java - 实例同步

Question

我编写了一小段代码来理解同步 block 的概念:

public class ObjectLevelSynchronized {

    public void run() {
        synchronized(this) {
            try {
                System.out.println(Thread.currentThread().getName());
                Thread.sleep(100);
                System.out.println(Thread.currentThread().getName() + " Finished.");
            }catch(Exception e) {

            }
        }
    }

    public static void main(String[] args) throws Exception {

        final ObjectLevelSynchronized c = new ObjectLevelSynchronized();
        final ObjectLevelSynchronized c1 = new ObjectLevelSynchronized();       

        Thread t = new Thread(new Runnable() {
            public void run() {
                c.run();
                c.run();
            }
        }, "MyThread1");

        Thread t1 = new Thread(new Runnable() {
            public void run() {
                c1.run();
                c1.run();
            }
        }, "MyThread2");

        t.start();
        t1.start();
    }
}

只有一个线程可以在同一监视器对象上同步的 Java 代码块内执行，我从两个不同的线程和两个不同的实例调用 run 方法。

预期结果:

MyThread1
MyThread2
MyThread1 Finished
MyThread2 Finished
MyThread1
MyThread2 
MyThread1 Finished
MyThread2 Finished

实际结果

MyThread1
MyThread1 Finished.
MyThread1
MyThread1 Finished.
MyThread2
MyThread2 Finished.
MyThread2
MyThread2 Finished.

为什么synchronized block 锁定了两个不同对象的代码？

Answer 1

你有两把不同的锁。

你使用synchronized(this)

你创建了两个对象

final ClassLevelSynchronized c = new ClassLevelSynchronized();
final ClassLevelSynchronized c1 = new ClassLevelSynchronized();     

所以，c和c1是你的锁，synchronized看起来是这些锁。您必须使用相同的对象来同步块(synchronized block)。对你来说最简单的方法是

private static Object lock = new Object();

并在同步中使用它

synchronized(lock)

或者，对于您的输出，这也应该有效。

public static void main(String[] args) throws Exception {

        final ObjectLevelSynchronized c = new ObjectLevelSynchronized();
        final ObjectLevelSynchronized c1 = new ObjectLevelSynchronized();       
        final Object lock = new Object();
        Thread t = new Thread(new Runnable() {
            public void run() {
                synchronized (lock) {
                    c.run();
                    c.run();
                }
            }
        }, "MyThread1");

        Thread t1 = new Thread(new Runnable() {
            public void run() {
                synchronized (lock) {
                    c1.run();
                    c1.run();
                }
            }
        }, "MyThread2");

        t.start();
        t1.start();
    }