[使用两个线程打印序列时出现Java死锁

Question

我尝试使用两个线程来打印奇数和偶数。但是程序进入了僵局。我不明白为什么它进入僵局。在调试模式下，程序的行为有所不同。它先打印1 2然后死锁。此行为是意外的。

预期输出

1
2
3
4
odd thread ends here    
even thread ends here    
main thread ends here

当前输出

1
2
3
4
odd thread ends here
(Deadlock)

这是Java代码

public class PrintSequence {    
    public static void main(String[] args) {
        EvenOddPrinter printer = new EvenOddPrinter(false, 1, 4);
        Thread odd = new Thread(new Runnable() {    
            @Override
            public void run() {
                printer.printOdd();    
            }
        });
        Thread even = new Thread(new Runnable() {
            @Override
            public void run() {
                printer.printEven();
            }
        });
        odd.start();
        even.start();
        try {
            odd.join();
            System.out.println("odd thread ends here");
            even.join();
            System.out.println("even thread ends here");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("main thread ends here");
    }

}

class EvenOddPrinter {

    private boolean isEven;
    private int index;
    private int maxNumber;

    public EvenOddPrinter(boolean isEven, int index, int maxNumber) {
        super();
        this.isEven = isEven;
        this.index = index;
        this.maxNumber = maxNumber;
    }

    public synchronized void printOdd () {
        while(index < maxNumber) {
            if(!isEven) {
                System.out.println(index);
                index++;
                isEven = true;
                notify();
            }
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }   
    public synchronized void printEven() {
        while(index <= maxNumber) {
            if(isEven) {
                System.out.println(index);
                index++;
                isEven = false;
                notify();
            }
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

有人可以帮我解决这个问题吗？

Answer 1

当奇数结束时，它永远不会通知偶数线程。这样做。

    public synchronized void printOdd () {
        while(index < maxNumber) {
            if(!isEven) {
                System.out.println(index);
                index++;
                isEven = true;
                notify();
            }
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        notify(); // add a notify here
    }   

Answer 2

打印最后一个偶数后，偶数线程释放锁定。并且奇数线程退出该方法而不通知偶数线程。因此陷入僵局。

public synchronized void printEven() {
while(index <= maxNumber) {
...
}
notify(); // add notify here
}


public synchronized void printOdd() {
while(index <= maxNumber) {
...
}
notify(); // add notify here
}