摘要：最近要做一个将K8s中的某组件UI通过反向代理映射到自定义规定的链接地址上，提供给用户拜访的需要。所以顺便钻研了一下Jetty的ProxyServlet。
本文分享自华为云社区《Jetty自定义ProxyServlet实现反向代理服务（含源码剖析）》，作者： 小焱 。
一、背景概述
最近要做一个将K8s中的某组件UI通过反向代理映射到自定义规定的链接地址上，提供给用户拜访的需要。所以顺便钻研了一下Jetty的ProxyServlet。在这里做一下剖析，如果有了解不到位的还心愿能够补充斧正。
二、Jetty 的根本架构
Jetty 是一个Servlet 引擎，它的架构比较简单，也是一个可扩展性和非常灵活的应用服务器，它有一个根本数据模型，这个数据模型就是 Handler，所有能够被扩大的组件都能够作为一个 Handler，增加到 Server 中，Jetty 就是帮你治理这些 Handler。
整个 Jetty 的外围组件由 Server 和 Connector 两个组件形成，整个 Server 组件是基于 Handler 容器工作的，Jetty 中另外一个比不可少的组件是 Connector，它负责承受客户端的连贯申请，并将申请调配给一个解决队列去执行。Jetty 中还有一些可有可无的组件，咱们能够在它上做扩大。如 JMX，咱们能够定义一些 Mbean 把它加到 Server 中，当 Server 启动的时候，这些 Bean 就会一起工作。
整个 Jetty 的外围是围绕着 Server 类来构建，Server 类继承了 Handler，关联了 Connector 和 Container。Container 是治理 Mbean 的容器。Jetty 的 Server 的扩大次要是实现一个个 Handler 并将 Handler 加到 Server 中，Server 中提供了调用这些 Handler 的拜访规定。整个 Jetty 的所有组件的生命周期治理是基于观察者模板设计，实现LifeCycle。
三、Handler 的体系结构
Jetty 次要是基于 Handler 来设计的，Handler 的体系结构影响着整个 Jetty 的方方面面。上面总结了一下 Handler 的品种及作用：
Jetty 次要提供了两种 Handler 类型，一种是 HandlerWrapper，它能够将一个 Handler 委托给另外一个类去执行，如咱们要将一个 Handler 加到 Jetty 中，那么就必须将这个 Handler 委托给 Server 去调用。配合 ScopeHandler 类咱们能够拦挡 Handler 的执行，在调用 Handler 之前或之后，能够做一些另外的事件，相似于 Tomcat 中的 Valve；另外一个 Handler 类型是 HandlerCollection，这个 Handler 类能够将多个 Handler 组装在一起，形成一个 Handler 链，不便咱们做扩大。
四、编码设计
这里我提供一个设计框架，具体内容能够依据须要自定义。
public class RestApi { private static final Logging LOGGER = Logging.getLogging(RestApi.class.getName()); private Server server; /** * 启动办法，须要在程序启动时调用该办法 */ public void start() { try { ContextHandlerCollection collection = new ContextHandlerCollection(); WebAppContext appContext = new WebAppContext(); appContext.setContextPath("/"); // 设置资源文件地址，可略 appContext.setResourceBase("/opt/appHome/myDemo/webapp"); // 设置web.xml，可在外面进行一些Servlet配置，可略 appContext.setDescriptor("/opt/appHome/myDemo/webapp/WEB-INF/web.xml"); appContext.setParentLoaderPriority(true); collection.addHandler(appContext); addProxyHandler(collection); server = new Server(8080); server.setHandler(collection); server.start(); } catch (Throwable t) { LOGGER.error("Start RESTful API server failed", t); } } private static void addProxyHandler(ContextHandlerCollection collection) { ProxyServlet proxyServlet = new WebProxyServlet(); // 增加自定义ProxyServlet ServletHolder holder = new ServletHolder(proxyServlet); holder.setInitParameter("idleTimeout", 120000); // 设置闲暇开释工夫 holder.setInitParameter("timeout", 300000); // 设置超时工夫 holder.setInitParameter("maxConnections", 256); // 设置最大连接数 ServletContextHandler contextHandler = new ServletContextHandler(); contextHandler.addServlet(holder, "/proxy/*"); contextHandler.setContextPath("/demo"); collection.addHandler(contextHandler); }}自定义 ProxyServlet，在此列出一部分罕用的重写办法，还有很多办法能够查问文档自行重写
public class WebProxyServlet extends ProxyServlet { private static final Logging LOGGING = Logging.getLogging(WebProxyServlet.class); /** * 自定义指标地址重写办法 */ @Override protected String rewriteTarget(HttpServletRequest request) { } /** * 自定义重写错误处理办法 */ @Override protected void onProxyRewriteFailed(HttpServletRequest clientRequest, HttpServletResponse clientResponse) { } /** * 自定义response错误处理办法 */ @Override protected void onProxyResponseFailure( HttpServletRequest clientRequest, HttpServletResponse proxyResponse, Response serverResponse, Throwable failure) { } /** * 自定义response头filter */ @Override protected String filterServerResponseHeader( HttpServletRequest clientRequest, Response serverResponse, String headerName, String headerValue) { } /** * 自定义头XForwarded设置 */ @Override protected void addXForwardedHeaders(HttpServletRequest clientRequest, Request proxyRequest) { }}五、申请处理过程
上面通过跟踪源码初步梳理了一下，从 request 申请进入到返回 response 的整个流程
六、源码剖析
1、Request 转发局部
当 Jetty 接管到一个申请时，Jetty 就把这个申请交给在 Server 中注册的 ContextHandlerCollection 去执行，查看 Service 的 handle 办法源码
public void handle(HttpChannel channel) throws IOException, ServletException { String target = channel.getRequest().getPathInfo(); Request request = channel.getRequest(); Response response = channel.getResponse(); if (LOG.isDebugEnabled()) { LOG.debug("{} {} {} on {}", new Object[]{request.getDispatcherType(), request.getMethod(), target, channel}); } if (!HttpMethod.OPTIONS.is(request.getMethod()) && !"*".equals(target)) { this.handle(target, request, request, response); } else if (!HttpMethod.OPTIONS.is(request.getMethod())) { request.setHandled(true); response.sendError(400); } else { this.handleOptions(request, response); if (!request.isHandled()) { this.handle(target, request, request, response); } } if (LOG.isDebugEnabled()) { LOG.debug("handled={} async={} committed={} on {}", new Object[]{request.isHandled(), request.isAsyncStarted(), response.isCommitted(), channel}); } }这里调用的 this.handle(target, request, request, response) 办法其实是父类 HandlerWrapper 的 handle 办法
public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { Handler handler = this._handler; if (handler != null) { handler.handle(target, baseRequest, request, response); } }创立 server 时曾调用过 server.setHandler(collection) ，所以这里就调用到了 ContextHandlerCollection 的 handle 办法
public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { ContextHandlerCollection.Mapping mapping = (ContextHandlerCollection.Mapping)this._handlers.get(); if (mapping != null) { Handler[] handlers = mapping.getHandlers(); if (handlers != null && handlers.length != 0) { if (handlers.length == 1) { handlers[0].handle(target, baseRequest, request, response); } else { HttpChannelState async = baseRequest.getHttpChannelState(); if (async.isAsync()) { ContextHandler context = async.getContextHandler(); if (context != null) { Handler branch = (Handler)mapping._contextBranches.get(context); if (branch == null) { context.handle(target, baseRequest, request, response); } else { branch.handle(target, baseRequest, request, response); } return; } } int limit; if (target.startsWith("/")) { Trie<Entry<String, ContextHandlerCollection.Branch[]>> pathBranches = mapping._pathBranches; if (pathBranches == null) { return; } int l; for(limit = target.length() - 1; limit >= 0; limit = l - 2) { Entry<String, ContextHandlerCollection.Branch[]> branches = (Entry)pathBranches.getBest(target, 1, limit); if (branches == null) { break; } l = ((String)branches.getKey()).length(); if (l == 1 || target.length() == l || target.charAt(l) == '/') { ContextHandlerCollection.Branch[] var12 = (ContextHandlerCollection.Branch[])branches.getValue(); int var13 = var12.length; for(int var14 = 0; var14 < var13; ++var14) { ContextHandlerCollection.Branch branch = var12[var14]; branch.getHandler().handle(target, baseRequest, request, response); if (baseRequest.isHandled()) { return; } } } } } else { Handler[] var17 = handlers; limit = handlers.length; for(int var19 = 0; var19 < limit; ++var19) { Handler handler = var17[var19]; handler.handle(target, baseRequest, request, response); if (baseRequest.isHandled()) { return; } } } } } }}从下面的源码能够看出 ContextHandlerCollection 的 handle 办法持续调用了 collection.addHandler 设置进来 ServletContextHandler 的 handle 办法，通过跟踪，能够找到其实这里调用了父类 ScopedHandler 的 handle --> doScope --> nextScope
public final void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { if (this.isStarted()) { if (this._outerScope == null) { this.doScope(target, baseRequest, request, response); } else { this.doHandle(target, baseRequest, request, response); } } } public void doScope(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { this.nextScope(target, baseRequest, request, response); } public final void nextScope(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { if (this._nextScope != null) { this._nextScope.doScope(target, baseRequest, request, response); } else if (this._outerScope != null) { this._outerScope.doHandle(target, baseRequest, request, response); } else { this.doHandle(target, baseRequest, request, response); } }查看 ServletContextHandler 能够找到次要注册了以下三个 handler，均为 ScopedHandler 的子类，也就是 nextScope 办法中的 this._nextScope
protected SessionHandler _sessionHandler;protected SecurityHandler _securityHandler;protected ServletHandler _servletHandler;SessionHandler 是对 ServletHandler 进行了一层包装（装璜器模式），用于一些session的预处理什么的，而SecurityHandler从名字剖析是做一些平安相干的，这两个具体就不剖析了，间接来看 ServletHandler 的 doScope 办法
public void doScope(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { String old_servlet_path = baseRequest.getServletPath(); String old_path_info = baseRequest.getPathInfo(); DispatcherType type = baseRequest.getDispatcherType(); ServletHolder servletHolder = null; Scope oldScope = null; MappedResource<ServletHolder> mapping = this.getMappedServlet(target); if (mapping != null) { servletHolder = (ServletHolder)mapping.getResource(); if (mapping.getPathSpec() != null) { PathSpec pathSpec = mapping.getPathSpec(); String servletPath = pathSpec.getPathMatch(target); String pathInfo = pathSpec.getPathInfo(target); if (DispatcherType.INCLUDE.equals(type)) { baseRequest.setAttribute("javax.servlet.include.servlet_path", servletPath); baseRequest.setAttribute("javax.servlet.include.path_info", pathInfo); } else { baseRequest.setServletPath(servletPath); baseRequest.setPathInfo(pathInfo); } } } if (LOG.isDebugEnabled()) { LOG.debug("servlet {}|{}|{} -> {}", new Object[]{baseRequest.getContextPath(), baseRequest.getServletPath(), baseRequest.getPathInfo(), servletHolder}); } try { oldScope = baseRequest.getUserIdentityScope(); baseRequest.setUserIdentityScope(servletHolder); this.nextScope(target, baseRequest, request, response); } finally { if (oldScope != null) { baseRequest.setUserIdentityScope(oldScope); } if (!DispatcherType.INCLUDE.equals(type)) { baseRequest.setServletPath(old_servlet_path); baseRequest.setPathInfo(old_path_info); } }}这里对 baseRequest 做了一些设置，将注册进来的 ServletHolder set 进了 baseRequest，之后又持续调用了 this.nextScope(target, baseRequest, request, response) ，依据下面的 nextScope 办法，所有 scope 执行完，则执行 doHandle 办法，持续跳过 SessionHandler 和 SecurityHandler，来看下ServletHandler 的 doHandle 办法
public void doHandle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { ServletHolder servletHolder = (ServletHolder)baseRequest.getUserIdentityScope(); FilterChain chain = null; if (servletHolder != null && this._filterMappings != null && this._filterMappings.length > 0) { chain = this.getFilterChain(baseRequest, target.startsWith("/") ? target : null, servletHolder); } if (LOG.isDebugEnabled()) { LOG.debug("chain={}", new Object[]{chain}); } try { if (servletHolder == null) { this.notFound(baseRequest, request, response); } else { ServletRequest req = request; if (request instanceof ServletRequestHttpWrapper) { req = ((ServletRequestHttpWrapper)request).getRequest(); } ServletResponse res = response; if (response instanceof ServletResponseHttpWrapper) { res = ((ServletResponseHttpWrapper)response).getResponse(); } servletHolder.prepare(baseRequest, (ServletRequest)req, (ServletResponse)res); if (chain != null) { chain.doFilter((ServletRequest)req, (ServletResponse)res); } else { servletHolder.handle(baseRequest, (ServletRequest)req, (ServletResponse)res); } } } finally { if (servletHolder != null) { baseRequest.setHandled(true); } }}doHandle 次要是取出注册的 FilterChain ServletHolder，如果存在 Filter，先执行 chain.doFilter 办法，否则执行 servletHolder.handle 我没有设置 filter 所有就间接看 ServletHolder 的 handle 办法了
public void handle(Request baseRequest, ServletRequest request, ServletResponse response) throws ServletException, UnavailableException, IOException { try { Servlet servlet = this.getServletInstance(); if (servlet == null) { throw new UnavailableException("Servlet Not Initialized"); } servlet.service(request, response); } catch (UnavailableException var5) { this.makeUnavailable(var5).service(request, response); } }这里调用了 ServletHolder 中 Servlet 的 service 办法，也就是走到了咱们自定义类 WebProxyServlet 类，因为没有重写，所以这里调用的是 ProxyServlet 的 service 办法
protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { int requestId = this.getRequestId(request); String rewrittenTarget = this.rewriteTarget(request); if (this._log.isDebugEnabled()) { StringBuffer uri = request.getRequestURL(); if (request.getQueryString() != null) { uri.append("?").append(request.getQueryString()); } if (this._log.isDebugEnabled()) { this._log.debug("{} rewriting: {} -> {}", new Object[]{requestId, uri, rewrittenTarget}); } } if (rewrittenTarget == null) { this.onProxyRewriteFailed(request, response); } else { Request proxyRequest = this.newProxyRequest(request, rewrittenTarget); this.copyRequestHeaders(request, proxyRequest); this.addProxyHeaders(request, proxyRequest); AsyncContext asyncContext = request.startAsync(); asyncContext.setTimeout(0L); proxyRequest.timeout(this.getTimeout(), TimeUnit.MILLISECONDS); if (this.hasContent(request)) { if (this.expects100Continue(request)) { DeferredContentProvider deferred = new DeferredContentProvider(new ByteBuffer[0]); proxyRequest.content(deferred); proxyRequest.attribute(CONTINUE_ACTION_ATTRIBUTE, () -> { try { ContentProvider provider = this.proxyRequestContent(request, response, proxyRequest); (new ProxyServlet.DelegatingContentProvider(request, proxyRequest, response, provider, deferred)).iterate(); } catch (Throwable var6) { this.onClientRequestFailure(request, proxyRequest, response, var6); } }); } else { proxyRequest.content(this.proxyRequestContent(request, response, proxyRequest)); } } this.sendProxyRequest(request, response, proxyRequest); }}至此调用到了咱们重写的最要害的办法 rewriteTarget 此办法能够自定义逻辑将 request 的地址解析，返回要代理到的指标地址，应用指标地址组成proxyRequest 最初调用 sendProxyRequest 实现代理转发。
2、Response 接管局部
如果持续跟 sendProxyRequest 会看到创立了一个 ProxyResponseListener，这里具体就不具体跟踪了，次要讲一下流程，有趣味的能够自行入手看一下。Response 返回会通过反射机制触发 onHeader 办法 ProxyServlet 重写了该办法并跳转到了 onServerResponseHeaders 办法
public void onHeaders(Response proxyResponse) { ProxyServlet.this.onServerResponseHeaders(this.request, this.response, proxyResponse); }这个办法是设置 Response 的 header 内容的，其中有获取 headerValue 调用了 this.filterServerResponseHeader 办法，咱们也能够通过重写此办法自定义返回体的 headerValue。
七、总结
到这里 Jetty 的 ProxyServlet 运行原理和自定义办法大抵梳理结束。还有许多漏掉的和了解不到位的中央，心愿大家能够提出斧正。工作中偶然抽出一点工夫读一下源码，既能够晋升对所用技术的了解，又能够学习观赏这些框架的奇妙设计，还是十分有意义的。
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