out:0

B----in:

B thread run i = 1;value = 1

B----out:1

B----in:

B thread run i = 2;value = 2

B----out:2

 

去掉B中的同步，或换为Object,结果如下：

A----in:

B----in:

A thread run i = 0;value = 0

B thread run i = 0;value = 1

A----out:0

B----out:1

A----in:

B----in:

A thread run i = 1;value = 2

B thread run i = 1;value = 3

B----out:3

B----in:

A----out:2

 

控制同步：

public class MyThread implements Runnable {

    private UnsafeSequence unsafe;

    private static int value;

    

    @Override

    public void run() {

        synchronized (MyThread.class) {

            unsafe = new UnsafeSequence();

            for(int i = 0; i < 60; i++)

            {

                System.out.println(Thread.currentThread().getName() + "----in:");

                System.out.println(Thread.currentThread().getName() + " thread run i = " + i + ";value = " + unsafe.getValue());

                System.out.println(Thread.currentThread().getName() + "----out:" + value++);

            }

        }

    }

}