什么是网关？为什么需要使用网关？


如图所示，在不使用网关的情况下，我们的服务是直接暴露给服务调用方。当调用方增多，势必需要添加定制化访问权限、校验等逻辑。当添加API网关后，再第三方调用端和服务提供方之间就创建了一面墙，这面墙直接与调用方通信进行权限控制。
本文所实现的网关源码抄袭了---Oh,不对，是借鉴。借鉴了Zuul网关的源码，提炼出其核心思路，实现了一套简单的网关源码，博主将其改名为Eatuul。
题外话


本文是业内能搜到的第一篇自己动手实现网关的文章。博主写的手把手系列的文章，目的是在以最简单的方式，揭露出中间件的核心原理，让读者能够迅速了解实现的核心。需要说明的是，这不是源码分析系列的文章，因此写出来的代码，省去了一些复杂的内容，毕竟大家能理解到该中间件的核心原理即可。如果想看源码分析系列的，请关注博主，后期会将spring、spring boot、dubbo、mybatis等开源框架一一揭示。
正文设计思路


先大致说一下，就是定义一个Servlet接收请求。然后经过preFilter(封装请求参数),routeFilter(转发请求)，postFilter(输出内容)。三个过滤器之间，共享request、response以及其他的一些全局变量。如下图所示



# 和真正的Zuul的区别?主要区别有如下几点
(1)Zuul中在异常处理模块，有一个ErrorFilter来处理，博主在实现的时候偷懒了，略去。
(2)Zuul中PreFilters,RoutingFilters,PostFilters默认都实现了一组，具体如下表所示



博主总不可能每一个都给你们实现一遍吧。所以偷懒了，每种只实现一个。但是调用顺序还是不变，按照PreFilters->RoutingFilters->PostFilters的顺序调用
(3)在routeFilters确实有转发请求的Filter,然而博主偷天换日了，改用RestTemplate实现.
代码结构


大家去spring官网上搭建一套springboot的项目，博主就不展示pom的代码了。直接将项目结构展示一下，如下图所示



# EatuulServlet.java。这个是网关的入口，逻辑也十分简单，分为三步
(1)将request,response放入threadlocal中
(2)执行三组过滤器
(3)清除threadlocal中的的环境变量
源码如下
package com.rjzheng.eatuul.http;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;@WebServlet(name = "eatuul", urlPatterns = "/*")public class EatuulServlet extends HttpServlet {   private EatRunner eatRunner = new EatRunner();   @Override   public void service(HttpServletRequest req, HttpServletResponse resp)           throws ServletException, IOException {       //将request，和response放入上下文对象中       eatRunner.init(req, resp);       try {           //执行前置过滤           eatRunner.preRoute();           //执行过滤           eatRunner.route();           //执行后置过滤           eatRunner.postRoute();       } catch (Throwable e) {           RequestContext.getCurrentContext().getResponse()                         .sendError(HttpServletResponse.SC_NOT_FOUND, e.getMessage());       } finally {           //清除变量           RequestContext.getCurrentContext().unset();       }   }}复制代码
EatuulRunner.java。
这个是具体的执行器。需要说明一下，在Zuul中，ZuulRunner在获取具体有哪些过滤器的时候，有一个FileLoader可以动态读取配置加载。博主在实现我们自己的EatuulRunner时候，略去动态读取的过程，直接静态写死。
源码如下

package com.rjzheng.eatuul.http;import java.util.ArrayList;import java.util.List;import java.util.concurrent.ConcurrentHashMap;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import com.rjzheng.eatuul.filter.EatuulFilter;import com.rjzheng.eatuul.filter.post.SendResponseFilter;import com.rjzheng.eatuul.filter.pre.RequestWrapperFilter;import com.rjzheng.eatuul.filter.route.RoutingFilter;public class EatRunner {   //静态写死过滤器   private ConcurrentHashMap<String, List<EatuulFilter>> hashFiltersByType = new ConcurrentHashMap<String, List<EatuulFilter>>(){{         put("pre",new ArrayList<EatuulFilter>(){{           add(new RequestWrapperFilter());       }});       put("route",new ArrayList<EatuulFilter>(){{           add(new RoutingFilter());       }});       put("post",new ArrayList<EatuulFilter>(){{           add(new SendResponseFilter());       }});    }};      public void init(HttpServletRequest req, HttpServletResponse resp) {       RequestContext ctx = RequestContext.getCurrentContext();       ctx.setRequest(req);       ctx.setResponse(resp);   }   public void preRoute() throws Throwable {       runFilters("pre");     }   public void route() throws Throwable{       runFilters("route");       }   public void postRoute() throws Throwable{       runFilters("post");   }       public void runFilters(String sType) throws Throwable {           List<EatuulFilter> list = this.hashFiltersByType.get(sType);           if (list != null) {               for (int i = 0; i < list.size(); i++) {                   EatuulFilter zuulFilter = list.get(i);                   zuulFilter.run();               }           }    }}复制代码
EatuulFilter.java。接下来就是一系列Filter的代码了，先上父类EatuulFilter的源码
package com.rjzheng.eatuul.filter;public abstract class EatuulFilter {   abstract public String filterType();   abstract public int filterOrder();   abstract public void run();}复制代码
RequestWrapperFilter.java。这个是PreFilter,前置执行过滤器，负责封装请求。步骤如下所示
(1)封装请求头
(2)封装请求体
(3)构造出RestTemplate能识别的RequestEntity
(4)将RequestEntity放入全局threadlocal之中
代码如下所示

package com.rjzheng.eatuul.filter.pre;import java.io.IOException;import java.io.InputStream;import java.net.URI;import java.net.URISyntaxException;import java.util.Collections;import java.util.List;import javax.servlet.http.HttpServletRequest;import org.springframework.http.HttpHeaders;import org.springframework.http.HttpMethod;import org.springframework.http.RequestEntity;import org.springframework.util.MultiValueMap;import org.springframework.util.StreamUtils;import com.rjzheng.eatuul.filter.EatuulFilter;import com.rjzheng.eatuul.http.RequestContext;public class RequestWrapperFilter extends EatuulFilter{   @Override   public String filterType() {       // TODO Auto-generated method stub       return "pre";   }   @Override   public int filterOrder() {       // TODO Auto-generated method stub       return -1;   }   @Override   public void run() {       String rootURL = "http://localhost:9090";       RequestContext ctx =RequestContext.getCurrentContext();       HttpServletRequest servletRequest = ctx.getRequest();       String targetURL = rootURL + servletRequest.getRequestURI();       RequestEntity<byte[]> requestEntity = null;       try {           requestEntity = createRequestEntity(servletRequest, targetURL);       } catch (Exception e) {           e.printStackTrace();       }       //4、将requestEntity放入全局threadlocal之中       ctx.setRequestEntity(requestEntity);   }      private RequestEntity createRequestEntity(HttpServletRequest request,String url) throws URISyntaxException, IOException {       String method = request.getMethod();       HttpMethod httpMethod = HttpMethod.resolve(method);       //1、封装请求头       MultiValueMap<String, String> headers =createRequestHeaders(request);       //2、封装请求体       byte[] body = createRequestBody(request);       //3、构造出RestTemplate能识别的RequestEntity       RequestEntity requestEntity = new RequestEntity<byte[]>(body,headers,httpMethod, new URI(url));       return requestEntity;   }      private byte[] createRequestBody(HttpServletRequest request) throws IOException {       InputStream inputStream = request.getInputStream();       return StreamUtils.copyToByteArray(inputStream);   }   private MultiValueMap<String, String> createRequestHeaders(HttpServletRequest request) {       HttpHeaders headers = new HttpHeaders();       List<String> headerNames = Collections.list(request.getHeaderNames());       for(String headerName:headerNames) {           List<String> headerValues = Collections.list(request.getHeaders(headerName));           for(String headerValue:headerValues) {               headers.add(headerName, headerValue);           }       }       return headers;   }}复制代码
RoutingFilter.java。这个是routeFilter,这里我偷懒了，直接做转发请求，并且将返回值ResponseEntity放入全局threadlocal中
package com.rjzheng.eatuul.filter.route;import org.springframework.http.RequestEntity;import org.springframework.http.ResponseEntity;import org.springframework.web.client.RestTemplate;import com.rjzheng.eatuul.filter.EatuulFilter;import com.rjzheng.eatuul.http.RequestContext;public class RoutingFilter extends EatuulFilter{   @Override   public String filterType() {       // TODO Auto-generated method stub       return "route";   }   @Override   public int filterOrder() {       // TODO Auto-generated method stub       return 0;   }      @Override   public void run(){       RequestContext ctx = RequestContext.getCurrentContext();       RequestEntity requestEntity = ctx.getRequestEntity();       RestTemplate restTemplate = new RestTemplate();       ResponseEntity responseEntity = restTemplate.exchange(requestEntity,byte[].class);       ctx.setResponseEntity(responseEntity);   }   }复制代码
SendResponseFilter.java。


这个是postFilters,将ResponseEntity输出即可
package com.rjzheng.eatuul.filter.post;import java.util.List;import java.util.Map;import javax.servlet.ServletOutputStream;import javax.servlet.http.HttpServletResponse;import org.springframework.http.HttpHeaders;import org.springframework.http.ResponseEntity;import com.rjzheng.eatuul.filter.EatuulFilter;import com.rjzheng.eatuul.http.RequestContext;public class SendResponseFilter extends EatuulFilter{   @Override   public String filterType() {       return "post";   }   @Override   public int filterOrder() {       return 1000;   }   @Override   public void run() {       try {           addResponseHeaders();           writeResponse();       } catch (Exception e) {           e.printStackTrace();       }   }   private void addResponseHeaders() {       RequestContext ctx = RequestContext.getCurrentContext();       HttpServletResponse servletResponse = ctx.getResponse();       ResponseEntity responseEntity = ctx.getResponseEntity();       HttpHeaders httpHeaders = responseEntity.getHeaders();       for(Map.Entry<String, List<String>> entry:httpHeaders.entrySet()) {           String headerName = entry.getKey();           List<String> headerValues = entry.getValue();           for(String headerValue:headerValues) {               servletResponse.addHeader(headerName, headerValue);           }       }   }   private void writeResponse()throws Exception {       RequestContext ctx = RequestContext.getCurrentContext();       HttpServletResponse servletResponse = ctx.getResponse();       if (servletResponse.getCharacterEncoding() == null) { // only set if not set           servletResponse.setCharacterEncoding("UTF-8");       }       ResponseEntity responseEntity = ctx.getResponseEntity();       if(responseEntity.hasBody()) {           byte[] body = (byte[]) responseEntity.getBody();           ServletOutputStream outputStream = servletResponse.getOutputStream();           outputStream.write(body);           outputStream.flush();       }   }}复制代码
RequestContext.java。


最后是一直在说的全局threadlocal变量
package com.rjzheng.eatuul.http;import java.util.HashMap;import java.util.Map;import java.util.concurrent.ConcurrentHashMap;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.springframework.http.RequestEntity;import org.springframework.http.ResponseEntity;public class RequestContext extends ConcurrentHashMap<String, Object> {   protected static Class<? extends RequestContext> contextClass = RequestContext.class;   protected static final ThreadLocal<? extends RequestContext> threadLocal = new ThreadLocal<RequestContext>() {       @Override       protected RequestContext initialValue() {           try {               return contextClass.newInstance();           } catch (Throwable e) {               throw new RuntimeException(e);           }       }   };   public static RequestContext getCurrentContext() {       RequestContext context = threadLocal.get();       return context;   }   public HttpServletRequest getRequest() {       return (HttpServletRequest) get("request");   }   public void setRequest(HttpServletRequest request) {       put("request", request);   }   public HttpServletResponse getResponse() {       return (HttpServletResponse) get("response");   }   public void setResponse(HttpServletResponse response) {       set("response", response);   }      public void setRequestEntity(RequestEntity requestEntity){       set("requestEntity",requestEntity);   }      public RequestEntity getRequestEntity() {       return (RequestEntity) get("requestEntity");   }      public void setResponseEntity(ResponseEntity responseEntity){       set("responseEntity",responseEntity);   }      public ResponseEntity getResponseEntity() {       return (ResponseEntity) get("responseEntity");   }      public void set(String key, Object value) {       if (value != null)           put(key, value);       else           remove(key);   }   public void unset() {       threadLocal.remove();   }}复制代码
# 如何测试？

自己另外起一个server端口为9090如下所示
package com.rjzheng.eatservice;import org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.boot.builder.SpringApplicationBuilder;import org.springframework.boot.web.servlet.ServletComponentScan;import com.rjzheng.eatservice.controller.IndexController;@SpringBootApplication@ServletComponentScan(basePackageClasses = IndexController.class)public class Application {   public static void main(String[] args) {       new SpringApplicationBuilder(Application.class).properties("server.port=9090").run(args);   }}复制代码
再来一个controller
package com.rjzheng.eatservice.controller;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.RestController;@RestControllerpublic class IndexController {      @RequestMapping("/index")   public String index() {       return "hello!world";   }}复制代码
然后，你就发现可以从localhost:8080/index进行跳转访问了
结论


本文模拟了一下zuul网关的源码，借鉴了一下其精髓的部分。希望大家能有所收获
出处： http://rjzheng.cnblogs.com/


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链接：https://juejin.im/post/5bcc8053e51d457a1179d4ba