4种方法实现多线程按着指定顺序执行

Java 多线程

目录

• 在子线程中通过join()方法指定顺序
• 在主线程中通过join()方法指定顺序
• 通过倒数计时器CountDownLatch实现

• 通过创建单一化线程池newSingleThreadExecutor()实现

  在子线程中通过join()方法指定顺序

  通过join()方法使当前线程“阻塞”，等待指定线程执行完毕后继续执行。
  举例：在线程thread2中，加上一句thread1.join()，其意义在于，当前线程2运行到此行代码时会进入阻塞状态，直到线程thread1执行完毕后，线程thread2才会继续运行，这就保证了线程thread1与线程thread2的运行顺序。

  public class ThreadJoinDemo {
    public static void main(String[] args) throws InterruptedException {
        final Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("打开冰箱！");
            }
        });
        final Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    thread1.join();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("拿出一瓶牛奶！");
            }
        });
        final Thread thread3 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    thread2.join();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("关上冰箱！");
            }
        });
        //下面三行代码顺序可随意调整，程序运行结果不受影响，因为在子线程中通过“join()方法”已经指定了运行顺序。
        thread3.start();
        thread2.start();
        thread1.start();
    }
  }

  运行结果：

  打开冰箱！
  拿出一瓶牛奶！
  关上冰箱！

  在主线程中通过join()方法指定顺序

  简单说一下子线程与主线程的区别，子线程指的是发生在Thread内部的代码，主线程指的是发生在main函数中的代码，可以在main函数中通过join()方法让主线程阻塞等待以达到指定顺序执行的目的。

  public class ThreadMainJoinDemo {
    public static void main(String[] args) throws InterruptedException {
        final Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("打开冰箱！");
            }
        });
        final Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("拿出一瓶牛奶！");
            }
        });
        final Thread thread3 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("关上冰箱！");
            }
        });
        thread1.start();
        thread1.join();
        thread2.start();
        thread2.join();
        thread3.start();
    }
  }

  输出结果：

  打开冰箱！
  拿出一瓶牛奶！
  关上冰箱！

  通过倒数计时器CountDownLatch实现

  CountDownLatch通过计数器提供了更灵活的控制，只要检测到计数器为0当前线程就可以往下执行而不用管相应的thread是否执行完毕。

  public class ThreadCountDownLatchDemo {
    private static CountDownLatch countDownLatch1 = new CountDownLatch(1);
    private static CountDownLatch countDownLatch2 = new CountDownLatch(1);
    public static void main(String[] args) {
        final Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("打开冰箱！");
                countDownLatch1.countDown();
            }
        });
        final Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    countDownLatch1.await();
                    System.out.println("拿出一瓶牛奶！");
                    countDownLatch2.countDown();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        final Thread thread3 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    countDownLatch2.await();
                    System.out.println("关上冰箱！");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        //下面三行代码顺序可随意调整，程序运行结果不受影响
        thread3.start();
        thread1.start();
        thread2.start();
    }
  }

  输出结果：

  打开冰箱！
  拿出一瓶牛奶！
  关上冰箱！

  通过创建单一化线程池newSingleThreadExecutor()实现

  单线程化线程池(newSingleThreadExecutor)的优点，串行执行所有任务。 ```java public class ThreadPoolDemo {

  static ExecutorService executorService = Executors.newSingleThreadExecutor();

  public static void main(String[] args) {

    final Thread thread1 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("打开冰箱！");
        }
    });
    final Thread thread2 =new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("拿出一瓶牛奶！");
        }
    });
    final Thread thread3 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("关上冰箱！");
        }
    });
    executorService.submit(thread1);
    executorService.submit(thread2);
    executorService.submit(thread3);
    executorService.shutdown();        //使用完毕记得关闭线程池

  }

}

输出结果：
```java
打开冰箱！
拿出一瓶牛奶！
关上冰箱！