线程同步synchronized，Class与Object

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