1 前言

如下图所示，现在有两个线程A,B；A打印12345，B打印abcde，结果为1a2b3c4d5e交替输出。

1.1 采用wait和notify

【分析】我们要求线程A始终先打印，因此在线程B先获得CPU使用时间时也应该阻塞。

 细节

• 线程A应该打印后，先notify，然后wait；
• 线程B先阻塞，然后打印后，先notity，再wait。
• 其线程B先阻塞的方法，采用countdownlatch实现
• 线程A在for循环后还需要在进行notify，因为线程B最后才能被打印。因此最后一定被阻塞。

package cn.itcast.n6.c1;import java.util.concurrent.CountDownLatch;/*** @author : msf* @date : 2022/12/2* 交替输出*/
public class AlteratePrint2 {public static void main(String[] args) {String numberA = "123456";String characterB = "abcdef";Object lock = new Object();CountDownLatch latch = new CountDownLatch(1);Thread threadA = new Thread(() -> {synchronized (lock) {for (char c : numberA.toCharArray()) {System.out.print(c);try {// 唤醒线程blatch.countDown();lock.notify();// 线程A阻塞lock.wait();} catch (InterruptedException e) {e.printStackTrace();}}// 是为了更好的退出，因为程序在结束时候肯定会有一个线程在阻塞住。lock.notify();}});Thread threadB = new Thread(() -> {try {latch.await();} catch (InterruptedException e) {e.printStackTrace();}synchronized (lock) {for (char c : characterB.toCharArray()) {try {System.out.print(c);lock.notify();lock.wait();} catch (InterruptedException e) {e.printStackTrace();}}}});threadB.start();threadA.start();}
}

上述代码结果如下：

1.2 采用park 和unpark

package cn.itcast.n6.c1;import java.util.concurrent.locks.LockSupport;/*** @author : msf* @date : 2022/12/2* 交替输出*/
public class AlteratePrint {static Thread threadA = null;static Thread threadB = null;public static void main(String[] args) {String numberA = "123456";String characterB = "abcdef";threadA = new Thread(()->{for (char c : numberA.toCharArray()) {System.out.print(c);LockSupport.unpark(threadB);LockSupport.park();}});threadB = new Thread(()->{for (char c : characterB.toCharArray()) {LockSupport.park();System.out.print(c);LockSupport.unpark(threadA);}});threadA.start();threadB.start();}
}

上述代码结果如下：

1.3 采用lock锁和condition

package cn.itcast.n6.c1;import java.util.concurrent.CountDownLatch;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;/*** @author : msf* @date : 2022/12/2* 交替输出*/
public class AlteratePrint3 {public static void main(String[] args) {String numberA = "123456";String characterB = "abcdef";ReentrantLock lock = new ReentrantLock();Condition conditionA = lock.newCondition();Condition conditionB = lock.newCondition();CountDownLatch latch = new CountDownLatch(1);Thread threadA = new Thread(() -> {lock.lock();try {for (char c : numberA.toCharArray()) {System.out.print(c);try {// 唤醒线程blatch.countDown();conditionB.signal();// 线程A阻塞conditionA.await();} catch (InterruptedException e) {e.printStackTrace();}}conditionB.signal();} finally {lock.unlock();}});Thread threadB = new Thread(() -> {try {latch.await();} catch (InterruptedException e) {e.printStackTrace();}lock.lock();try {for (char c : characterB.toCharArray()) {try {System.out.print(c);// 唤醒线程aconditionA.signal();// 线程b阻塞conditionB.await();} catch (InterruptedException e) {e.printStackTrace();}}} finally {lock.unlock();}});threadB.start();threadA.start();}
}

上述代码结果如下：

1.4 采用transferQueue实现

为什么可以用transferQueue实现呢？

原因

其的take 和 transfer命令都是同步阻塞的。

package cn.itcast.n6.c1;import java.util.concurrent.CountDownLatch;
import java.util.concurrent.LinkedTransferQueue;
import java.util.concurrent.TransferQueue;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;/*** @author : msf* @date : 2022/12/2* 交替输出*/
public class AlteratePrint4 {public static void main(String[] args) {String numberA = "123456";String characterB = "abcdef";TransferQueue queue = new LinkedTransferQueue<>();Thread threadA = new Thread(() -> {try {for (char c : numberA.toCharArray()) {queue.transfer(c);System.out.print(queue.take());}} catch (InterruptedException e) {e.printStackTrace();}});Thread threadB = new Thread(() -> {try {for (char c : characterB.toCharArray()) {System.out.print(queue.take());queue.transfer(c);}} catch (InterruptedException e) {e.printStackTrace();}});threadB.start();threadA.start();}
}