Hadoop源码解读-Http服务器Jetty的使用
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Hadoop内嵌了Http服务器Jetty，主要有以下两方面的作用
1、Web访问接口，用于展示Hadoop的内部状态
2、参与Hadoop集群的运行和管理
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以Namenode为例
Namenode通过
startHttpServer(conf);
?来启动HttpServer（Jetty），具体代码如下
httpServer = new HttpServer("hdfs", infoHost, infoPort, infoPort == 0, conf, SecurityUtil.getAdminAcls(conf, DFSConfigKeys.DFS_ADMIN));? 深入HttpServer可以看到，上面的代码是将HADOOP_HOME\webapp下面的hdfs目录作为了jetty的默认Context（datanode就用datanode目录，jobtracker为job目录，tasktracker为task目录，secondarynamenode为secondary目录）
webAppContext = new WebAppContext(); webAppContext.setDisplayName("WepAppsContext"); webAppContext.setContextPath("/"); webAppContext.setWar(appDir + "/" + name); webAppContext.getServletContext().setAttribute(CONF_CONTEXT_ATTRIBUTE, conf); webAppContext.getServletContext().setAttribute(ADMINS_ACL, adminsAcl); webServer.addHandler(webAppContext);?同时添加了对Log和webapps下面static资源（css、js、pic）的访问
protected void addDefaultApps(ContextHandlerCollection parent, final String appDir) throws IOException { // set up the context for "/logs/" if "hadoop.log.dir" property is defined. String logDir = System.getProperty("hadoop.log.dir"); if (logDir != null) { Context logContext = new Context(parent, "/logs"); logContext.setResourceBase(logDir); logContext.addServlet(AdminAuthorizedServlet.class, "/"); logContext.setDisplayName("logs"); setContextAttributes(logContext); defaultContexts.put(logContext, true); } // set up the context for "/static/*" Context staticContext = new Context(parent, "/static"); staticContext.setResourceBase(appDir + "/static"); staticContext.addServlet(DefaultServlet.class, "/*"); staticContext.setDisplayName("static"); setContextAttributes(staticContext); defaultContexts.put(staticContext, true); }以及一些状态信息的访问
/** * Add default servlets. */ protected void addDefaultServlets() { // set up default servlets addServlet("stacks", "/stacks", StackServlet.class); addServlet("logLevel", "/logLevel", LogLevel.Servlet.class); addServlet("metrics", "/metrics", MetricsServlet.class); addServlet("conf", "/conf", ConfServlet.class); addServlet("jmx", "/jmx", JMXJsonServlet.class); }?最后，返回namenode，又添加了一些namenode特有的访问接口，例如
/fsck用于文件系统的检查
/getimage是SecondaryNamenode获取image的入口
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httpServer.addInternalServlet("getDelegationToken", GetDelegationTokenServlet.PATH_SPEC, GetDelegationTokenServlet.class, true); httpServer.addInternalServlet("renewDelegationToken", RenewDelegationTokenServlet.PATH_SPEC, RenewDelegationTokenServlet.class, true); httpServer.addInternalServlet("cancelDelegationToken", CancelDelegationTokenServlet.PATH_SPEC, CancelDelegationTokenServlet.class, true); httpServer.addInternalServlet("fsck", "/fsck", FsckServlet.class, true); httpServer.addInternalServlet("getimage", "/getimage", GetImageServlet.class, true); httpServer.addInternalServlet("listPaths", "/listPaths/*", ListPathsServlet.class, false); httpServer.addInternalServlet("data", "/data/*", FileDataServlet.class, false); httpServer.addInternalServlet("checksum", "/fileChecksum/*", FileChecksumServlets.RedirectServlet.class, false); httpServer.addInternalServlet("contentSummary", "/contentSummary/*", ContentSummaryServlet.class, false); httpServer.start(); // The web-server port can be ephemeral... ensure we have the correct info infoPort = httpServer.getPort(); httpAddress = new InetSocketAddress(infoHost, infoPort); conf.set("dfs.http.address", infoHost + ":" + infoPort); LOG.info("Web-server up at: " + infoHost + ":" + infoPort); return httpServer;?
再打开hdfs目录，会发现index页面会直接转跳到dfshealth.jsp，查看web.xml
<servlet-mapping> <servlet-name>org.apache.hadoop.hdfs.server.namenode.dfshealth_jsp</servlet-name> <url-pattern>/dfshealth.jsp</url-pattern> </servlet-mapping>
?dfshealth_jsp.class可以从hadoop-core-xxx.jar里面找到
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java.lang.ClassNotFoundException: org.apache.hadoop.hdfs.server.namenode.dfshealth_jspat java.net.URLClassLoader$1.run(URLClassLoader.java:200)at java.security.AccessController.doPrivileged(Native Method)at java.net.URLClassLoader.findClass(URLClassLoader.java:188)at java.lang.ClassLoader.loadClass(ClassLoader.java:307)at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)at java.lang.ClassLoader.loadClass(ClassLoader.java:252)