使用synchronized和Condition三线程轮流打印A,B,C示例

    ReentrantLock并不是一种替代内置加锁的方法，而是当内置加锁机制不适用时，作为一种可选择的高级功能。当需要一些高级功能时才使用ReentrantLock，包括定时的锁等待，可中断的锁等待，公平性，锁绑定多个条件以及实现非块结构的加锁。否则，还是应该优先使用内置锁synchronized。synchronized是JVM的内置特性，可以进行一些优化。比如对线程封闭锁对象的锁消除优化，通过增加锁粒度来消除内置锁的同步等。
   

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

public class MultiThreadPrint
{

    public static void main(String[] args)
    {
        init();
    }

    private static void init()
    {
        ExecutorService executor = Executors.newFixedThreadPool(3);
        final Print print = new SynchronizedPrint();
        // final Print print = new ConditionPrint();

        executor.execute(new Runnable()
        {
            @Override
            public void run()
            {
                try
                {
                    Thread.currentThread().setName("thread A");
                    print.printA();
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
            }
        });
        executor.execute(new Runnable()
        {
            @Override
            public void run()
            {
                try
                {
                    Thread.currentThread().setName("thread B");
                    print.printB();
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
            }
        });

        executor.execute(new Runnable()
        {
            @Override
            public void run()
            {
                try
                {
                    Thread.currentThread().setName("thread C");

                    print.printC();
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
            }
        });
        try
        {
            Thread.sleep(20*1000);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        executor.shutdownNow();

    }

    interface Print
    {
        void printA() throws InterruptedException;

        void printB() throws InterruptedException;

        void printC() throws InterruptedException;

    }

    static class SynchronizedPrint implements Print
    {
        int flag = 1;

        @Override
        public void printA() throws InterruptedException
        {
            while (true)
            {
                synchronized (this)
                {
                    while (flag != 1)
                    {
                        this.wait();
                    }
                    System.out.println(Thread.currentThread().getName() + " prints " + "A");
                    Thread.sleep(800);
                    flag = 2;
                    notifyAll();
                }
            }

        }

        @Override
        public void printB() throws InterruptedException
        {
            while (true)
            {
                synchronized (this)
                {
                    while (flag != 2)
                    {
                        this.wait();
                    }
                    System.out.println(Thread.currentThread().getName() + " prints " + "B");
                    Thread.sleep(800);
                    flag = 3;
                    notifyAll();
                }
            }
        }

        @Override
        public void printC() throws InterruptedException
        {
            while (true)
            {
                synchronized (this)
                {
                    while (flag != 3)
                    {
                        this.wait();
                    }
                    System.out.println(Thread.currentThread().getName() + " prints " + "C");
                    Thread.sleep(800);
                    flag = 1;
                    notifyAll();
                }
            }

        }

    }

    static class ConditionPrint implements Print
    {
        final ReentrantLock lock = new ReentrantLock();
        final Condition cond1 = lock.newCondition();
        final Condition cond2 = lock.newCondition();
        final Condition cond3 = lock.newCondition();
        int flag = 1;

        public void printA() throws InterruptedException
        {
            while (true)
            {
                lock.lock();
                try
                {
                    while (flag != 1)
                    {
                        cond1.await();
                    }
                    System.out.println(Thread.currentThread().getName() + " prints " + "A");
                    Thread.sleep(800);
                    flag = 2;
                    cond2.signal();
                }
                finally
                {
                    lock.unlock();
                }
            }
        }

        public void printC() throws InterruptedException
        {
            while (true)
            {
                lock.lock();
                try
                {
                    while (flag != 2)
                    {
                        cond2.await();
                    }
                    System.out.println(Thread.currentThread().getName() + " prints " + "B");
                    Thread.sleep(800);
                    flag = 3;
                    cond3.signal();
                }
                finally
                {
                    lock.unlock();
                }
            }

        }

        public void printB() throws InterruptedException
        {
            while (true)
            {
                lock.lock();
                try
                {
                    while (flag != 3)
                    {
                        cond3.await();
                    }
                    System.out.println(Thread.currentThread().getName() + " prints " + "C");
                    Thread.sleep(800);
                    flag = 1;
                    cond1.signal();
                }
                finally
                {
                    lock.unlock();
                }
            }
        }
    }
}

控制台输出：
thread A prints A
thread B prints B
thread C prints C
thread A prints A
thread B prints B
thread C prints C
thread A prints A
thread B prints B
thread C prints C
。。。。