交替打印ABC的多种方法

这是一道经典的笔试题，最近团队招聘，大家又把它翻出来了。这里稍微梳理下几种做法。

synchronized

todo

 

ReentrantLock

jdk1.5并发包引入了lock方式，通过ReentrantLock我们可以很方便的进行显式的锁操作，即获取锁和释放锁，对于同一个对象锁而言，同一时刻只可能有一个线程拿到了这个锁，此时其他线程通过lock.lock()来获取对象锁时都会被阻塞，直到这个线程通过lock.unlock()操作释放这个锁后，其他线程才能拿到这个锁。
 

package reentrantlock;

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

public class LockABC {

    private static Lock lock = new ReentrantLock();// 通过JDK5中的Lock锁来保证线程的访问的互斥
    private static int state = 0;//通过state的值来确定是否打印

    public static void main(String[] args) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 10;) {
                    lock.lock();
                    try {
                        while (state % 3 == 0) {
                        System.out.println("A");
                        state++;
                        i++;
                    }
                    } finally {
                        lock.unlock();
                    }
                }
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 10; ) {
                    lock.lock();
                    try {
                        while (state % 3 == 1) {
                            System.out.println("B");
                            state++;
                            i++;
                        }
                    } finally {
                        lock.unlock();
                    }

                }

            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 10; ) {
                    lock.lock();
                    try {
                        while (state % 3 == 2) {
                            System.out.println("C");
                            state++;
                            i++;
                        }
                    } finally {
                        lock.unlock();
                    }

                }

            }
        }).start();
    }
}

 

 

ReentrantLock+Condition

生成多个对象锁，通过控制各个对象锁的获取和释放。

package reentrantlock;

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

public class ConditionABC {

    private static Lock lock = new ReentrantLock();
    static Condition conditionA = lock.newCondition();
    static Condition conditionB = lock.newCondition();
    static Condition conditionC = lock.newCondition();

    private static String state = "A";

    public static void main(String[] args) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                lock.lock();
                try {
                    for (int i = 0; i < 10; i++) {
                        if (!state.equals("A")){
                            conditionA.await();
                        }
                        state = "B";
                        System.out.println("A");
                        conditionB.signal();
                    }
                }catch (Exception e){

                }finally {
                    lock.unlock();
                }
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                lock.lock();
                try {
                    for (int i = 0; i < 10; i++) {
                        if (!state.equals("B")){
                            conditionB.await();
                        }
                        state = "C";
                        System.out.println("B");
                        conditionC.signal();
                    }
                }catch (Exception e){

                }finally {
                    lock.unlock();
                }
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                lock.lock();
                try {
                    for (int i = 0; i < 10; i++) {
                        if (!state.equals("C")){
                            conditionC.await();
                        }
                        state = "A";
                        System.out.println("C");
                        conditionA.signal();
                    }
                }catch (Exception e){

                }finally {
                    lock.unlock();
                }
            }
        }).start();
    }
}