我使用2个线程最多打印10个奇数的实现有什么问题

Question

I am trying to print even and odd numbers using two different threads. Can some one point out the mistake I am making. Currently this is what the output looks like:

Odd: 1

going to wait 1

notify 2

going to wait 2

Please find the code below:

 public class EvenOdd {
        public static void main(String[] args) {        
            PrintEvenOdd p1=new PrintEvenOdd(false);
            PrintEvenOdd p2=new PrintEvenOdd(true);
            p1.start();
            p2.start();         
        }   
   }    
class PrintEvenOdd extends Thread{    
    boolean isEven;
    boolean isOdd=true;

    public PrintEvenOdd(boolean e) {
        isEven=e;
    }

    public synchronized void run() {
            if(isEven) {
            for(int i=2;i<=10;i+=2)
            {
                while(isOdd) {
                    try {
                        System.out.println("going to wait 1");
                        wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                System.out.println("Even: "+i);
                isOdd=true;
                System.out.println("notify 1");
                notifyAll();

            }
            }
            if(!isEven) {
            for(int i=1;i<=10;i+=2) {
                while(!isOdd) {
                    try {
                        System.out.println("going to wait 2");
                        wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                System.out.println("Odd: "+ i);
                isOdd=false;
                System.out.println("notify 2");
                notifyAll();
            }
        }
    }
}

Answer 1

As the main idea of your algorithm is not definitely clear for me, I'll just point out where is the bug & you can try to fix it by yourself.

When you start your threads one after another: p1.start(); p2.start(); p1.start(); p2.start(); they are executing in the same order as they we started.

When the first thread calls synchronized void run() method, than it locks the monitor on PrintEvenOdd.class . That means the second thread will wait until the first releases the monitor (say will not execute any single line of code).

Going further in debugger by the first thread execution flow you'll see operations below:

• drop down into if(!isEven) condition
• set isOdd=false during i=1 loop iteration
• goes to endless wait() during i=2 loop iteration

As wait operation does not release the class monitor you're getting situation when both threads are suspended.