Undertow 作为一个web服务器，是支持反向代理的，比较适用小型平台或者开发测试场景， 以下是使用记录

首先在pom中引用undertow作为web服务器，我使用的springboot项目，配置参考

<dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><exclusions><exclusion><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-tomcat</artifactId></exclusion></exclusions></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-undertow</artifactId></dependency><!--  <dependency><groupId>org.mitre.dsmiley.httpproxy</groupId><artifactId>smiley-http-proxy-servlet</artifactId><version>1.11</version></dependency>--></dependencies>

注： 上面注释的部分，是因为开始用httpclient 做代理，使用也比较简单，有兴趣可以百度一下，后来猜想undertow应该具有反向代理的功能，再引入smilley就有些多余了

undertow 默认的反向代理只能代理整个服务，不能类似nginx 代理某个具体路径，而恰巧我需要该项目能够将 /base路径开始的请求代理转发到其他服务，故而这里取了个巧，先看代码

package com.iflytek.research.datawood.config;import java.net.URI;import javax.annotation.Resource;import org.springframework.boot.web.embedded.undertow.UndertowServletWebServerFactory;
import org.springframework.boot.web.server.WebServerFactoryCustomizer;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;import io.undertow.server.HandlerWrapper;
import io.undertow.server.HttpHandler;
import io.undertow.server.HttpServerExchange;
import io.undertow.server.handlers.DisallowedMethodsHandler;
import io.undertow.server.handlers.proxy.ProxyClient;
import io.undertow.server.handlers.proxy.ProxyHandler;
import io.undertow.server.handlers.proxy.SimpleProxyClientProvider;
import io.undertow.util.HttpString;/*** @author ljgeng**/
@Configuration
public class UndertowConfiguration {@Resourceprivate ProxyProperties ProxyProperties;@Beanpublic WebServerFactoryCustomizer<UndertowServletWebServerFactory> custom(){return new WebServerFactoryCustomizer<UndertowServletWebServerFactory>() {@Overridepublic void customize(UndertowServletWebServerFactory factory) {factory.addDeploymentInfoCustomizers(deploymentInfo -> {deploymentInfo.addInitialHandlerChainWrapper(new GatewayProxyHandlerWrapper());});}}; } /*** Gateway  反向代理*  http://undertow.io/undertow-docs/undertow-docs-2.0.0/index.html#reverse-proxy* @author ljgeng**/public class GatewayProxyHandlerWrapper implements HandlerWrapper {@Overridepublic HttpHandler wrap(HttpHandler handler) {// 代理路径ProxyClient proxyClientProvider = new ProxyClientProvider(URI.create(ProxyProperties.getGateway().getUrl()));HttpString[] disallowedHttpMethods = { HttpString.tryFromString("TRACE"),HttpString.tryFromString("TRACK") };return new ProxyHandler(proxyClientProvider, 0,  new DisallowedMethodsHandler(handler, disallowedHttpMethods));}}public class ProxyClientProvider extends SimpleProxyClientProvider {public ProxyClientProvider(URI uri) {super(uri);}/***  重写findTarget 方法： 指定转发的路径。*/@Overridepublic ProxyTarget findTarget(HttpServerExchange exchange) {// 代理/base路径if (exchange.getRequestPath().startsWith("/base")) {// 修改路径，去除baseexchange.setResolvedPath("/base");return super.findTarget(exchange);}return null;}}}

主要看ProxyClientProvider，它重新实现了SimpleProxyClientProvider findTarget方法，当请求路径以/base开始时调用父类方法，否则返回null. 方法exchange.setResolvedPath("/base") 是让代理的路径不再包含base。 可能有人会好奇，为什么这里返回null就可以绕过代理，弄清这个问题的话就要看下ProxyHandler的handleRequest方法到底做了什么（undertow通过handler的handleRequest对请求进行下一步处理）

public void handleRequest(final HttpServerExchange exchange) throws Exception {final ProxyClient.ProxyTarget target = proxyClient.findTarget(exchange);if (target == null) {log.debugf("No proxy target for request to %s", exchange.getRequestURL());next.handleRequest(exchange);return;}if(exchange.isResponseStarted()) {//we can't proxy a request that has already started, this is basically a server configuration errorUndertowLogger.REQUEST_LOGGER.cannotProxyStartedRequest(exchange);exchange.setStatusCode(StatusCodes.INTERNAL_SERVER_ERROR);exchange.endExchange();return;}final long timeout = maxRequestTime > 0 ? System.currentTimeMillis() + maxRequestTime : 0;int maxRetries = maxConnectionRetries;if(target instanceof ProxyClient.MaxRetriesProxyTarget) {maxRetries = Math.max(maxRetries, ((ProxyClient.MaxRetriesProxyTarget) target).getMaxRetries());}final ProxyClientHandler clientHandler = new ProxyClientHandler(exchange, target, timeout, maxRetries, idempotentRequestPredicate);if (timeout > 0) {final XnioExecutor.Key key = WorkerUtils.executeAfter(exchange.getIoThread(), new Runnable() {@Overridepublic void run() {clientHandler.cancel(exchange);}}, maxRequestTime, TimeUnit.MILLISECONDS);exchange.putAttachment(TIMEOUT_KEY, key);exchange.addExchangeCompleteListener(new ExchangeCompletionListener() {@Overridepublic void exchangeEvent(HttpServerExchange exchange, NextListener nextListener) {key.remove();nextListener.proceed();}});}exchange.dispatch(exchange.isInIoThread() ? SameThreadExecutor.INSTANCE : exchange.getIoThread(), clientHandler);}

可以很明显看到，如果target是null, 就走下一个handler 不再继续走代理逻辑了。