java - 为什么同步在第二个代码中不起作用？

Question

同步在此代码中正常工作:

    class PrintNumbers {
        synchronized public void display() {
            System.out.println("in display");
            for (int i = 0; i < 3; i++) {
                System.out.println("Thread name : "+ Thread.currentThread().getName() + " i= " + i);
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.getMessage();
                }
            }
            System.out.println("out of display");
        }
    }

    class MyThread implements Runnable {
        Thread t;
        PrintNumbers printNumbers;

        MyThread(PrintNumbers printNumbers, String s) {
            this.printNumbers = printNumbers;
            t = new Thread(this,s);
            t.start();
        }

        public void run() {
            printNumbers.display();
        }
    }

    class SyncExample {
        public static void main(String[] args) {
            PrintNumbers printNumbers = new PrintNumbers();

            new MyThread(printNumbers, "My Thread 1");
            new MyThread(printNumbers, "My Thread 2");
        }
    }

输出:

in display  
Thread name : My Thread 1 i= 0  
Thread name : My Thread 1 i= 1  
Thread name : My Thread 1 i= 2  
out of display
in display  
Thread name : My Thread 2 i= 0  
Thread name : My Thread 2 i= 1  
Thread name : My Thread 2 i= 2  
out of display

但不在这段代码中:

    class PrintNumbers {
        synchronized public void display() {
            System.out.println("in display");
            for (int i = 0; i < 3; i++) {
                System.out.println("Thread name : "+ Thread.currentThread().getName() + " i= " + i);
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.getMessage();
                }
            }
            System.out.println("out of display");
        }
    }

    class MyThread implements Runnable {
        Thread t;
        PrintNumbers printNumbers;

        MyThread(String s) {
            this.printNumbers = new PrintNumbers();
            t = new Thread(this,s);
            t.start();
        }

        public void run() {
            printNumbers.display();
        }
    }

    class SyncExample {
        public static void main(String[] args) {
            new MyThread("My Thread 1");
            new MyThread("My Thread 2");
        }
    }

输出:

in display  
Thread name : My Thread 1 i= 0  
in display  
Thread name : My Thread 2 i= 0  
Thread name : My Thread 1 i= 1  
Thread name : My Thread 2 i= 1  
Thread name : My Thread 2 i= 2  
Thread name : My Thread 1 i= 2  
out of display  
out of display  

我无法理解在 Runnable MyThread 和 SyncExample 类中初始化 PrintNumbers 与 Synchronization 有什么区别。请解释。

Answer 1

I cannot understand what difference wrt Synchronization does it make to initialize PrintNumbers in the Runnable MyThread and in the SyncExample class.

事实并非如此。 重要的是，在您的第一个示例中，您只有两个线程共享的 one PrintNumbers 实例。但在第二个示例中，您有两个单独的 PrintNumbers 实例，每个线程一个。

由于 PrintNumbers#display 在实例上同步(synchronized 实例方法在 this 上同步)，它只在实例内同步，而不是跨实例同步多个实例。

当两个线程共享一个实例时，对 display 的两次调用被序列化。但是当每个线程都有自己的实例时，对 display 的两次调用是在不同的实例上进行的，因此调用没有序列化，它们可以重叠。