AsyncContext优雅实现HTTP长轮询接口
一、背景
接到一个需求，实现方案时需要提供一个HTTP接口，接口需要hold住5-8秒，轮询查询数据库，一旦数据库中值有变化，取出变化的值进行处理，处理完成后返回响应。这不就是长轮询吗，如何优雅的实现呢？
二、方案设计
在 Spring 中，AsyncContext 是用于支持异步处理的一个重要的特性。它允许我们在 servlet 请求处理过程中，将长时间运行的操作放在一个单独的线程中执行，而不会阻塞其他请求的处理。
AsyncContext 在以下两种情况下特别有用：
-
长时间运行的操作：当我们需要执行一些耗时的操作，例如网络请求、数据库查询或其他 I/O 操作时，通过将这些操作放在一个新的线程中，可以避免阻塞 servlet 容器中的线程，提高应用的并发性能。
-
推送异步响应：有时候，我们可能需要推送异步产生的响应，而不是等到所有操作都完成后再下发响应。通过 AsyncContext，我们可以在任何时间点上触发异步响应，将结果返回给客户端。
使用 AsyncContext 的步骤如下：
- 在 servlet 中启用异步模式：在 servlet 中，通过调用
startAsync()方法，可以获取到当前请求的 AsyncContext 对象，从而启用异步处理模式。
HttpServletRequest request = ...;
AsyncContext asyncContext = request.startAsync();
- 指定异步任务：通过调用 AsyncContext 对象的
start()方法，在新的线程中执行需要异步处理的任务。
asyncContext.start(() -> {
// 异步任务逻辑
});
- 提交响应：在异步任务完成后，可以调用 AsyncContext 对象的
complete()方法，以表示异步操作完成。
asyncContext.complete();
需要注意的是，我们在使用 AsyncContext 时需要特别注意线程安全。由于异步任务在单独的线程中执行，所以可能存在并发问题。因此，在编写异步任务逻辑时，需要注意线程安全性，使用合适的同步措施。
另外，AsyncContext 也支持超时设置、错误处理、事件监听等功能，这些可以通过相应的方法和回调进行配置。可以根据具体的需求使用这些功能来优化异步处理的逻辑。
总结来说，Spring 的 AsyncContext 提供了方便的异步处理机制，可以提高应用的并发性能，并支持推送异步响应，使得应用更具有响应性和可伸缩性。
三、代码1
import com.google.common.util.concurrent.ThreadFactoryBuilder;
import lombok.extern.slf4j.Slf4j;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import javax.annotation.Resource;
import javax.servlet.AsyncContext;
import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.concurrent.*;
@RestController
@RequestMapping("/api/byai/transform")
@Slf4j
public class AsyncTestController {
@Resource
private RedisTemplate redisTemplate;
private final ExecutorService timeoutChecker = new ThreadPoolExecutor(1,1,1000,TimeUnit.SECONDS,new ArrayBlockingQueue<>(1000));
private static final ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat("longPolling-timeout-checker-%d").build();
// private static boolean result = false;
@PostMapping("/async")
public void async(HttpServletRequest request, HttpServletResponse response) {
// 创建AsyncContext
AsyncContext asyncContext = request.startAsync(request, response);
// 设置处理超时时间8s
asyncContext.setTimeout(8000L);
// asyncContext监听
JdAsyncTestListener asyncListener = new JdAsyncTestListener(redisTemplate,asyncContext);
asyncContext.addListener(asyncListener);
// 定时处理业务，处理成功后asyncContext.complete();完成异步请求
asyncContext.start(asyncListener);
}
// 模拟业务处理完成
@PostMapping("/set")
public ResultModel notify(String key, String value) {
redisTemplate.opsForValue().set(key, value);
return ResultModel.success();
}
@PostMapping("/get")
public ResultModel get(String key) {
String s = redisTemplate.opsForValue().get(key);
return ResultModel.success(s);
}
@PostMapping("/del")
public ResultModel del(String key) {
redisTemplate.delete(key);
return ResultModel.success();
}
} import com.google.common.util.concurrent.ThreadFactoryBuilder;
import lombok.extern.slf4j.Slf4j;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import javax.annotation.Resource;
import javax.servlet.AsyncContext;
import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.concurrent.*;
@RestController
@RequestMapping("/api/test")
@Slf4j
public class AsyncTestController {
@Resource
private RedisTemplate redisTemplate;
private final ExecutorService timeoutChecker = new ThreadPoolExecutor(1,1,1000,TimeUnit.SECONDS,new ArrayBlockingQueue<>(1000));
private static final ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat("longPolling-timeout-checker-%d").build();
// private static boolean result = false;
@PostMapping("/async")
public void async(HttpServletRequest request, HttpServletResponse response) {
// 创建AsyncContext
AsyncContext asyncContext = request.startAsync(request, response);
// 设置处理超时时间8s
asyncContext.setTimeout(8000L);
// asyncContext监听
JdAsyncTestListener asyncListener = new JdAsyncTestListener(redisTemplate,asyncContext);
asyncContext.addListener(asyncListener);
// 定时处理业务，处理成功后asyncContext.complete();完成异步请求
asyncContext.start(asyncListener);
}
// 模拟业务处理完成
@PostMapping("/set")
public ResultModel notify(String key, String value) {
redisTemplate.opsForValue().set(key, value);
return ResultModel.success();
}
@PostMapping("/get")
public ResultModel get(String key) {
String s = redisTemplate.opsForValue().get(key);
return ResultModel.success(s);
}
@PostMapping("/del")
public ResultModel del(String key) {
redisTemplate.delete(key);
return ResultModel.success();
}
} import lombok.extern.slf4j.Slf4j;
import org.springframework.data.redis.core.RedisTemplate;
import javax.servlet.AsyncContext;
import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
import java.io.IOException;
@Slf4j
public class JdAsyncTestListener implements AsyncListener,Runnable {
boolean isComplete;
private RedisTemplate redisTemplate;
private AsyncContext asyncContext;
public JdAsyncTestListener(RedisTemplate redisTemplate, AsyncContext asyncContext) {
this.redisTemplate = redisTemplate;
this.asyncContext = asyncContext;
}
@Override
public void run() {
try {
while(true){
if(isComplete){
log.info("已经退出");
break;
}
boolean b = redisTemplate.opsForValue().get(1) != null;
log.info("获取标志位:"+b);
Thread.sleep(300);
if (b) {
asyncContext.getResponse().getWriter().print(1);
asyncContext.complete();
}
}
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
@Override
public void onComplete(AsyncEvent asyncEvent) throws IOException {
log.info("结束了");
isComplete = true;
}
@Override
public void onTimeout(AsyncEvent asyncEvent) throws IOException {
log.info("超时了");
isComplete = true;
}
@Override
public void onError(AsyncEvent asyncEvent) throws IOException {
}
@Override
public void onStartAsync(AsyncEvent asyncEvent) throws IOException {
}
} 四、代码二
import com.alibaba.fastjson.JSON;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.util.concurrent.ThreadFactoryBuilder;
import lombok.extern.slf4j.Slf4j;
import org.apache.commons.lang3.StringUtils;
import org.apache.dubbo.config.annotation.Reference;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.validation.annotation.Validated;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import javax.annotation.Resource;
import javax.servlet.AsyncContext;
import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.List;
import java.util.Map;
import java.util.concurrent.*;
@Validated
@RestController
@RequestMapping("/api/test")
@Slf4j
public class TestController {
@Resource
private RedisTemplate redisTemplate;
private final ScheduledExecutorService timeoutChecker = new ScheduledThreadPoolExecutor(10, threadFactory);
private static final ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat("longPolling-timeout-checker-%d").build();
private static boolean result = false;
private final boolean isTimeout = false;
/**
* 消息
*
* @return
*/
@PostMapping("/test")
public void callback(@RequestBody TestLongPollRequest testLongPollRequest, HttpServletRequest request, HttpServletResponse response) {
// 创建AsyncContext
AsyncContext asyncContext = request.startAsync(request, response);
String jdCustomerId = jdLongPollRequest.getJdCustomerId();
// 设置处理超时时间8s
asyncContext.setTimeout(8000L);
// asyncContext监听
asyncContext.addListener(new AsyncListener() {
@Override
public void onComplete(AsyncEvent asyncEvent) throws IOException {
log.info("onComplete={}", asyncEvent);
}
@Override
public void onTimeout(AsyncEvent asyncEvent) throws IOException {
log.info("onTimeout={}", asyncEvent);
ConcurrentHashMap map = new ConcurrentHashMap<>();
map.put("code", "500"); asyncContext.getResponse().getWriter().print(JSON.toJSONString(map));
asyncContext.complete();
}
@Override
public void onError(AsyncEvent asyncEvent) throws IOException {
log.info("onError={}", asyncEvent);
}
@Override
public void onStartAsync(AsyncEvent asyncEvent) throws IOException {
log.info("onStartAsync={}", asyncEvent);
}
});
// 定时处理业务，处理成功后asyncContext.complete();完成异步请求
timeoutChecker.scheduleAtFixedRate(() -> {
try {
String redisKey = getcustomerProcessRes(customerId);
String redisValue = redisTemplate.opsForValue().get(redisKey);
result = StringUtils.isNotBlank(redisValue);
if (result) {
//todo 长轮询查询数据库。通过customerId查询
send(customerId, redisValue);
ConcurrentHashMap map = new ConcurrentHashMap<>();
map.put("code", "200");
map.put("msg", redisValue);
asyncContext.getResponse().getWriter().print(JSON.toJSONString(map));
asyncContext.complete();
}
} catch (IOException e) {
e.printStackTrace();
}
}, 0, 100L, TimeUnit.MILLISECONDS);
}
/**
* 发送消息
*/
private void send(String customerId, String content) {
}
}