Java五种方式实现多线程循环打印问题 Java五种方式实现多线程循环打

wait-notify
join方式
ReentrantLock
ReentrantLock+Condition
Semaphore

三个线程T1、T2、T3轮流打印ABC，打印n次，如ABCABCABCABC…
N个线程循环打印1-100…

wait-notify
循环打印问题可以通过设置目标值，每个线程想打印目标值，如果拿到锁后这次轮到的数不是它想要的就进入wait

class Wait_Notify_ABC {private int num; private static final Object Lock = new Object(); private void print_ABC(int target) {synchronized (Lock) {//循环打印for (int i = 0; i < 10; i++) {while (num % 3 != target) {try {Lock.wait(); } catch (InterruptedException e) {e.printStackTrace(); }}num++; System.out.print(Thread.currentThread().getName()); Lock.notifyAll(); }}}public static void main(String[] args) {Wait_Notify_ABC wait_notify_abc = new Wait_Notify_ABC(); new Thread(() -> {wait_notify_abc.print_ABC(0); }, "A").start(); new Thread(() -> {wait_notify_abc.print_ABC(1); }, "B").start(); new Thread(() -> {wait_notify_abc.print_ABC(2); }, "C").start(); }}

【Java五种方式实现多线程循环打印问题】打印1-100问题可以理解为有个全局计数器记录当前打印到了哪个数，其它就和循环打印ABC问题相同。

class Wait_Notify_100 {private int num; private static final Object LOCK = new Object(); private int maxnum = 100; private void printABC(int targetNum) {while (true) {synchronized (LOCK) {while (num % 3 != targetNum) {if (num >= maxnum) {break; }try {LOCK.wait(); } catch (InterruptedException e) {e.printStackTrace(); }}if (num >= maxnum) {break; }num++; System.out.println(Thread.currentThread().getName() + ": " + num); LOCK.notifyAll(); }}}public static void main(String[] args) {Wait_Notify_100 wait_notify_100 = new Wait_Notify_100(); new Thread(() -> {wait_notify_100.printABC(0); }, "thread1").start(); new Thread(() -> {wait_notify_100.printABC(1); }, "thread2").start(); new Thread(() -> {wait_notify_100.printABC(2); }, "thread3").start(); }}

join方式
一个线程内调用另一个线程的join()方法可以让另一个线程插队执行，比如Main方法里调用了A.join()，那么此时cpu会去执行A线程中的任务，执行完后再看Main是否能抢到运行权。所以对于ABC，我们可以对B说让A插队，对C说让B插队

class Join_ABC {static class printABC implements Runnable {private Thread beforeThread; public printABC(Thread beforeThread) {this.beforeThread = beforeThread; }@Overridepublic void run() {if (beforeThread != null) {try {beforeThread.join(); } catch (InterruptedException e) {e.printStackTrace(); }}System.out.print(Thread.currentThread().getName()); }}public static void main(String[] args) throws InterruptedException {for (int i = 0; i < 10; i++) {Thread t1 = new Thread(new printABC(null), "A"); Thread t2 = new Thread(new printABC(t1), "B"); Thread t3 = new Thread(new printABC(t2), "C"); t1.start(); t2.start(); t3.start(); Thread.sleep(100); }}}

ReentrantLock
同理，synchronized和reentrantlock都是我们常用的加锁方式，不过后者可以中断，可以实现公平锁，可以使用condition…但是需要我们手动释放锁。jdk8后二者性能差不多，毕竟synchronized有锁升级的过程嘛。

class ReentrantLock_ABC {private int num; private Lock lock = new ReentrantLock(); private void printABC(int targetNum) {for (int i = 0; i < 100; ) {lock.lock(); if (num % 3 == targetNum) {num++; i++; System.out.print(Thread.currentThread().getName()); }lock.unlock(); }}public static void main(String[] args) {Lock_ABC lockABC = new Lock_ABC(); new Thread(() -> {lockABC.printABC(0); }, "A").start(); new Thread(() -> {lockABC.printABC(1); }, "B").start(); new Thread(() -> {lockABC.printABC(2); }, "C").start(); }}

ReentrantLock+Condition
以上方式如果线程抢到锁后发现自己无法执行任务，那么就释放，然后别的线程再抢占再看是不是自己的…这种方式比较耗时，如果我们能实现精准唤醒锁呢，即A完成任务后唤醒它的下一个即B，这就用到我们的Condition啦

class ReentrantLock_Condition_ABC {private int num; private static Lock lock = new ReentrantLock(); private static Condition c1 = lock.newCondition(); private static Condition c2 = lock.newCondition(); private static Condition c3 = lock.newCondition(); private void printABC(int targetNum, Condition currentThread, Condition nextThread) {for (int i = 0; i < 100; ) {lock.lock(); try {while (num % 3 != targetNum) {currentThread.await(); //阻塞当前线程}num++; i++; System.out.print(Thread.currentThread().getName()); nextThread.signal(); //唤醒下一个线程} catch (Exception e) {e.printStackTrace(); } finally {lock.unlock(); }}}public static void main(String[] args) {ReentrantLock_Condition_ABC reentrantLockConditionAbc = new ReentrantLock_Condition_ABC(); new Thread(() -> {reentrantLockConditionAbc.printABC(0, c1, c2); }, "A").start(); new Thread(() -> {reentrantLockConditionAbc.printABC(1, c2, c3); }, "B").start(); new Thread(() -> {reentrantLockConditionAbc.printABC(2, c3, c1); }, "C").start(); }}

Semaphore
小伙伴们有没有想到过，在生产者消费者模型中我们有哪几种实现方式呢？wait\notify，ReentrantLock，Semaphone，阻塞队列，管道输入输出流。
对的就是Semaphone。
Semaphore有acquire方法和release方法。当调用acquire方法时线程就会被阻塞，直到获得许可证为止。当调用release方法时将向Semaphore中添加一个许可证。如果没有获取许可证的线程， Semaphore只是记录许可证的可用数量。
使用Semaphore也可以实现精准唤醒。

class SemaphoreABC {private static Semaphore s1 = new Semaphore(1); //因为先执行线程A，所以这里设s1的计数器为1private static Semaphore s2 = new Semaphore(0); private static Semaphore s3 = new Semaphore(0); private void printABC(Semaphore currentThread, Semaphore nextThread) {for (int i = 0; i < 10; i++) {try {currentThread.acquire(); //阻塞当前线程，即信号量的计数器减1为0System.out.print(Thread.currentThread().getName()); nextThread.release(); //唤醒下一个线程，即信号量的计数器加1} catch (InterruptedException e) {e.printStackTrace(); }}}public static void main(String[] args) throws InterruptedException {SemaphoreABC printer = new SemaphoreABC(); new Thread(() -> {printer.printABC(s1, s2); }, "A").start(); Thread.sleep(100); new Thread(() -> {printer.printABC(s2, s3); }, "B").start(); Thread.sleep(100); new Thread(() -> {printer.printABC(s3, s1); }, "C").start(); }}

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