如何从0写一个服务网关？

作者：孤独烟

来自：rjzheng.cnblogs.com

0 引言

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。

1 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();        }    }

}

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();                }            }     }}

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();}

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;    }}

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);    }

}

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();        }    }

}

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();    }

}

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);    }}

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";    }}

然后，你就发现可以从localhost:8080/index进行跳转访问了

3 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

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