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方案1 Semaphore 机制

通过信息号机制来 协调两个线程，一个线程打印后，给另一个线程释放一个信号量

  Semaphore semaphorea = new Semaphore(1);Semaphore semaphoreb = new Semaphore(0);Thread threada = new Thread(new Runnable() {@Overridepublic void run() {for (int i = 0; i < 50; i++) {try {semaphorea.acquire();} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.println("a");semaphoreb.release();}}});Thread threadb = new Thread(new Runnable() {@Overridepublic void run() {for (int i = 0; i < 50; i++) {try {semaphoreb.acquire();} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.println("b");semaphorea.release();}}});threada.start();threadb.start();

方案2

通过锁的condition来两个线程协作

public class AlternatePrintAB {private static boolean printA = true; // 初始化为true，用于控制打印的起始public static void main(String[] args) {Lock lock = new ReentrantLock();Condition conditiona = lock.newCondition();Condition conditionb = lock.newCondition();Thread thread = new Thread(new Runnable() {@Overridepublic void run() {for (int i = 0; i < 10; i++) {try {lock.lock();while (!printA) {try {//不打印A时，A等待conditiona.await();//await 会释放锁，唤醒时，要重新获取锁} catch (InterruptedException e) {e.printStackTrace();}}System.out.println("A");printA = false;conditionb.signal();} catch (Exception e) {throw new RuntimeException(e);} finally {lock.unlock();}}}});Thread thread1 = new Thread(new Runnable() {@Overridepublic void run() {for (int i = 0; i < 10; i++) {lock.lock();while (printA) {try {//打印A时，B等待conditionb.await();} catch (InterruptedException e) {e.printStackTrace();}}System.out.println("B");printA = true;conditiona.signal();}}});thread.start();thread1.start();}}