如何确保三个线程顺序执行

于 2021-07-01 16:47:24 发布
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分类专栏： Thread 文章标签： java
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三个线程t1、t2、t3。确保三个线程，t1 执行完后 t2 执行，t2 执行完后 t3 执行。

一、使用 CountDownLatch

二、使用 join

thread.join 把指定的线程加入到当前线程，可以将两个交替执行的线程合并为顺序执行的线程。比如在线程 A 中调用了线程 B 的 join()，直到线程 B 执行完毕后，才会继续执行线程 A。

public class ThreadTest1 {
	// T1、T2、T3三个线程顺序执行
	public static void main(String[] args) {
		Thread t1 = new Thread(new Work(null));
		Thread t2 = new Thread(new Work(t1));
		Thread t3 = new Thread(new Work(t2));
		t1.start();
		t2.start();
		t3.start();
	}

	static class Work implements Runnable {
		private Thread beforeThread;
		public Work(Thread beforeThread) {
			this.beforeThread = beforeThread;
		}
		public void run() {
			if (beforeThread != null) {
				try {
					beforeThread.join();
					System.out.println("thread start:" + Thread.currentThread().getName());
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			} else {
				System.out.println("thread start:" + Thread.currentThread().getName());
			}
		}
	}
}

三、CachedThreadPool

FutureTask 一个可取消的异步计算。FutureTask 实现了 Future 的基本方法，提空 start cancel 操作，可以查询计算是否已经完成，并且可以获取计算的结果。结果只可以在计算完成之后获取， get 方法会阻塞当计算没有完成的时候，一旦计算已经完成，那么计算就不能再次启动或是取消。

一个 FutureTask 可以用来包装一个 Callable 或是一个 runnable 对象。因为 FurtureTask 实现了 Runnable 方法，所以一个 FutureTask 可以提交(submit)给一个 Excutor 执行(excution)。

import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;

public class ThreadTest3 {

	// T1、T2、T3三个线程顺序执行
	public static void main(String[] args) {
		FutureTask<Integer> future1 = new FutureTask<Integer>(new Work(null));
		Thread t1 = new Thread(future1);

		FutureTask<Integer> future2 = new FutureTask<Integer>(new Work(future1));
		Thread t2 = new Thread(future2);

		FutureTask<Integer> future3 = new FutureTask<Integer>(new Work(future2));
		Thread t3 = new Thread(future3);

		t1.start();
		t2.start();
		t3.start();
	}

	static class Work implements Callable<Integer> {
		private FutureTask<Integer> beforeFutureTask;

		public Work(FutureTask<Integer> beforeFutureTask) {
			this.beforeFutureTask = beforeFutureTask;
		}

		public Integer call() throws Exception {
			if (beforeFutureTask != null) {
				Integer result = beforeFutureTask.get();//阻塞等待
				System.out.println("thread start:" + Thread.currentThread().getName());
			} else {
				System.out.println("thread start:" + Thread.currentThread().getName());
			}
			return 0;
		}
	}
}

四、使用阻塞队列(BlockingQueue)

阻塞队列(BlockingQueue)是java util.concurrent包下重要的数据结构。BlockingQueue 提供了线程安全的队列访问方式：当阻塞队列进行插入数据时，如果队列已满，线程将会阻塞等待直到队列非满；从阻塞队列取数据时，如果队列已空，线程将会阻塞等待直到队列非空。并发包下很多高级同步类的实现都是基于 BlockingQueue 实现的。

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;

public class ThreadTest4 {
	
	// T1、T2、T3三个线程顺序执行
	public static void main(String[] args) {
		//blockingQueue保证顺序
		BlockingQueue<Thread> blockingQueue = new LinkedBlockingQueue<Thread>();
		Thread t1 = new Thread(new Work());
		Thread t2 = new Thread(new Work());
		Thread t3 = new Thread(new Work());

		blockingQueue.add(t1);
		blockingQueue.add(t2);
		blockingQueue.add(t3);

		for (int i = 0; i < 3; i++) {
			Thread t = null;
			try {
				t = blockingQueue.take();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			t.start();
			//检测线程是否还活着
			while (t.isAlive()) ;
		}
	}

	static class Work implements Runnable {
		public void run() {
			System.out.println("thread start:" + Thread.currentThread().getName());
		}
	}
}

五、使用单个线程池

newSingleThreadExecutor 返回一个包含单线程的 Executor，将多个任务交给此 Executor 时，这个线程处理完一个任务后接着处理下一个任务，若该线程出现异常，将会有一个新的线程来替代。

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ThreadTest5 {

	public static void main(String[] args) {
		final Thread t1 = new Thread(new Runnable() {
			public void run() {
				System.out.println(Thread.currentThread().getName() + " run 1");
			}
		}, "T1");
		final Thread t2 = new Thread(new Runnable() {
			public void run() {
				System.out.println(Thread.currentThread().getName() + " run 2");
				try {
					t1.join(10);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
		}, "T2");
		final Thread t3 = new Thread(new Runnable() {
			public void run() {
				System.out.println(Thread.currentThread().getName() + " run 3");
				try {
					t2.join(10);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
		}, "T3");

		//使用 单个任务的线程池来实现。保证线程的依次执行
		ExecutorService executor = Executors.newSingleThreadExecutor();
		executor.submit(t1);
		executor.submit(t2);
		executor.submit(t3);
		executor.shutdown();
	}
}

三个线程轮流打印1-100

六、synchronized关键字实现

public class MyThread1 implements Runnable {

    private static Object lock = new Object();
    private static int count;
    private int no;

    public MyThread1(int no) {
        this.no = no;
    }

    @Override
    public void run() {
        while (true) {
            synchronized (lock) {
                if (count > 100) {
                    break;
                }
                if (count % 3 == this.no) {
                    System.out.println(Thread.currentThread().getName() + "\t" + this.no + "\t" + count);
                    count++;
                } else {
                    try {
                        lock.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                lock.notifyAll();
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(new MyThread1(0), "A");
        Thread t2 = new Thread(new MyThread1(1), "B");
        Thread t3 = new Thread(new MyThread1(2), "C");
        t1.start();
        t2.start();
        t3.start();
        t1.join();
        t2.join();
        t3.join();
    }
}

七、ReentrantLock实现一

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class MyThread2 implements Runnable {

    private static ReentrantLock lock = new ReentrantLock();
    private static Condition condition = lock.newCondition();
    private static int count;
    private int no;

    public MyThread2(int no) {
        this.no = no;
    }

    @Override
    public void run() {
        while (true) {
            lock.lock();
            if (count > 100) {
                break;
            } else {
                if (count % 3 == this.no) {
                    System.out.println(this.no + "-->" + count);
                    count++;
                } else {
                    try {
                        condition.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
            condition.signalAll();
            lock.unlock();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(new MyThread2(0));
        Thread t2 = new Thread(new MyThread2(1));
        Thread t3 = new Thread(new MyThread2(2));
        t1.start();
        t2.start();
        t3.start();
        t1.join();
        t2.join();
        t3.join();
    }
}

八、ReentrantLock 实现二

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Testmain {
    public static void main(String[] args) {

        Alternate an = new Alternate();
        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 1; i <= 33; i++) {
                    an.logA(i);
                }
            }
        }, "A").start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 1; i <= 33; i++) {
                    an.logB(i);
                }
            }
        }, "B").start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 1; i <= 33; i++) {
                    an.logC(i);
                    System.out.println("--------------------------------");
                }
            }
        }, "C").start();
    }
}

class Alternate {

    private static int num = 1;
    private int tempA = 0;
    private int tempB = 0;
    private int tempC = 0;
    Lock lock = new ReentrantLock();
    private Condition condition1 = lock.newCondition();
    private Condition condition2 = lock.newCondition();
    private Condition condition3 = lock.newCondition();

    public void logA(int total) {
        lock.lock();
        try {
            if (num != 1 && num != (tempA + 3)) {
                condition1.await();
            }
            System.out.println(Thread.currentThread().getName() + "\t" + num + "\t" + total);
            tempA = num;
            num++;
            condition2.signal();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void logB(int total) {
        lock.lock();
        try {
            if (num != 2 && num != (tempB + 3)) {
                condition2.await();
            }
            System.out.println(Thread.currentThread().getName() + "\t" + num + "\t" + total);
            tempB = num;
            num++;
            condition3.signal();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void logC(int total) {
        lock.lock();
        try {
            if (num != 3 && num != (tempC + 3)) {
                condition3.await();
            }
            System.out.println(Thread.currentThread().getName() + "\t" + num + "\t" + total);
            tempC = num;
            num++;
            condition1.signal();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}
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