【原创】自己动手写一个服务网关

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。

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;

@RestController
public 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

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