Java经典面试题-多线程打印

thread+synchronized

就好像一个圆圈，A->B->C->A。。。。。

synchronized能够保证多个线程进入实，只用一个线程能进入。

/**多线程交替打印* */
public class Task {private final Object lock = new Object();private int count = 0;public static void main(String[] args) {Task task = new Task();new Thread(()->{for(int i=0;i<10;i++){task.printA();}}).start();new Thread(()->{for(int i=0;i<10;i++){task.printB();}}).start();new Thread(()->{for(int i=0;i<10;i++){task.printC();}}).start();}public void printA(){synchronized (lock){while(count!=0){try {lock.wait();} catch (InterruptedException e) {e.printStackTrace();}}System.out.print("A");count++;lock.notifyAll();}}public void printB(){synchronized (lock){while(count!=1){try{lock.wait();}catch (InterruptedException e){e.printStackTrace();}}System.out.print("B");count++;lock.notifyAll();}}public void printC(){synchronized (lock){while(count!=2){try {lock.wait();} catch (InterruptedException e) {e.printStackTrace();}}System.out.print("C");count=0;lock.notifyAll();}}
}

ReentrantLock

public class Task2 {private  final Lock lock=new ReentrantLock();private final Condition condition=lock.newCondition();private final Condition condition1=lock.newCondition();private final Condition condition2=lock.newCondition();private int state=0;public static void main(String[] args) {Task2 tash2 = new Task2();Thread thread = new Thread(tash2::printA);Thread thread1 = new Thread(tash2::printB);thread.start();thread1.start();}private void printA(){// get locklock.lock();try{for(int i=0;i<10;i++){while(state!=0){condition.await();}System.out.print("a");state=1;condition1.signal();}} catch (InterruptedException e) {throw new RuntimeException(e);}finally {lock.unlock();}}private  void printB(){lock.lock();try{for(int i=0;i<10;i++){while(state!=1){condition1.await();}System.out.print("b");state=0;condition.signal();}} catch (InterruptedException e) {throw new RuntimeException(e);}finally {lock.unlock();}}
}

Semaphore

信号量机制，设置刚开始的值为1，如果减去1的结果>=0,则执行该线程，否则不执行，执行完释放，值加1

public final void acquireShared(int arg) {if (tryAcquireShared(arg) < 0)acquire(null, arg, true, false, false, 0L);}/*** Acquires in shared mode, aborting if interrupted.  Implemented* by first checking interrupt status, then invoking at least once* {@link #tryAcquireShared}, returning on success.  Otherwise the* thread is queued, possibly repeatedly blocking and unblocking,* invoking {@link #tryAcquireShared} until success or the thread* is interrupted.* @param arg the acquire argument.* This value is conveyed to {@link #tryAcquireShared} but is* otherwise uninterpreted and can represent anything* you like.* @throws InterruptedException if the current thread is interrupted*/public final void acquireSharedInterruptibly(int arg)throws InterruptedException {if (Thread.interrupted() ||(tryAcquireShared(arg) < 0 &&acquire(null, arg, true, true, false, 0L) < 0))throw new InterruptedException();}

package com.r.ThreadTask;import javax.swing.text.Segment;
import java.util.concurrent.Semaphore;public class Task3 {private final Semaphore semaphore=new Semaphore(1);private final Semaphore semaphore1=new Semaphore(0);private final Semaphore semaphore2=new Semaphore(0);public static void main(String[] args) {Task3 task3 = new Task3();Thread thread = new Thread(task3::printA);Thread thread1 = new Thread(task3::printB);Thread thread2 = new Thread(task3::printC);thread.start();thread1.start();thread2.start();}private void printA(){for(int i=0;i<10;i++){try {semaphore.acquire();} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.print("a");semaphore1.release();}}private void printB(){for(int i=0;i<10;i++){try {semaphore1.acquire();} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.print("b");semaphore2.release();}}private void printC(){for(int i=0;i<10;i++){try {semaphore2.acquire();} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.print("c");semaphore.release();}}
}