异步转同步

业务需求

有些接口查询反馈结果是异步返回的，无法立刻获取查询结果。

• 正常处理逻辑

触发异步操作，然后传递一个唯一标识。

等到异步结果返回，根据传入的唯一标识，匹配此次结果。

• 如何转换为同步

正常的应用场景很多，但是有时候不想做数据存储，只是想简单获取调用结果。

即想达到同步操作的结果，怎么办呢？

思路

1. 发起异步操作

2. 在异步结果返回之前，一直等待(可以设置超时)

3. 结果返回之后，异步操作结果统一返回

循环等待

• LoopQuery.java

使用 query()，将异步的操作 remoteCallback() 执行完成后，同步返回。

public class LoopQuery implements Async {private String result;private static final Logger LOGGER = LogManager.getLogger(LoopQuery.class.getName());@Overridepublic String query(String key) {startQuery(key);new Thread(new Runnable() {@Overridepublic void run() {remoteCallback(key);}}).start();final String queryResult = endQuery();LOGGER.info("查询结果: {}", queryResult);return queryResult;}/*** 开始查询* @param key 查询条件*/private void startQuery(final String key) {LOGGER.info("执行查询: {}", key);}/*** 远程的回调是等待是随机的** @param key 查询条件*/private void remoteCallback(final String key) {try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}this.result = key + "-result";LOGGER.info("remoteCallback set result: {}", result);}/*** 结束查询* @return 返回结果*/private String endQuery() {while (true) {if (null == result) {try {TimeUnit.MILLISECONDS.sleep(100);} catch (InterruptedException e) {e.printStackTrace();}} else {return result;}}}
}

• main()

public static void main(String[] args) {new LoopQuery().query("12345");
}

• 测试结果

18:14:16.491 [main] INFO  com.github.houbb.thread.learn.aysnc.loop.LoopQuery - 执行查询: 12345
18:14:21.498 [Thread-1] INFO  com.github.houbb.thread.learn.aysnc.loop.LoopQuery - remoteCallback set result: 12345-result
18:14:21.548 [main] INFO  com.github.houbb.thread.learn.aysnc.loop.LoopQuery - 查询结果: 12345-result

CountDownLatch

• AsyncQuery.java

使用 CountDownLatch 类达到同步的效果。

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;public class AsyncQuery {private static final Logger LOGGER = LogManager.getLogger(AsyncQuery.class.getName());/*** 结果*/private String result;/*** 异步转同步查询* @param key*/public void asyncQuery(final String key) {final CountDownLatch latch = new CountDownLatch(1);this.startQuery(key);new Thread(new Runnable() {@Overridepublic void run() {LOGGER.info("远程回调线程开始");remoteCallback(key, latch);LOGGER.info("远程回调线程结束");}}).start();try {latch.await();} catch (InterruptedException e) {e.printStackTrace();}this.endQuery();}private void startQuery(final String key) {LOGGER.info("执行查询: {}", key);}/*** 远程的回调是等待是随机的* @param key*/private void remoteCallback(final String key, CountDownLatch latch) {try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}this.result = key + "-result";latch.countDown();}private void endQuery() {LOGGER.info("查询结果: {}", result);}}

• main()

public static void main(String[] args) {AsyncQuery asyncQuery = new AsyncQuery();final String key = "123456";asyncQuery.asyncQuery(key);
}

• 日志

18:19:12.714 [main] INFO  com.github.houbb.thread.learn.aysnc.countdownlatch.AsyncQuery - 执行查询: 123456
18:19:12.716 [Thread-1] INFO  com.github.houbb.thread.learn.aysnc.countdownlatch.AsyncQuery - 远程回调线程开始
18:19:17.720 [main] INFO  com.github.houbb.thread.learn.aysnc.countdownlatch.AsyncQuery - 查询结果: 123456-result
18:19:17.720 [Thread-1] INFO  com.github.houbb.thread.learn.aysnc.countdownlatch.AsyncQuery - 远程回调线程结束

Spring EventListener

使用观察者模式也可以。（对方案一的优化）

此处结合 spring 进行使用。

• BookingCreatedEvent.java

定义一个传输属性的对象。

public class BookingCreatedEvent extends ApplicationEvent {private static final long serialVersionUID = -1387078212317348344L;private String info;public BookingCreatedEvent(Object source) {super(source);}public BookingCreatedEvent(Object source, String info) {super(source);this.info = info;}public String getInfo() {return info;}
}

• BookingService.java

说明：当 this.context.publishEvent(bookingCreatedEvent); 触发时，
会被 @EventListener 指定的方法监听到。

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Service;import java.util.concurrent.TimeUnit;@Service
public class BookingService {@Autowiredprivate ApplicationContext context;private volatile BookingCreatedEvent bookingCreatedEvent;/*** 异步转同步查询* @param info* @return*/public String asyncQuery(final String info) {query(info);new Thread(new Runnable() {@Overridepublic void run() {remoteCallback(info);}}).start();while(bookingCreatedEvent == null) {//.. 空循环// 短暂等待。try {TimeUnit.MILLISECONDS.sleep(1);} catch (InterruptedException e) {//...}//2. 使用两个单独的 event...}final String result = bookingCreatedEvent.getInfo();bookingCreatedEvent = null;return result;}@EventListenerpublic void onApplicationEvent(BookingCreatedEvent bookingCreatedEvent) {System.out.println("监听到远程的信息: " + bookingCreatedEvent.getInfo());this.bookingCreatedEvent = bookingCreatedEvent;System.out.println("监听到远程消息后: " + this.bookingCreatedEvent.getInfo());}/*** 执行查询* @param info*/public void query(final String info) {System.out.println("开始查询: " + info);}/*** 远程回调* @param info*/public void remoteCallback(final String info) {System.out.println("远程回调开始: " + info);try {TimeUnit.SECONDS.sleep(2);} catch (InterruptedException e) {e.printStackTrace();}// 重发结果事件String result = info + "-result";BookingCreatedEvent bookingCreatedEvent = new BookingCreatedEvent(this, result);//触发eventthis.context.publishEvent(bookingCreatedEvent);}
}

• 测试方法

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = SpringConfig.class)
public class BookServiceTest {@Autowiredprivate BookingService bookingService;@Testpublic void asyncQueryTest() {bookingService.asyncQuery("1234");}}

• 日志

2018-08-10 18:27:05.958  INFO  [main] com.github.houbb.spring.lean.core.ioc.event.BookingService:84 - 开始查询:1234
2018-08-10 18:27:05.959  INFO  [Thread-2] com.github.houbb.spring.lean.core.ioc.event.BookingService:93 - 远程回调开始:1234
接收到信息: 1234-result
2018-08-10 18:27:07.964  INFO  [Thread-2] com.github.houbb.spring.lean.core.ioc.event.BookingService:73 - 监听到远程的信息: 1234-result
2018-08-10 18:27:07.964  INFO  [Thread-2] com.github.houbb.spring.lean.core.ioc.event.BookingService:75 - 监听到远程消息后: 1234-result
2018-08-10 18:27:07.964  INFO  [Thread-2] com.github.houbb.spring.lean.core.ioc.event.BookingService:106 - 已经触发event
2018-08-10 18:27:07.964  INFO  [main] com.github.houbb.spring.lean.core.ioc.event.BookingService:67 - 查询结果: 1234-result
2018-08-10 18:27:07.968  INFO  [Thread-1] org.springframework.context.support.GenericApplicationContext:993 - Closing org.springframework.context.support.GenericApplicationContext@5cee5251: startup date [Fri Aug 10 18:27:05 CST 2018]; root of context hierarchy

超时和空循环

空循环

空循环会导致 cpu 飙升

while(true) {
}

• 解决方式

while(true) {// 小睡即可TimeUnit.sleep(1);
}

超时编写

不可能一直等待反馈，可以设置超时时间。

/*** 循环等待直到获取结果* @param key key* @param timeoutInSeconds 超时时间* @param <T> 泛型* @return 结果。如果超时则抛出异常*/
public <T> T loopWaitForValue(final String key, long timeoutInSeconds) {long startTime = System.nanoTime();long deadline = startTime + TimeUnit.SECONDS.toNanos(timeoutInSeconds);//1. 如果没有新回调，或者 key 对应元素不存在。则一直循环while(ObjectUtil.isNull(map.get(key))) {try {TimeUnit.MILLISECONDS.sleep(5);} catch (InterruptedException e) {LOGGER.warn("Loop meet InterruptedException, just ignore it.", e);}// 超时判断long currentTime = System.nanoTime();if(currentTime >= deadline) {throw new BussinessException(ErrorCode.READ_TIME_OUT);}}final T target = (T) map.get(key);LOGGER.debug("loopWaitForValue get value:{} for key:{}", JSON.toJSON(target), key);//2. 获取到元素之后，需要移除掉对应的值map.remove(key);return target;
}

代码地址

loop

countdownlatch

spring-event-listener