查看“多线程交替打印”的源代码

https://www.bilibili.com/video/BV1ez421C7Fz/?spm_id_from=333.999.0.0&vd_source=92b1acbb084e38873b05857f8f1b9289

有两个线程，分别打印a字符和b字符

=== 代码1： ===
<syntaxhighlight lang="java">
import java.util.concurrent.atomic.AtomicInteger;

public class MultiPrint2 {

	public static void main(String[] args) throws InterruptedException {
		Object monitor = new Object();
		AtomicInteger i = new AtomicInteger(0);
		PrintRunnable pr = new PrintRunnable(monitor, i, 100);
		Thread t1 = new Thread(pr);
		Thread t2 = new Thread(pr);
		t1.start();
		t2.start();
		//得t1和t2都执行完才能结束
		t1.join();
		System.out.println(t1.getId() + "结束");
		//得t1和t2都执行完才能结束
		t2.join();
		System.out.println(t2.getId() + "结束");
	}
}

class PrintRunnable implements Runnable {
	private Object monitor;
	private AtomicInteger i;
	private int end;
	/**
	 * @param monitor
	 * @param i
	 * @param end
	 */
	public PrintRunnable(Object monitor, AtomicInteger i, int end) {
		super();
		this.monitor = monitor;
		this.i = i;
		this.end = end;
	}
	@Override
	public void run() {
		long id = Thread.currentThread().getId();
		synchronized (monitor) {
			while (i.get() < end) {
				System.out.println(id + ":" + i.incrementAndGet());
				try {
					monitor.notify();
					monitor.wait();
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				/**
				 * notify要在synchronized里，不然会抛IllegalMonitorStateException
				 * 这单独的一个notify调用是为了通知另一个还在while里wait的线程
				 */
			}
			monitor.notify();
		}
	}
}
</syntaxhighlight>会有一个线程，我们假定它是A，先进入synchronized块，然后执行到wait释放锁（执行notify的时候并有其他在wait的线程，所以无所谓）；

这时候另一个线程，我们叫它B，就可以进入synchronized一直执行到notify通知 wait的线程A 继续执行，之后线程B wait；

然后就是A、B两个线程交替执行，直到有一个线程，我们假定是线程A 执行完wait之后，又被线程B nofity之后 继续执行，到 while 中的判断不再成立，

于是线程A跳出while继续执行while外面的notify，然后通知在while中notify了线程A之后wait的线程B继续执行，然后线程A执行结束，线程B也在while判断不成立后跳出循环，执行notify后（因为此时并没有 wait 的线程，所以这个B线程执行的 notify 没什么影响）结束


代码2：

<syntaxhighlight lang="java">
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class MultiPrint3 {

	public static void main(String[] args) throws InterruptedException {
		AtomicInteger i = new AtomicInteger(0);
		Lock lock = new ReentrantLock();
		Condition condition = lock.newCondition();
		Runnable pr = new NewPrintRunnable(lock, condition, i, 100);
		Thread t1 = new Thread(pr);
		Thread t2 = new Thread(pr);
		t1.start();
		t2.start();
		// 得t1和t2都执行完才能结束
		t1.join();
		System.out.println(t1.getId() + "结束");
		// 得t1和t2都执行完才能结束
		t2.join();
		System.out.println(t2.getId() + "结束");
	}
}

class NewPrintRunnable implements Runnable {

	private final Lock lock;
	private final Condition condition;
	private AtomicInteger i;
	private int end;

	/**
	 * @param monitor
	 * @param i
	 * @param end
	 */
	public NewPrintRunnable(Lock lock, Condition condition, AtomicInteger i, int end) {
		super();
		this.lock = lock;
		this.condition = condition;
		this.i = i;
		this.end = end;
	}

	@Override
	public void run() {
		long id = Thread.currentThread().getId();
		lock.lock();
		while (i.get() < end) {
			System.out.println(id + ":" + i.incrementAndGet());
			try {
				condition.signal();
				condition.await();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		condition.signal();
		lock.unlock();
	}
}
</syntaxhighlight>代码2与代码1的实现思路是一样的，代码2使用了JDK1.5加入的Lock和Condition这些JUC工具类。