Java中将异步调用转为同步的五种方法

异步与同步的核心区别

• 同步调用：调用方阻塞等待结果返回
• 异步调用：调用方立即返回，通过回调/轮询等方式获取结果

本文重点讨论如何将异步调用转为同步阻塞模式，以下是五种实现方案：

方法一：使用wait/notify + synchronized

代码示例

 public class ProducerConsumerExample {
     private static final int BUFFER_SIZE = 5;
     private final Object lock = new Object();
     private int[] buffer = new int[BUFFER_SIZE];
     private int count = 0;
 
     // 生产者线程
     public void produce() throws InterruptedException {
         int value = 0;
         while (true) {
             synchronized (lock) {
                 while (count == BUFFER_SIZE) {
                     System.out.println("缓冲区已满，生产者等待...");
                     lock.wait();
                 }
                 buffer[count++] = value++;
                 System.out.println("生产数据: " + value + "，缓冲区数量: " + count);
                 lock.notify();
             }
             Thread.sleep(1000);
         }
     }
 
     // 消费者线程
     public void consume() throws InterruptedException {
         while (true) {
             synchronized (lock) {
                 while (count == 0) {
                     System.out.println("缓冲区为空，消费者等待...");
                     lock.wait();
                 }
                 int value = buffer[--count];
                 System.out.println("消费数据: " + value + "，缓冲区数量: " + count);
                 lock.notify();
             }
             Thread.sleep(1500);
         }
     }
 
     public static void main(String[] args) {
         ProducerConsumerExample example = new ProducerConsumerExample();
     
         // 启动生产者和消费者线程
         new Thread(example::produce).start();
         new Thread(example::consume).start();
     }
 }

关键要点

1. 共享资源保护：通过synchronized(lock)保证线程安全

2. 条件判断：

   • while循环而非if防止虚假唤醒
   • 缓冲区满时生产者等待(wait())
   • 缓冲区空时消费者等待(wait())

3. 协作机制：每次操作后通过notify()唤醒等待线程

4. 方法对比：

   • notify()：唤醒单个等待线程
   • notifyAll()：唤醒所有等待线程（适用于多生产者场景）

方法二：使用ReentrantLock + Condition

代码示例

 import java.util.concurrent.locks.Condition;
 import java.util.concurrent.locks.ReentrantLock;
 
 public class TestReentrantLock4 {
     static ReentrantLock lock = new ReentrantLock();
     static Condition moneyCondition = lock.newCondition();
     static Condition ticketCondition = lock.newCondition();
     static boolean haveMoney = false;
     static boolean haveTicket = false;
 
     public static void main(String[] args) throws InterruptedException {
         // 农民1（等钱）
         new Thread(() -> {
             lock.lock();
             try {
                 while (!haveMoney) {
                     System.out.println("农民1等待资金...");
                     moneyCondition.await();
                 }
                 System.out.println("农民1获得资金，回家！");
             } finally {
                 lock.unlock();
             }
         }, "Farmer1").start();
 
         // 农民2（等票）
         new Thread(() -> {
             lock.lock();
             try {
                 while (!haveTicket) {
                     System.out.println("农民2等待车票...");
                     ticketCondition.await();
                 }
                 System.out.println("农民2获得车票，回家！");
             } finally {
                 lock.unlock();
             }
         }, "Farmer2").start();
 
         // 主线程模拟发放条件
         Thread.sleep(1000);
         lock.lock();
         try {
             haveMoney = true;
             moneyCondition.signal();
             System.out.println("资金已发放！");
 
             haveTicket = true;
             ticketCondition.signal();
             System.out.println("车票已发放！");
         } finally {
             lock.unlock();
         }
     }
 }

核心特性

1. 多条件支持：

   • 一个锁对象可绑定多个Condition（如moneyCondition/ticketCondition）

2. 精准唤醒：

   • await()：释放锁并等待特定条件
   • signal()：唤醒满足条件的等待线程

3. 代码结构：

   • 必须在lock.lock()和finally unlock()之间操作
   • 条件判断使用while循环防止虚假唤醒

方法三：Future（Callable + ExecutorService）

代码示例

 import java.util.concurrent.*;
 
 public class FutureExample {
     public static void main(String[] args) {
         ExecutorService executor = Executors.newSingleThreadExecutor();
 
         Future<Integer> future = executor.submit(() -> {
             int sum = 0;
             for (int i = 1; i <= 100; i++) {
                 sum += i;
                 Thread.sleep(10);
             }
             return sum;
         });
 
         System.out.println("主线程执行其他任务...");
     
         try {
             Integer result = future.get(2, TimeUnit.SECONDS);
             System.out.println("计算结果: 1+2+...+100 = " + result);
         } catch (TimeoutException e) {
             System.err.println("计算超时！");
             future.cancel(true);
         } catch (Exception e) {
             e.printStackTrace();
         } finally {
             executor.shutdown();
         }
     }
 }

关键API

执行流程

1. 提交Callable任务到线程池
2. 主线程继续执行其他操作
3. 调用future.get()阻塞等待结果
4. 处理可能出现的异常情况
5. 最终关闭线程池资源

方法四：CountDownLatch（多线程同步）

代码示例

 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.TimeUnit;
 
 public class CountDownLatchExample {
     private static final int RUNNERS = 5;
     private static final CountDownLatch startSignal = new CountDownLatch(1);
     private static final CountDownLatch readySignal = new CountDownLatch(RUNNERS);
 
     public static void main(String[] args) throws InterruptedException {
         ExecutorService executor = Executors.newFixedThreadPool(RUNNERS);
 
         for (int i = 1; i <= RUNNERS; i++) {
             executor.execute(() -> {
                 try {
                     System.out.println("运动员" + i + "正在准备...");
                     TimeUnit.MILLISECONDS.sleep(300);
                     readySignal.countDown();
 
                     startSignal.await();
                     System.out.println("运动员" + i + "起跑！");
                 
                     TimeUnit.MILLISECONDS.sleep((long)(Math.random() * 1000));
                     System.out.println("运动员" + i + "到达终点！");
                 } catch (InterruptedException e) {
                     e.printStackTrace();
                 }
             });
         }
 
         System.out.println("裁判等待运动员就位...");
         readySignal.await();
         System.out.println("\n所有运动员就位！");
 
         TimeUnit.SECONDS.sleep(1);
         System.out.println("发令枪响！");
         startSignal.countDown();
 
         executor.shutdown();
         executor.awaitTermination(5, TimeUnit.SECONDS);
         System.out.println("\n比赛结束！");
     }
 }

应用场景

1. 多线程初始化后统一执行：如服务启动时等待所有组件就绪
2. 并发测试控制：模拟固定数量请求同时发起
3. 事件驱动编程：等待多个前置条件完成

方法五：CyclicBarrier（可重用同步屏障）

代码示例

 import java.util.concurrent.BrokenBarrierException;
 import java.util.concurrent.CyclicBarrier;
 
 public class CyclicBarrierExample {
     private static final CyclicBarrier barrier = 
         new CyclicBarrier(3, () -> System.out.println("\n===== 进入下一阶段 ====="));
 
     public static void main(String[] args) {
         for (int i = 1; i <= 3; i++) {
             new Thread(new TeamMember(i)).start();
         }
     }
 
     static class TeamMember implements Runnable {
         private int id;
 
         public TeamMember(int id) {
             this.id = id;
         }
 
         @Override
         public void run() {
             try {
                 doWork("需求分析", 1000);
                 barrier.await();
             
                 doWork("开发编码", 1500);
                 barrier.await();
             
                 doWork("测试部署", 800);
                 barrier.await();
             } catch (Exception e) {
                 e.printStackTrace();
             }
         }
 
         private void doWork(String phase, int baseTime) throws InterruptedException {
             int time = baseTime + (int)(Math.random() * 500);
             System.out.printf("%s 完成%s（%dms）\n", 
                 Thread.currentThread().getName(), phase, time);
             Thread.sleep(time);
         }
     }
 }

核心特性

总结对比表

最佳实践建议：

• 简单同步场景优先使用CountDownLatch
• 需要结果返回时使用Future
• 多条件或多阶段场景推荐CyclicBarrier
• 避免使用过时的Object.wait/notify直接控制