在Java中，通常不保证多个线程按特定顺序执行，因为线程的调度是由操作系统管理的，并且是不可预测的。然而，如果需要确保线程按特定顺序执行，可以使用以下方法：

1. 使用线程的join()方法：确保一个线程在另一个线程完成之后才开始。
2. 使用同步工具类：如CountDownLatch、CyclicBarrier和Semaphore。
3. 使用ReentrantLock和条件变量（Condition）。

方法一：使用join()方法

通过调用一个线程的join()方法，当前线程会等待这个线程完成之后再继续执行。

public class JoinExample {public static void main(String[] args) {Thread t1 = new Thread(() -> {System.out.println("Thread 1 is running");});Thread t2 = new Thread(() -> {System.out.println("Thread 2 is running");});Thread t3 = new Thread(() -> {System.out.println("Thread 3 is running");});try {t1.start();t1.join(); // 等待t1完成t2.start();t2.join(); // 等待t2完成t3.start();t3.join(); // 等待t3完成} catch (InterruptedException e) {e.printStackTrace();}}
}

 

方法二：使用CountDownLatch

CountDownLatch允许一个或多个线程等待其他线程完成某些操作。

import java.util.concurrent.CountDownLatch;public class CountDownLatchExample {public static void main(String[] args) {CountDownLatch latch = new CountDownLatch(1);Thread t1 = new Thread(() -> {System.out.println("Thread 1 is running");latch.countDown(); // 计数器减1});Thread t2 = new Thread(() -> {try {latch.await(); // 等待计数器减到0System.out.println("Thread 2 is running");} catch (InterruptedException e) {e.printStackTrace();}});t1.start();t2.start();}
}

 

方法三：使用CyclicBarrier

CyclicBarrier可以让一组线程相互等待，直到到达一个共同的障碍点。

 

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;public class CyclicBarrierExample {public static void main(String[] args) {CyclicBarrier barrier = new CyclicBarrier(3, () -> System.out.println("All threads are ready"));Thread t1 = new Thread(() -> {try {System.out.println("Thread 1 is waiting");barrier.await();System.out.println("Thread 1 is running");} catch (InterruptedException | BrokenBarrierException e) {e.printStackTrace();}});Thread t2 = new Thread(() -> {try {System.out.println("Thread 2 is waiting");barrier.await();System.out.println("Thread 2 is running");} catch (InterruptedException | BrokenBarrierException e) {e.printStackTrace();}});Thread t3 = new Thread(() -> {try {System.out.println("Thread 3 is waiting");barrier.await();System.out.println("Thread 3 is running");} catch (InterruptedException | BrokenBarrierException e) {e.printStackTrace();}});t1.start();t2.start();t3.start();}
}

 

方法四：使用ReentrantLock和条件变量（Condition）

使用ReentrantLock和Condition可以更灵活地控制线程执行顺序。

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;public class ReentrantLockExample {public static void main(String[] args) {Lock lock = new ReentrantLock();Condition condition = lock.newCondition();boolean[] t1Done = {false};Thread t1 = new Thread(() -> {lock.lock();try {System.out.println("Thread 1 is running");t1Done[0] = true;condition.signal();} finally {lock.unlock();}});Thread t2 = new Thread(() -> {lock.lock();try {while (!t1Done[0]) {condition.await();}System.out.println("Thread 2 is running");} catch (InterruptedException e) {e.printStackTrace();} finally {lock.unlock();}});t1.start();t2.start();}
}