Java死锁挑衅

Question

我正在用java做一些练习(你们中的一些人可能会猜想代码来自哪里)。我尝试使用以下代码引发死锁情况:

class Resource {

    public Integer value = 42;
}

public class DeadLockRisk implements Runnable {

    private Resource resourceA = new Resource();
    private Resource resourceB = new Resource();

    public void write(int a, int b) {

        System.out.println(Thread.currentThread().getName() + " try write Lock A");

        synchronized(resourceA) {

            System.out.println(Thread.currentThread().getName() + " write Lock A");
            System.out.println(Thread.currentThread().getName() + " try write Lock B");

            synchronized(resourceB) {

                System.out.println(Thread.currentThread().getName() + " write Lock B");

                resourceA.value = a;
                resourceB.value = b;
        //sit on it!
        //try { Thread.sleep(5000); } catch (Exception e) {}

        System.out.println(Thread.currentThread().getName() + " release write B");
        }
        System.out.println(Thread.currentThread().getName() + " release write A");
    }
    }

    public int read() {

    Integer retVal;
        System.out.println(Thread.currentThread().getName() + " try read Lock B");

        synchronized(resourceB) {

            System.out.println(Thread.currentThread().getName() + " read Lock B");
            System.out.println(Thread.currentThread().getName() + " try read Lock A");

            synchronized(resourceA) {

                System.out.println(Thread.currentThread().getName() + " read Lock A");
                retVal = resourceB.value + resourceA.value;
        System.out.println(Thread.currentThread().getName() + " release read A");
            }
        System.out.println(Thread.currentThread().getName() + " release read B");
        }
    return retVal;
    }

    public void run() {


    if (Thread.currentThread().getName().equals("Thread-1")) {
        write(1,2);
    }

    System.out.println(read());
    }

    public static void main(String[] args) {

        Thread ta = new Thread(new DeadLockRisk());
        Thread tb = new Thread(new DeadLockRisk());
        ta.start(); tb.start();
    }
}

(javac -版本 javac 1.6.0_0uname -a Linux inspiron 2.6.29 #1 SMP 5 月 16 日星期六 10:56:17 CEST 2009 i686 GNU/Linux)

并获取(在其他情况下)以下输出:

Thread-1 try write Lock AThread-0 try read Lock BThread-1 write Lock A      <=====Thread-0 read Lock BThread-1 try write Lock BThread-0 try read Lock AThread-1 write Lock BThread-0 read Lock A       <=====Thread-1 release write BThread-0 release read AThread-1 release write AThread-0 release read BThread-1 try read Lock B154Thread-1 read Lock BThread-1 try read Lock AThread-1 read Lock AThread-1 release read AThread-1 release read B3

我看到了什么错误？在线程 1 释放锁之前，线程 0 如何进入 read() 中的临界区并获得 A 上的锁。这可能是因为输出不同步吗？此代码永远不会发生死锁。

Answer 1

您将不同的 DeadLockRisk 实例传递给两个线程，因此它们在同一对象上不会同步。