按序打印

按序打印

一开始这样写的，错了，大家知道为什么吗？

class Foo {

    private int volatile n = 0;
    public Foo() {

    }

    public void first(Runnable printFirst) throws InterruptedException {

        synchronized (this){
            if(n % 3 == 0){
                // printFirst.run() outputs "first". Do not change or remove this line.
                printFirst.run();
                n += 1;
                notifyAll();
            }else {
                wait();
            }
        }

    }

    public void second(Runnable printSecond) throws InterruptedException {

        synchronized (this) {
            if (n % 3 == 1) {
                // printSecond.run() outputs "second". Do not change or remove this line.
                printSecond.run();
                n += 1;
                notifyAll();
            } else {
                wait();
            }
        }
    }

    public void third(Runnable printThird) throws InterruptedException {

        synchronized (this) {
            if (n % 3 == 2) {
                // printThird.run() outputs "third". Do not change or remove this line.
                printThird.run();
                n += 1;
                notifyAll();
            } else {
                wait();
            }
        }
    }
}

 其实错就错在：

else {
       wait();
            }

这里了，因为如果second和third先执行，然后first执行，导致只能输出first，不能输出second和third，因为wait被notify后，不会再次执行函数体。

改成下面的代码就ok了：

package 多线程;

class Foo {

    private volatile  int n = 0;
    public Foo() {

    }

    public void first(Runnable printFirst) throws InterruptedException {

        synchronized (this){
            while(n % 3 != 0){
                wait();
            }
            if(n % 3 == 0){
                // printFirst.run() outputs "first". Do not change or remove this line.
                printFirst.run();
                n += 1;
                notifyAll();
            }
        }

    }

    public void second(Runnable printSecond) throws InterruptedException {

        synchronized (this) {

            while(n % 3 != 1){
                wait();
            }
            if (n % 3 == 1) {
                // printSecond.run() outputs "second". Do not change or remove this line.
                printSecond.run();
                n += 1;
                notifyAll();
            }
        }
    }

    public void third(Runnable printThird) throws InterruptedException {

        synchronized (this) {

            while(n % 3 != 2){
                wait();
            }
            if (n % 3 == 2) {
                // printThird.run() outputs "third". Do not change or remove this line.
                printThird.run();
                n += 1;
                notifyAll();
            }
        }
    }

    public static void main(String[] args) {
        Foo foo = new Foo();

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    foo.first();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    foo.second();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    foo.third();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }
}

这里能够保证，拿到锁，继续执行下面的方法。

 while(n % 3 != 1){
                wait();
            }