java线程同步问题

Question

想提出一些关于线程同步的疑问。我理解了以下概念同步。但是当我通过多线程实现一个示例java程序时，我无法达到我想要的结果。但最终我通过更改一行代码就达到了结果。但我想知道下面的代码有什么问题。

在这里，当我使用同步块(synchronized block)，synchronized(SharedResource.class)时，我可以达到我想要的结果。为什么它不适用于同步方法和同步(this) block ？

public class SharedResource {   
    public  synchronized void access(String name){      

            System.out.println(name+" :accessed shared resoure");
            System.out.println(name+" doing his job: ");
            for(int i = 0; i < 5;i++){
                try {
                    System.out.println(name+": "+ i);
                    Thread.sleep(500);
                } catch (InterruptedException e) {              
                    e.printStackTrace();
                }
            }
            System.out.println(name+" :finished doing his job..");

        }
}


public class SharedAccessThread implements Runnable {

    private String name ;
    public SharedAccessThread(String name) {
            this.name = name ;
    }
    @Override
    public void run() {
        SharedResource resource = new SharedResource();
        resource.access(Thread.currentThread().getName());

    }

}



public class MultiThreading {

    public static void main(String[] args) {

        SharedAccessThread thread = new SharedAccessThread(Thread.currentThread().getName());

        Thread t1 = new Thread(thread);
        t1.setName("A");
        Thread t2 = new Thread(thread);
        t2.setName("b");
        Thread t3 = new Thread(thread);
        t3.setName("C");

        t1.start();
        t2.start();
        t3.start();

    }

}

   The expected out put is below:

A :accessed shared resoure
A doing his job: 
A: 0
A: 1
A: 2
A: 3
A: 4
A :finished doing his job..
C :accessed shared resoure
C doing his job: 
C: 0
C: 1
C: 2
C: 3
C: 4
C :finished doing his job..
b :accessed shared resoure
b doing his job: 
b: 0
b: 1
b: 2
b: 3
b: 4
b :finished doing his job..

但它以一种困惑的方式给出。这意味着 A 访问了共享资源..B 正在做他的工作...等我不想要的。

Answer 1

问题是您正在每个线程内创建一个新的 SharedResource:

@Override
public void run() {
    SharedResource resource = new SharedResource();
    resource.access(Thread.currentThread().getName());

}

因此，同步 this 或方法本身(与同步 this 相同)会导致每个线程看到不同的锁，因此它没有影响。

在类本身上进行同步是有效的，因为它充当全局锁，因为类本身对于所有线程都是相同的。

为了能够在 this 上同步，您应该使资源成为静态成员，例如:

public class SharedAccessThread implements Runnable {

    private String name;
    private static SharedResource resource = new SharedResource();

    public SharedAccessThread(String name) {
            this.name = name ;
    }
    @Override
    public void run() {          
        resource.access(Thread.currentThread().getName());   
    }    
}

这保证了所有线程都能看到相同的 SharedResource 实例。