为什么synchronized方法不适用于多线程

Question

我写了一个程序,其中两个线程正在运行,这就是我使用synchronized关键字的原因.我的代码如下 -

public class TestThreadAnother {
    public synchronized void decrement(){
        try {
            for (int i = 4; i > 0; i--) {
                System.out.println("Thread " + i);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}


public class TestThreadClass extends Thread {
    private String threadName;
    TestThreadAnother obj1;

    public TestThreadClass(String threadName, TestThreadAnother Obj1) {
        this.threadName = threadName;
        obj1 = Obj1;
        System.out.println("Creating " + threadName);
    }

    @Override
    public void run() {
        System.out.println("Running " + threadName);
        obj1.decrement();
        System.out.println("End of " + threadName);
    }    
}

public class TestThreadMain {

    public static void main(String[] args) {
        TestThreadAnother obj1 = new TestThreadAnother();

        TestThreadClass t1 = new TestThreadClass("Thread 1", obj1);
        TestThreadClass t2 = new TestThreadClass("Thread 2", obj1);

        t1.start();
        t2.start();
    }
}

输出应该是 -

Creating Thread 1
Creating Thread 2
Running Thread 1
Thread 4
Thread 3
Thread 2
Thread 1
End of Thread 1
Running Thread 2
Thread 4
Thread 3
Thread 2
Thread 1
End of Thread 2

但我得到以下结果 -

Creating Thread 1
Creating Thread 2
Running Thread 2
Thread 4
Thread 3
Running Thread 1
Thread 2
Thread 1
Thread 4
Thread 3
Thread 2
Thread 1
End of Thread 1
End of Thread 2

但我的问题是,因为我使用同步方法然后为什么会发生这种情况？因为知道我是否使用synchronized,所以在第一个线程将执行然后第二个将执行.但在我的情况下,有时这不会发生.Java专家需要您的帮助来解决这个问题.

Answer 1

输出是正确的.只有你的decrement()方法是同步的.这意味着decrement()一次不能通过多个线程调用此对象的方法.但这并不妨碍其他线程在执行一个线程时执行其他操作decrement().

您的输出显示,虽然线程2正在执行decrement(),线程1是执行第一System.out.println()中run()方法中,然后可能等待线程2已经完成了其到呼叫decrement(),然后执行decrement().

Answer 2

你必须要了解的重要事情是.Synchronization只保证两个线程不会同时锁定同一个对象.在您的情况下,一个线程(thread2或thread1)将获得锁定并将调用decrement()方法.直到此线程结束,其他threda将被阻止.

现在,分析你的输出:

Creating Thread 1
Creating Thread 2
Running Thread 2 // Thread -2 has the lock now.
Thread 4         // thrad-2's decrement()
Thread 3
Running Thread 1  // thread-1 is blocked. So, only this line is printed
Thread 2         // still thread2
Thread 1      // still thread2
Thread 4        // now thread1 decrement() starts
Thread 3
Thread 2
Thread 1
End of Thread 1
End of Thread 2

打印thread.getName()你的减量方法,以获得更好的主意.