百度Android面试经历之串行执行三个线程

接着上一篇内容，这次面试主要围绕多线程进行提问。接下来让我编程：串行执行三个线程，任务是每一个线程将i加1。我当时使用的是join()方法，但是由于对join()方法理解的不透彻。对自己的代码信心不大。我在这再写一遍：

public class SerialDemo {
    public static void main(String[] args) throws InterruptedException {
        System.out.println("main thread start");
        SerialThread serialThread_1 = new SerialThread("thread_1");
        SerialThread serialThread_2 = new SerialThread("thread_2");
        SerialThread serialThread_3 = new SerialThread("thread_3");
        serialThread_1.start();
        serialThread_1.join();
        serialThread_2.start();
        serialThread_2.join();
        serialThread_3.start();
        serialThread_3.join();
        System.out.println("main thread end");

    }

}
class SerialThread extends Thread{
    private String name;
    public SerialThread(String name){
        this.name = name;
    }
    static int i=0;
    @Override
    public void run() {
        System.out.println("SerialThread run start name:"+name+",i="+i);
        i++;
        System.out.println("SerialThread run end"+name+",i="+i);

    }
}

结果：

main thread start
SerialThread run start name:thread_1,i=0
SerialThread run endthread_1,i=1
SerialThread run start name:thread_2,i=1
SerialThread run endthread_2,i=2
SerialThread run start name:thread_3,i=2
SerialThread run endthread_3,i=3
main thread end

thread.join()方法的意义是抢占当前线程的执行时间切片，也就是说阻塞当前线程，运行thread，当thread运行结束后，再运行当前线程。以实现线程的串行执行。在上述代码中，首先运行主线程，接着运行thread_1.start()运行线程1，随后调用thread_1.join()。阻塞主线程，那主线程中的下一句thread_2.start()就不会执行，直到thread_1执行完毕。主线程继续运行调用thread_2.start()，实现线程的串行。其实，这不是完全意义的thread_1 thread_2 thread_3串行执行，而是thread_1执行完回到主线程，接着执行thread_2。也就是说串行执行是由主线程控制并调度。我想这不是面试官考我的真实考点。于是，下面一种解决方案：

public class SerialThreadDemo_2 {
    static int i=0;
    public static void main(String[] args) throws InterruptedException {

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                add();
            }
        });
        Thread t2 = new Thread(new Runnable() {

            @Override
            public void run() {
                try {
                    //引用t1线程，等待t1线程执行完
                    t1.start();
                    t1.join();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                add();
            }
        });
        Thread t3 = new Thread(new Runnable() {

            @Override
            public void run() {
                try {
                    //引用t2线程，等待t2线程执行完,一定要在此处t2.start()
                    //在主线程中调用t2.start()执行顺序不确定，因为若t2.join()先于t2.start()运行
                    //导致结果不确定
                    t2.start();
                    t2.join();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                add();
            }
        });
        t3.start();
//        t2.start();
//        t1.start();
    }
    public static void add(){
        System.out.println("add start in thread name:"+Thread.currentThread().getName()+",i:"+i);
        i++;
        System.out.println("add end in thread name:"+Thread.currentThread().getName()+"i,:"+i);
    }
}

费了些力气才写了出来，说明自己水平还是欠修炼

结果：

add start in thread name:Thread-0,i:0
add end in thread name:Thread-0，i:1
add start in thread name:Thread-1,i:1
add end in thread name:Thread-1，i:2
add start in thread name:Thread-2,i:2
add end in thread name:Thread-2，i:3