java - 以下代码中的同步有什么问题

Question

同步

我已经声明了一个类b，它有一个在类c中访问的同步方法:

class b {
    String msg;

    public synchronized void foo() {
        System.out.print("[" + msg);
        try {
            Thread.sleep(1000); // Threads go to sleeep

        } catch (InterruptedException e) {
            System.out.println("Caught" + e);
        }
        System.out.println("]");
    }
}

class a implements Runnable {
    b ob;

    Thread t;

    a(String msg, b obb) {
        ob = obb;
        ob.msg = msg;
        t = new Thread(this); // creating a thread
        t.start();
    }

    public void run() {
        ob.foo(); // calling method of class b
    }

    public static void main(String... a) {
        b obb = new b();
        a ob = new a("Hello", obb); /* PASSING */
        a ob1 = new a("Synch", obb); /* THE */
        a ob2 = new a("World", obb);/* MESSAGE */

        try {
            ob.t.join();
            ob1.t.join();
            ob2.t.join();
        } catch (InterruptedException e) {
            System.out.println("Caught" + e);
        }
    }
}

我期待输出:

[Hello]
[Synch]
[World]

但是代码给出:

[World]
[World]
[World]

帮我提一些建议。我是一个天真的JAVA用户。

Answer 1

使用以下代码获得预期的答案。

b类{ //字符串消息；

public void foo(String msg) {
    System.out.print("[" + msg);
    try {
        Thread.sleep(1000); // Threads go to sleeep

    } catch (InterruptedException e) {
        System.out.println("Caught" + e);
    }
    System.out.println("]");
}

}

公共(public)类 Threading 实现 Runnable {

    b ob;
String msg;
Thread t;

Threading(String message, b obb) {
    ob = obb;
    msg = message;
    t = new Thread(this); // creating a thread
    t.start();
}

public void run() {
    synchronized (ob) {
        ob.foo(msg); // calling method of class b
    }

}

public static void main(String... a) {
    b obb = new b();
    Threading ob = new Threading("Hello", obb); /* PASSING */
    Threading ob2 = new Threading("World", obb); /* THE */
    Threading ob1 = new Threading("Synch", obb);/* MESSAGE */

    try {
        ob.t.join();
        ob1.t.join();
        ob2.t.join();
    } catch (InterruptedException e) {
        System.out.println("Caught" + e);
    }
}

}