[Java多线程]ABC三个线程顺序输出的问题

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原文链接：https://www.cnblogs.com/icejoywoo/archive/2012/10/15/2724674.html

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本文介绍了一种使用Java实现多线程同步的方法，确保三个线程按ABC顺序循环输出10次。通过ReentrantLock和Condition实现了线程间的同步，利用Semaphore简化了同步逻辑。

大概的问题是这样的:

有A,B,C三个线程, A线程输出A, B线程输出B, C线程输出C

要求, 同时启动三个线程, 按顺序输出ABC, 循环10次

这是一个多线程协同的问题, 本身多线程是没有执行顺序的, 顺序不一定, Java在concurrent里面提供了多线程同步的支持

使用ReentrantLock来解决, 还有个state整数用来判断轮到谁执行了

  import java.util.concurrent.locks.Lock;
  import java.util.concurrent.locks.ReentrantLock;
  
  public class ABC {
      private static Lock lock = new ReentrantLock();//通过JDK5中的锁来保证线程的访问的互斥
      private static int state = 0;
      
      static class ThreadA extends Thread {
          @Override
         public void run() {
             for (int i = 0; i < 10;) {
                 lock.lock();
                 if (state % 3 == 0) {
                     System.out.print("A");
                     state++;
                     i++;
                 }
                 lock.unlock();
             }
         }
     }
     
     static class ThreadB extends Thread {
         @Override
         public void run() {
             for (int i = 0; i < 10;) {
                 lock.lock();
                 if (state % 3 == 1) {
                     System.out.print("B");
                     state++;
                     i++;
                 }
                 lock.unlock();
             }
         }
     }
     
     static class ThreadC extends Thread {
         @Override
         public void run() {
             for (int i = 0; i < 10;) {
                 lock.lock();
                 if (state % 3 == 2) {
                     System.out.print("C");
                     state++;
                     i++;
                 }
                 lock.unlock();
             }
         }
     }
     
     public static void main(String[] args) {
         new ThreadA().start();
         new ThreadB().start();
         new ThreadC().start();
     }
     
 }

使用lock来保证只有一个线程在输出操作, 要保证了state不会被两个线程同时修改, 思路简单

还可以使用condition, condition的效率可能会更高一些, await会释放lock锁, condition的await和signal与object的wait和notify方法作用类似

package moreThread;

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

/**
 */
public class ABC2 {

    private static Lock lock = new ReentrantLock();
    private static int count = 0;
    private static Condition A = lock.newCondition();
    private static Condition B = lock.newCondition();
    private static Condition C = lock.newCondition();


    static class ThreadA extends Thread {

        @Override
        public void run() {
            lock.lock();
            try {
                for (int i = 0; i < 10; i++) {
                    while (count % 3 != 0) {
                        A.await(); // 会释放lock锁
                        System.out.print("A");
                        count++;
                        B.signal(); // 唤醒相应线程
                    }
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }


        }

    }

    static class ThreadB extends Thread {

        @Override
        public void run() {
            lock.lock();
            try {
                for (int i = 0; i < 10; i++) {
                    while (count % 3 != 0) {
                        B.await(); // 会释放lock锁
                        System.out.print("B");
                        count++;
                        C.signal(); // 唤醒相应线程
                    }
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }


        }

    }

    static class ThreadC extends Thread {

        @Override
        public void run() {
            lock.lock();
            try {
                for (int i = 0; i < 10; i++) {
                    while (count % 3 != 0) {
                        C.await(); // 会释放lock锁
                        System.out.print("C");
                        count++;
                        A.signal(); // 唤醒相应线程
                    }
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }


        }

    }

     public static void main(String[] args) throws InterruptedException {
         new ThreadA().start();
         new ThreadB().start();
         ThreadC threadC = new ThreadC();
         threadC.start();
         threadC.join();
         System.out.println(count);
     }

}

使用信号量也可以, 这个思路最简单, 整个代码也比较简洁

import java.util.concurrent.Semaphore;
  
  public class ABC3 {
      private static Semaphore A = new Semaphore(1);
      private static Semaphore B = new Semaphore(1);
      private static Semaphore C = new Semaphore(1);
      
      static class ThreadA extends Thread {
  
         @Override
         public void run() {
             try {
                 for (int i = 0; i < 10; i++) {
                     A.acquire();
                     System.out.print("A");
                     B.release();
                 }
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }
         }
         
     }
     
     static class ThreadB extends Thread {
 
         @Override
         public void run() {
             try {
                 for (int i = 0; i < 10; i++) {
                     B.acquire();
                     System.out.print("B");
                     C.release();
                 }
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }
         }
         
     }
     
     static class ThreadC extends Thread {
 
         @Override
         public void run() {
             try {
                 for (int i = 0; i < 10; i++) {
                     C.acquire();
                     System.out.println("C");
                     A.release();
                 }
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }
         }
         
     }
     
     public static void main(String[] args) throws InterruptedException {
         B.acquire(); C.acquire(); // 开始只有A可以获取, BC都不可以获取, 保证了A最先执行
         new ThreadA().start();
         new ThreadB().start();
         new ThreadC().start();
     }
 }

注意:

lock是需要lock所有者去释放的, 即谁lock, 谁释放, 不可以跨线程, 会报java.lang.IllegalMonitorStateException;

semaphore是没有所有者的说法, 可以跨线程释放和获取.