一道多线程题目的解决方案

一道多线程题目的解决方案

 

在iteye上看到的一道多线程的题目，参考了一下网友的实现，那Eclipse调试通过，算是对JAVA5的并发库有个大致的了解，分享出来，欢迎拍砖。

题目：

要求用三个线程，按顺序打印1,2,3,4,5.... 71,72,73,74, 75.

线程1先打印1,2,3,4,5, 然后是线程2打印6,7,8,9,10, 然后是线程3打印11,12,13,14,15. 接着再由线程1打印16,17,18,19,20....以此类推, 直到线程3打印到75。

分析：感觉出题人是要考察一下你是否能够很好的控制多线程，让他们有序的进行。

1、线程池：3个线程，需要使用并发库的线程池

2、锁（lcok）：在打印的时候，只允许一个线程进入，其他的线程等待

下面的主要的代码：

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class NumberPrinter {

    private Lock lock = new ReentrantLock();

    private Condition c1 = lock.newCondition();
    private Condition c2 = lock.newCondition();
    private Condition c3 = lock.newCondition();

    private Map<Integer, Condition> condtionContext =
        new HashMap<Integer, Condition>();

    public NumberPrinter() {
        condtionContext.put(Integer.valueOf(0), c1);
        condtionContext.put(Integer.valueOf(1), c2);
        condtionContext.put(Integer.valueOf(2), c3);
    }

    private int count = 0;

    public void print(int id) {
        lock.lock();
        try {
            while(count*5 < 75) {
                int curID = calcID();
                if (id == curID) {
                    for (int i = 1; i<=5; i++) {
                        System.out.print(count*5 +i+ ",");
                    }
                    System.out.println();
                    count++;
                    int nextID = calcID();
                    Condition nextCondition = condtionContext.get(
                            Integer.valueOf(nextID));
                    //通知下一线程
                    nextCondition.signal();
                } else {
                    Condition condition = condtionContext.get(
                            Integer.valueOf(id));
                    condition.await();
                }
            }
            //通知线程结束
            for(Condition c : condtionContext.values()) {
                c.signal();
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }


    private int calcID() {
        // TODO Auto-generated method stub
        return count % 3;
    }


    /**
     * @param args
     */
    public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(3);
        final CountDownLatch latch = new CountDownLatch(1);
        final NumberPrinter printer = new NumberPrinter();
        for (int i = 0; i < 3; i++) {
            final int id = i;
            executor.submit(new Runnable() {
                @Override
                public void run() {
                    // TODO Auto-generated method stub
                    try {
                        latch.await();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }

                    printer.print(id);
                }

            });
        }

        System.out.println("三个任务开始顺序打印数字。。。。。。");
        latch.countDown();
        executor.shutdown();
    }

}