Java多线程面试题--保证多个线程顺序执行

public class A implements Runnable {
	
	@Override
	public void run() {
		// TODO Auto-generated method stub
		System.out.println("A");
	}

}

public class B implements Runnable {

	@Override
	public void run() {
		// TODO Auto-generated method stub
		System.out.println("B");
	}

}

public class C implements Runnable {

	@Override
	public void run() {
		// TODO Auto-generated method stub
		System.out.println("C");
	}

}

/**
	 * 方法一：
	 * @param args
	 * @throws InterruptedException
	 */
	public static void main(String[] args) throws InterruptedException {
		// TODO Auto-generated method stub
		Thread thread1 = new Thread(new A());
		thread1.start();
		thread1.join();
		Thread thread2 = new Thread(new B());
		thread2.start();
		thread2.join();
		Thread thread3 = new Thread(new C());
		thread3.start();
	}

　　使用join()方法，等前一个线程执行完毕，下一个线程才能执行

import java.util.ArrayList;
import java.util.List;

/**
 * 保证多个线程顺序执行
 * @author 尘世间迷茫的小书童
 *
 */
public class TestJoin {
    
    public static void main(String[] args) {
        
        List<Thread> threads = new ArrayList<>();
        
        for(int i=0; i<100; i++) {
            threads.add(new Thread(() -> {
                System.out.println(Thread.currentThread().getName());
            }, "线程"+i));
        }
        
        threads.forEach(thread -> {
            thread.start();
            try {
                thread.join(); //具体实现调用Object wait方法
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        
    }
    
}

/**
	 *    方法二：
	 */
	@Test
	public void test2() {
		//创建只有一根线程的线程池
		ExecutorService executorService = Executors.newSingleThreadExecutor();
		executorService.submit(new A());
		executorService.submit(new B());
		executorService.submit(new C());
		executorService.shutdown();
	}

　　创建一个只有一根线程的线程池，保证所有任务按照指定顺序执行

/**
 * 线程顺序执行类
 * @author 15735400536
 *
 */
public class StepOver {
	
	private volatile int count = 1;
	
	public void a() throws InterruptedException {
		synchronized (this) {
			while(count != 1) {
				wait();
			}
			System.out.println("线程A");
			count = 2;
			notifyAll();
			
//			if(count == 1) {
//				System.out.println("线程A");
//				count = 2;
//				//Thread.currentThread().notifyAll();
//				notifyAll();
//			}else {
//				//Thread.currentThread().wait(1000);
//			    wait();
//			}
		}
	}
	
	public void b() throws InterruptedException {
		synchronized (this) {
			while(count != 2) {
				wait();
			}
			System.out.println("线程B");
			count = 3;
			notifyAll();
			
//			if(count == 2) {
//				System.out.println("线程B");
//				count = 3;
//				//Thread.currentThread().notifyAll();
//				notifyAll();
//			}else {
//				//Thread.currentThread().wait(1000);
//				wait();
//			}
		}
	}
	
	public void c() throws InterruptedException {
		synchronized (this) {
			while(count != 3) {
				wait();
			}
			System.out.println("线程C");
			count = 1;
			notifyAll();
			
//			if(count == 3) {
//				System.out.println("线程C");
//				count = 0;
//				//Thread.currentThread().notifyAll();
//				notifyAll();
//			}else {
//				//Thread.currentThread().wait(1000);
//				wait();
//			}
		}
	}
	
}

public class ThreadA implements Runnable {
	
	private StepOver stepOver;
	
	public ThreadA(StepOver stepOver) {
		this.stepOver = stepOver;
	}
	
	@Override
	public void run() {
		// TODO Auto-generated method stub
		try {
			stepOver.a();
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

}

public class ThreadB implements Runnable {
	
	private StepOver stepOver;
	
	public ThreadB(StepOver stepOver) {
		this.stepOver = stepOver;
	}
	
	@Override
	public void run() {
		// TODO Auto-generated method stub
		try {
			stepOver.b();
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

}

public class ThreadC implements Runnable {
	
	private StepOver stepOver;
	
	public ThreadC(StepOver stepOver) {
		this.stepOver = stepOver;
	}
	
	@Override
	public void run() {
		// TODO Auto-generated method stub
		try {
			stepOver.c();
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

}

/**
	 * 方法三
	 */
	@Test
	public void test3() {
		StepOver stepOver = new StepOver();
		for(int i=0; i<10 ; i++) {
			Thread b = new Thread(new ThreadB(stepOver));
			b.start();
			Thread a = new Thread(new ThreadA(stepOver));
			a.start();
			Thread c = new Thread(new ThreadC(stepOver));
			c.start();
		}
	}

　　面试的时候，面试官问你怎么保证多个线程顺序执行多半都是考察多线程的join方法，join方法的实现调用了Object的wait方法，所以上面说到的方法1和方法3本质上是相通的。