線程執(zhí)行順序

我們先來看一個簡單的例子，我們顯示地創(chuàng)建三個線程t1、t2执虹、t3，按照聲明的順序聪姿，依次分別調(diào)用線程的start方法，線程的執(zhí)行順序是怎樣的乙嘀？程序執(zhí)行的結(jié)果會如何末购？：

public class ThreadOrderDemo {
    // 顯示地創(chuàng)建三個線程，觀察執(zhí)行順序
    static Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("This is thread1");
        }
    });
    static Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("This is thread2");
        }
    });
    static Thread t3 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("This is thread3");
        }
    });
    public static void main(String[] args) throws Exception {
        t1.start();
        t2.start();
        t3.start();
    }
}

猜測程序執(zhí)行結(jié)果：
1.按照代碼順序執(zhí)行虎谢，依次為t1盟榴、t2、t3：

This is thread1
This is thread2
This is thread3

2.按照CPU調(diào)度規(guī)則婴噩，順序隨機：

This is thread1
This is thread3
This is thread2

我們經(jīng)過編輯器執(zhí)行擎场，會發(fā)現(xiàn)結(jié)果是2，按照CPU調(diào)度几莽，執(zhí)行順序隨機迅办。
那我們?nèi)绾巫尵€程有序執(zhí)行呢？以下有兩種方式章蚣。

方式一：使用join讓主線程等待

public static void main(String[] args) throws Exception {
        // 方式一：使用join讓主線程等待
        t1.start();
        t1.join();

        t2.start();
        t2.join();
        
        t3.start();
    }

上述改寫主線程main方法站欺，使用Thread類提供的join方法，可以達到讓線程有序執(zhí)行的效果纤垂。

public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

查看Thread類源碼矾策，可知，此處join方法峭沦，實質(zhì)上是通過調(diào)用Object的wait方法贾虽，讓主線程等待子線程執(zhí)行，子線程執(zhí)行完之后熙侍，主線程再執(zhí)行榄鉴。

方式二：使用線程池隊列

public class ThreadOrderDemo {
    // 顯示地創(chuàng)建三個線程履磨，觀察執(zhí)行順序
    static Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("This is thread1");
        }
    });
    static Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("This is thread2");
        }
    });
    static Thread t3 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("This is thread3");
        }
    });
    static ExecutorService executor = Executors.newSingleThreadExecutor();
  
    public static void main(String[] args) throws Exception {
        // 方式二：使用線程池隊列
       executor.submit(t1);
       executor.submit(t2);
       executor.submit(t3);
    }
}

上述使用J.U.C（java.util.concurrent）包中的ExecutorService建立線程池隊列蛉抓，以保持線程有序執(zhí)行。

public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>()));
    }

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    }

上述Executors.newSingleThreadExecutor()實質(zhì)是LinkedBlockingQueue剃诅，創(chuàng)建了一個阻塞的安全線程隊列巷送，F(xiàn)IFO先進先出，所以可以達到線程有序執(zhí)行的目的矛辕。

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