Spring MVC的困境

我们先看一段工作中大家常见的代码

@RestController
public class TestAController {
@RequestMapping(value ="resource",method = RequestMethod.GET)
public Object processResult(){RestTemplate restTemplateew RestTemplate();∥请求外部资源String result =  restTemplate. getForObject("http://example.com/api/resource2", String.class)return processResultFurther(result);
}private String processResultFurther(String result){return "resource here"
}

Tomcat处理请求，线程状态的变化如下：
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我们发现这里的请求和响应事实上是 同步阻塞。

再深入想一下，如果每个线程的执行时间是不可控的，而Tomcat线程池中的线程数量是有限的…

那该怎么办呢？

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Servlet 异步请求缓解线程池压力

我们来算一下：

TPS : 2000/s
请求耗时:250ms

那么在这种情况下：

tomcat最大线程数配置: 2000/s * 0.25s=500

因此 server. tomcat. threads. max=500 基本能满足需求

那假设 tps 到了 4000 呢？

虽然我们可以扩大线程数量，但线程是要消耗操作系统资源的，也并非越多越好，当然了还有其他很多影响因素。

那怎么办呢？
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Servlet 3.0 异步请求处理

Filter/Servlet在生成响应之前可能要等待一些资源的响应以完成请求处理,比如一个jdbc查询,或者远程服务rpc调用。

在Servlet阻塞等待是一个低效的操作,这将导致受限系统资源急剧紧张,比如线程数、连接数等等

Servlet 3.0引入了异步处理请求的能力,使得线程可以不用阻塞等待,提早返回到容器,从而执行更多的任务请求。把耗时的任务提交给另一个异步线程去执行,以及产生响应。

Code 演示

工程

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pom

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><groupId>com.artisan</groupId><artifactId>servlet-asyn</artifactId><version>1.0-SNAPSHOT</version><properties><maven.compiler.source>8</maven.compiler.source><maven.compiler.target>8</maven.compiler.target><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding></properties><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><version>2.7.4</version></dependency><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>1.18.24</version></dependency></dependencies></project>

配置文件

server.tomcat.threads.max=1

启动类

package com.artisan;import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.ServletComponentScan;/*** @author 小工匠* @version 1.0* @description: 启动类* @date 2022/10/6 12:03* @mark: show me the code , change the world*/@SpringBootApplication
// 使用@ServletComponentScan注解后，Servlet、Filter、Listener可以直接通过@WebServlet、@WebFilter、@WebListener注解自动注册，无需其他代码
@ServletComponentScan
public class ServletAsyncApplication {public static void main(String[] args) {SpringApplication.run(ServletAsyncApplication.class,args);}
}

同步servlet

package com.artisan.servlet;import lombok.extern.slf4j.Slf4j;import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.concurrent.TimeUnit;/*** @author 小工匠* @version 1.0* @description: 同步Servlet请求* @date 2022/10/6 12:06* @mark: show me the code , change the world*/@WebServlet(value = "/sync")
@Slf4j
public class SyncServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {log.info("request: {}, currentThread: {}", req.getQueryString(), Thread.currentThread().getName());processFuture(req, resp);}private void processFuture(HttpServletRequest req, HttpServletResponse resp) {try {TimeUnit.SECONDS.sleep(10);resp.getWriter().println("sync handler");} catch (IOException  | InterruptedException e) {throw new RuntimeException(e);}}}

演示

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异步servlet

package com.artisan.servlet;import com.artisan.handler.AsyncRequestWrapper;
import com.artisan.handler.AsyncServletRejectedHandler;
import com.artisan.handler.AsyncThreadFactory;
import lombok.extern.slf4j.Slf4j;import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;/*** @author 小工匠* @version 1.0* @description: 异步Servlet请求* @date 2022/10/6 15:23* @mark: show me the code , change the world*/@WebServlet(value = "/async", asyncSupported = true)
@Slf4j
public class AsyncServlet extends HttpServlet {private ThreadPoolExecutor executor = new ThreadPoolExecutor(1, 1, 0L, TimeUnit.SECONDS,new LinkedBlockingDeque<>(1),AsyncThreadFactory.builder().threadName("async-thread-pool").build(),new AsyncServletRejectedHandler());@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {log.info("request: {}, currentThread: {}", req.getQueryString(), Thread.currentThread().getName());AsyncContext asyncContext = req.startAsync();AsyncRequestWrapper wrapper = AsyncRequestWrapper.builder().asyncContext(asyncContext).servletRequest(req).servletResponse(resp).thread(Thread.currentThread()).build();executor.execute(wrapper);}
}

辅助Code

【AsyncThreadFactory 】

package com.artisan.handler;import lombok.Builder;import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.atomic.AtomicInteger;/*** @author 小工匠* @version 1.0* @description: 线程池工厂* @date 2022/10/6 15:35* @mark: show me the code , change the world*/@Builder
public class AsyncThreadFactory implements ThreadFactory {private final ThreadFactory threadFactory = Executors.defaultThreadFactory();private String threadName;private final AtomicInteger atomicInteger = new AtomicInteger(1);@Overridepublic Thread newThread(Runnable r) {Thread thread = threadFactory.newThread(r);thread.setName(this.threadName + "-" + atomicInteger.getAndIncrement());return thread;}
}

【AsyncRequestWrapper 】

package com.artisan.handler;import lombok.Builder;
import lombok.Data;
import lombok.extern.slf4j.Slf4j;import javax.servlet.AsyncContext;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import java.io.IOException;
import java.util.concurrent.TimeUnit;/*** @author 小工匠* @version 1.0* @description: 线程包装对象* @date 2022/10/6 15:42* @mark: show me the code , change the world*/@Slf4j
@Data
@Builder
public class AsyncRequestWrapper implements Runnable {private AsyncContext asyncContext;private ServletRequest servletRequest;private ServletResponse servletResponse;private Thread thread;@Overridepublic void run() {processFuture(asyncContext, servletRequest, servletResponse);}private void processFuture(AsyncContext asyncContext, ServletRequest servletRequest, ServletResponse servletResponse) {HttpServletRequest httpServletRequest = (HttpServletRequest)servletRequest;try {TimeUnit.SECONDS.sleep(10);log.info("processFuture 当前处理线程 {}",  Thread.currentThread().getName());servletResponse.getWriter().println("async handler -->" + httpServletRequest.getQueryString());} catch (IOException | InterruptedException e) {throw new RuntimeException(e);}// 完成asyncContext.complete();}
}

【AsyncServletRejectedHandler】

package com.artisan.handler;import lombok.extern.slf4j.Slf4j;import javax.servlet.http.HttpServletRequest;
import java.io.IOException;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;/*** @author 小工匠* @version 1.0* @description: 拒绝策略* @date 2022/10/6 15:24* @mark: show me the code , change the world*/
@Slf4j
public class AsyncServletRejectedHandler  implements RejectedExecutionHandler {@Overridepublic void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {AsyncRequestWrapper wrapper = (AsyncRequestWrapper) r ;HttpServletRequest httpServletRequest = (HttpServletRequest) wrapper.getServletRequest();try {String queryString = httpServletRequest.getQueryString();String threadName = wrapper.getThread().getName();log.info("当前线程:{} , 当前线程请求参数:{}", threadName, queryString);wrapper.getServletResponse().getWriter().println("too many request , current thread:" + threadName + " , current param:" + queryString );} catch (IOException e) {throw new RuntimeException(e);}// 别忘了 completewrapper.getAsyncContext().complete();}
}

演示

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2022-10-06 21:30:09.700  INFO 7860 --- [nio-8080-exec-1] com.artisan.servlet.AsyncServlet         : request: i=1, currentThread: http-nio-8080-exec-1
2022-10-06 21:30:11.277  INFO 7860 --- [nio-8080-exec-1] com.artisan.servlet.AsyncServlet         : request: i=2, currentThread: http-nio-8080-exec-1
2022-10-06 21:30:12.813  INFO 7860 --- [nio-8080-exec-1] com.artisan.servlet.AsyncServlet         : request: i=3, currentThread: http-nio-8080-exec-1
2022-10-06 21:30:12.813  INFO 7860 --- [nio-8080-exec-1] c.a.handler.AsyncServletRejectedHandler  : 当前线程:http-nio-8080-exec-1 , 当前线程请求参数:i=3
2022-10-06 21:30:15.355  INFO 7860 --- [nio-8080-exec-1] com.artisan.servlet.AsyncServlet         : request: i=4, currentThread: http-nio-8080-exec-1
2022-10-06 21:30:15.355  INFO 7860 --- [nio-8080-exec-1] c.a.handler.AsyncServletRejectedHandler  : 当前线程:http-nio-8080-exec-1 , 当前线程请求参数:i=4
2022-10-06 21:30:19.712  INFO 7860 --- [c-thread-pool-1] com.artisan.handler.AsyncRequestWrapper  : processFuture 当前处理线程 async-thread-pool-1
2022-10-06 21:30:29.719  INFO 7860 --- [c-thread-pool-1] com.artisan.handler.AsyncRequestWrapper  : processFuture 当前处理线程 async-thread-pool-1