1、文件的加载
Bootstrap的load方法是加载Tomcat的server.xml的入口，load方法实际通过反射调用catalinaDaemon（类型为Catalina）的load方法：
/** * Load daemon. */private void load(String[] arguments) throws Exception { // Call the load() method String methodName = "load"; Object param[]; Class paramTypes[]; if (arguments==null || arguments.length==0) { paramTypes = null; param = null; } else { paramTypes = new Class[1]; paramTypes[0] = arguments.getClass(); param = new Object[1]; param[0] = arguments; } Method method = catalinaDaemon.getClass().getMethod(methodName, paramTypes);//通过反射机制调用Catalina类的load方法。 if (log.isDebugEnabled()) log.debug("Calling startup class " + method); method.invoke(catalinaDaemon, param);} Catalina类的load方法：
/* * Load using arguments */public void load(String args[]) { try { if (arguments(args)) { load();//调用自身的load方法 } } catch (Exception e) { e.printStackTrace(System.out); }} /** * Start a new server instance. */public void load() { long t1 = System.nanoTime(); initDirs(); //用于对catalina.home和catalina.base的一些检查工作 // Before digester - it may be needed initNaming();//给系统设置java.naming.factory.url.pkgs和java.naming.factory.initial // Create and execute our Digester Digester digester = createStartDigester();//实例化Digester InputSource inputSource = null; InputStream inputStream = null; File file = null; try { try { file = configFile();//获取conf/server.xml配置文件 inputStream = new FileInputStream(file);获取conf/server.xml配置文件输入流 inputSource = new InputSource(file.toURI().toURL().toString()); } catch (Exception e) { if (log.isDebugEnabled()) { log.debug(sm.getString("catalina.configFail", file), e); } } if (inputStream == null) { try { inputStream = getClass().getClassLoader() .getResourceAsStream(getConfigFile()); inputSource = new InputSource (getClass().getClassLoader() .getResource(getConfigFile()).toString()); } catch (Exception e) { if (log.isDebugEnabled()) { log.debug(sm.getString("catalina.configFail", getConfigFile()), e); } } } // This should be included in catalina.jar // Alternative: don't bother with xml, just create it manually. if( inputStream==null ) { try { inputStream = getClass().getClassLoader() .getResourceAsStream("server-embed.xml"); inputSource = new InputSource (getClass().getClassLoader() .getResource("server-embed.xml").toString()); } catch (Exception e) { if (log.isDebugEnabled()) { log.debug(sm.getString("catalina.configFail", "server-embed.xml"), e); } } } if (inputStream == null || inputSource == null) { if (file == null) { log.warn(sm.getString("catalina.configFail", getConfigFile() + "] or [server-embed.xml]")); } else { log.warn(sm.getString("catalina.configFail", file.getAbsolutePath())); if (file.exists() && !file.canRead()) { log.warn("Permissions incorrect, read permission is not allowed on the file."); } } return; } try { inputSource.setByteStream(inputStream);//将FileInputStream封装为InputSource digester.push(this); digester.parse(inputSource);//调用Digester的parse方法进行解析 } catch (SAXParseException spe) { log.warn("Catalina.start using " + getConfigFile() + ": " + spe.getMessage()); return; } catch (Exception e) { log.warn("Catalina.start using " + getConfigFile() + ": " , e); return; } } finally { if (inputStream != null) { try { inputStream.close(); } catch (IOException e) { // Ignore } } } getServer().setCatalina(this); // Stream redirection initStreams();//initStreams对输出流、错误流重定向 // Start the new server try { getServer().init();//初始化server } catch (LifecycleException e) { if (Boolean.getBoolean("org.apache.catalina.startup.EXIT_ON_INIT_FAILURE")) { throw new java.lang.Error(e); } else { log.error("Catalina.start", e); } } long t2 = System.nanoTime(); if(log.isInfoEnabled()) { log.info("Initialization processed in " + ((t2 - t1) / 1000000) + " ms"); }} 下面开始分析整个过程：
1.1、initDirs（）方法用于对catalina.home和catalina.base的一些检查工作
protected void initDirs() { String catalinaHome = System.getProperty(Globals.CATALINA_HOME_PROP); if (catalinaHome == null) { // Backwards compatibility patch for J2EE RI 1.3 String j2eeHome = System.getProperty("com.sun.enterprise.home"); if (j2eeHome != null) { catalinaHome=System.getProperty("com.sun.enterprise.home"); } else if (System.getProperty(Globals.CATALINA_BASE_PROP) != null) { catalinaHome = System.getProperty(Globals.CATALINA_BASE_PROP); } } // last resort - for minimal/embedded cases. if(catalinaHome==null) { catalinaHome=System.getProperty("user.dir"); } if (catalinaHome != null) { File home = new File(catalinaHome); if (!home.isAbsolute()) { try { catalinaHome = home.getCanonicalPath(); } catch (IOException e) { catalinaHome = home.getAbsolutePath(); } } System.setProperty(Globals.CATALINA_HOME_PROP, catalinaHome); } if (System.getProperty(Globals.CATALINA_BASE_PROP) == null) { System.setProperty(Globals.CATALINA_BASE_PROP, catalinaHome); } else { String catalinaBase = System.getProperty(Globals.CATALINA_BASE_PROP); File base = new File(catalinaBase); if (!base.isAbsolute()) { try { catalinaBase = base.getCanonicalPath(); } catch (IOException e) { catalinaBase = base.getAbsolutePath(); } } System.setProperty(Globals.CATALINA_BASE_PROP, catalinaBase); } String temp = System.getProperty("java.io.tmpdir"); if (temp == null || (!(new File(temp)).exists()) || (!(new File(temp)).isDirectory())) { log.error(sm.getString("embedded.notmp", temp)); }} 1.2、initNaming()方法给系统设置java.naming.factory.url.pkgs和java.naming.factory.initial
protected void initNaming() { // Setting additional variables if (!useNaming) { log.info( "Catalina naming disabled"); System.setProperty("catalina.useNaming", "false"); } else { System.setProperty("catalina.useNaming", "true"); String value = "org.apache.naming"; String oldValue = System.getProperty(javax.naming.Context.URL_PKG_PREFIXES); if (oldValue != null) { value = value + ":" + oldValue; } System.setProperty(javax.naming.Context.URL_PKG_PREFIXES, value); if( log.isDebugEnabled() ) { log.debug("Setting naming prefix=" + value); } value = System.getProperty (javax.naming.Context.INITIAL_CONTEXT_FACTORY); if (value == null) { System.setProperty (javax.naming.Context.INITIAL_CONTEXT_FACTORY, "org.apache.naming.java.javaURLContextFactory"); } else { log.debug( "INITIAL_CONTEXT_FACTORY already set " + value ); } }} 在创建JNDI上下文时，使用Context.INITIAL _ CONTEXT _ FACTORY（"java.naming.factory.initial"）属性，来指定创建JNDI上下文的工厂类；Context.URL _ PKG _ PREFIXES("java.naming.factory.url.pkgs")用在查询url中包括scheme方法id时创建对应的JNDI上下文.
1.3、createStartDigester()创建并配置将要用来启动的Digester实例，并且设置一些列Rule，具体映射到server.xml
/** * Create and configure the Digester we will be using for startup. */protected Digester createStartDigester() { long t1=System.currentTimeMillis(); // Initialize the digester Digester digester = new Digester(); digester.setValidating(false); digester.setRulesValidation(true); HashMap , List > fakeAttributes = new HashMap , List >(); ArrayList attrs = new ArrayList (); attrs.add("className"); fakeAttributes.put(Object.class, attrs); digester.setFakeAttributes(fakeAttributes); digester.setUseContextClassLoader(true); // Configure the actions we will be using digester.addObjectCreate("Server", "org.apache.catalina.core.StandardServer", "className"); digester.addSetProperties("Server"); digester.addSetNext("Server", "setServer", "org.apache.catalina.Server"); digester.addObjectCreate("Server/GlobalNamingResources", "org.apache.catalina.deploy.NamingResources"); digester.addSetProperties("Server/GlobalNamingResources"); digester.addSetNext("Server/GlobalNamingResources", "setGlobalNamingResources", "org.apache.catalina.deploy.NamingResources"); digester.addObjectCreate("Server/Listener", null, // MUST be specified in the element "className"); digester.addSetProperties("Server/Listener"); digester.addSetNext("Server/Listener", "addLifecycleListener", "org.apache.catalina.LifecycleListener"); digester.addObjectCreate("Server/Service", "org.apache.catalina.core.StandardService", "className"); digester.addSetProperties("Server/Service"); digester.addSetNext("Server/Service", "addService", "org.apache.catalina.Service"); digester.addObjectCreate("Server/Service/Listener", null, // MUST be specified in the element "className"); digester.addSetProperties("Server/Service/Listener"); digester.addSetNext("Server/Service/Listener", "addLifecycleListener", "org.apache.catalina.LifecycleListener"); //Executor digester.addObjectCreate("Server/Service/Executor", "org.apache.catalina.core.StandardThreadExecutor", "className"); digester.addSetProperties("Server/Service/Executor"); digester.addSetNext("Server/Service/Executor", "addExecutor", "org.apache.catalina.Executor"); digester.addRule("Server/Service/Connector", new ConnectorCreateRule()); digester.addRule("Server/Service/Connector", new SetAllPropertiesRule(new String[]{"executor"})); digester.addSetNext("Server/Service/Connector", "addConnector", "org.apache.catalina.connector.Connector"); digester.addObjectCreate("Server/Service/Connector/Listener", null, // MUST be specified in the element "className"); digester.addSetProperties("Server/Service/Connector/Listener"); digester.addSetNext("Server/Service/Connector/Listener", "addLifecycleListener", "org.apache.catalina.LifecycleListener"); // Add RuleSets for nested elements digester.addRuleSet(new NamingRuleSet("Server/GlobalNamingResources/")); digester.addRuleSet(new EngineRuleSet("Server/Service/")); digester.addRuleSet(new HostRuleSet("Server/Service/Engine/")); digester.addRuleSet(new ContextRuleSet("Server/Service/Engine/Host/")); addClusterRuleSet(digester, "Server/Service/Engine/Host/Cluster/"); digester.addRuleSet(new NamingRuleSet("Server/Service/Engine/Host/Context/")); // When the 'engine' is found, set the parentClassLoader. digester.addRule("Server/Service/Engine", new SetParentClassLoaderRule(parentClassLoader)); addClusterRuleSet(digester, "Server/Service/Engine/Cluster/"); long t2=System.currentTimeMillis(); if (log.isDebugEnabled()) { log.debug("Digester for server.xml created " + ( t2-t1 )); } return (digester);} 从上面的代码可以看出：首先创建Digester，Digester继承了DefaultHandler，而DefaultHandler默认实现了ContentHander、DTDHander、ErrorHandler及EntityResolver 这4个接口，代码如下：
public class DefaultHandler implements EntityResolver, DTDHandler, ContentHandler, ErrorHandler
通过源码可以发现DefaultHandler的所有实现都是空实现，所以解析还需要Digester。（具体分析后面在说）。
1.4、configFile()获取配置文件conf/server.xml，并使用FileInputStream获取conf/server.xml配置文件输入流
/** * Return a File object representing our configuration file. */protected File configFile() { File file = new File(configFile); if (!file.isAbsolute()) { file = new File(System.getProperty(Globals.CATALINA_BASE_PROP), configFile); } return (file);} 1.5、将FileInputStream封装为InputSource，并且调用Digester的parse方法进行解析
inputSource.setByteStream(inputStream);digester.push(this);digester.parse(inputSource);
1.6、initStreams()对输出流、错误流重定向
protected void initStreams() { // Replace System.out and System.err with a custom PrintStream System.setOut(new SystemLogHandler(System.out)); System.setErr(new SystemLogHandler(System.err));} 1.7、初始化server
// Start the new server try { getServer().init(); } catch (LifecycleException e) { if (Boolean.getBoolean("org.apache.catalina.startup.EXIT_ON_INIT_FAILURE")) { throw new java.lang.Error(e); } else { log.error("Catalina.start", e); } } 2、文件的解析
当加载server.xml配置文件到内存后，开始对XML文件中的内容进行解析，主要包含两个步骤：
- 构造server.xml的规则，这些规则即可以用于构造Tomcat内部的容器（如StandardServer，StandardService等），也可以对server.xml进行合法性检查。如果server.xml不符合Tomcat内置的规则，在解析时将抛出异常，进而导致Tomcat无法启动。
- 使用SAX解析server.xml，边解析边应用规则，最终使用server.xml中的配置构建好Tomcat所需的各种容器。
Tomcat将server.xml文件中的所有元素上的属性都抽象为Rule，以Server元素为例，在内存中对应Server实例，Server实例的属性值就来自于Server元素的属性值。通过对规则（Rule）的应用，最终改变Server实例的属性值。Rule是一个抽象类，其中定义了以下方法：
- getDigester：获取Digester实例；
- setDigester：设置Digester实例；
- getNamespaceURI：获取Rule所在的相对命名空间URI；
- setNamespaceURI：设置Rule所在的相对命名空间URI；
- begin(String namespace, String name, Attributes attributes)：此方法在遇到一个匹配的XML元素的开头时被调用，如<Server>；
- body(String namespace, String name, String text)：在遇到匹配XML元素的body时，此方法被调用，如进入标签内部时；
- end(String namespace, String name)：此方法在遇到一个匹配的XML元素的末尾时被调用。如：< /Server>；
Rule目前有很多实现类，如：NodeCreateRule、AbsoluteOrderingRule、CallParamRule、ConnectorCreateRule等。下图展示了Rule的部分实现类：
Tomcat使用SAX（Simple API for XML)解析XML:
SAX解析XML采用的是从上而下的基于事件驱动的解析方式，在解析过程中会视情况自动调用ContentHandler接口中的startDocument()、startElement()、characters()、endElement()、endDocument()等相关的方法。
由编译执行的结果分析：
- startDocument()方法只会在文档开始解析的时候被调用，每次解析只会调用一次。
- startElement()方法每次在开始解析一个元素，即遇到元素标签开始的时候都会调用。
- characters()方法也是在每次解析到元素标签携带的内容时都会调用，即使该元素标签的内容为空或换行。而且如果元素内嵌套元素，在父元素结束标签前， characters()方法会再次被调用，此处需要注意。
- endElement()方法每次在结束解析一个元素，即遇到元素标签结束的时候都会调用。
- endDocument()方法只会在文档解析结束的时候被调用，每次解析只会调用一次。
使用SAX解析XML的好处：
- SAX 不用解析完整个文档
- 相比于 DOM 而言 SAX 是一种速度更快，更有效，占用内存更少的解析 XML 文件的方法
- 逐行扫描，可以做到边扫描边解析，因此 SAX 可以在解析文档的任意时刻停止解析
由于SAX是基于事件驱动的，不用解析完整个文档，在按内容顺序解析文档过程中， SAX 会判断当前读到的字符是否符合 XML 文件语法中的某部分。如果符合某部分，则会触发事件。所谓触发事件，就是调用一些回调方法。在用 SAX 解析 xml 文档时候，在读取到文档开始和结束标签时候就会回调一个事件，在读取到其他节点与内容时候也会回调一个事件。在 SAX 接口中，事件源是 org.xml.sax 包中的 XMLReader ，它通过 parser() 方法来解析 XML 文档，并产生事件。事件处理器是 org.xml.sax 包中 ContentHander 、 DTDHander 、 ErrorHandler ，以及 EntityResolver 这4个接口：
- ContentHander （ XML 文档的开始与结束）setContentHandler(ContentHandler h)
- DTDHander （ 处理 DTD 解析） setDTDHandler(DTDHandler h)
- ErrorHandler （ 处理 XML 时产生的错误）setErrorHandler(ErrorHandler h)
- EntityResolver （处理外部实体） setEntityResolver(EntityResolver e)
回调方法一般都定义在ContentHandler接口中，上面已经对这些回调方法的加载顺序已经说了就不在介绍了。
使用 SAX 解析 XML 文件一般有以下五个步骤：
- 创建一个 SAXParserFactory 对象；
- 调用 SAXParserFactory 中的 newSAXParser 方法创建一个 SAXParser 对象；
- 然后在调用 SAXParser 中的 getXMLReader 方法获取一个 XMLReader 对象；
- 实例化一个 DefaultHandler 对象；
- 连接事件源对象 XMLReader 到事件处理类 DefaultHandler 中；
- 调用 XMLReader 的 parse 方法从输入源中获取到的 xml 数据；
- 通过 DefaultHandler 返回我们需要的数据集合。
我们通过源码可以发现DefaultHandler的所有实现都是空实现，所以解析还需要Digester自身，代码如下：
@Overridepublic void startDocument() throws SAXException { if (saxLog.isDebugEnabled()) { saxLog.debug("startDocument()"); } // ensure that the digester is properly configured, as // the digester could be used as a SAX ContentHandler // rather than via the parse() methods. configure();}@Overridepublic void startElement(String namespaceURI, String localName, String qName, Attributes list) throws SAXException { boolean debug = log.isDebugEnabled(); if (saxLog.isDebugEnabled()) { saxLog.debug("startElement(" + namespaceURI + "," + localName + "," + qName + ")"); } // Parse system properties list = updateAttributes(list); // Save the body text accumulated for our surrounding element bodyTexts.push(bodyText); bodyText = new StringBuilder(); // the actual element name is either in localName or qName, depending // on whether the parser is namespace aware String name = localName; if ((name == null) || (name.length() < 1)) { name = qName; } // Compute the current matching rule StringBuilder sb = new StringBuilder(match); if (match.length() > 0) { sb.append('/'); } sb.append(name); match = sb.toString(); if (debug) { log.debug(" New match='" + match + "'"); } // Fire "begin" events for all relevant rules List rules = getRules().match(namespaceURI, match); matches.push(rules); if ((rules != null) && (rules.size() > 0)) { for (int i = 0; i < rules.size(); i++) { try { Rule rule = rules.get(i); if (debug) { log.debug(" Fire begin() for " + rule); } rule.begin(namespaceURI, name, list); } catch (Exception e) { log.error("Begin event threw exception", e); throw createSAXException(e); } catch (Error e) { log.error("Begin event threw error", e); throw e; } } } else { if (debug) { log.debug(" No rules found matching '" + match + "'."); } }}@Overridepublic void endDocument() throws SAXException { if (saxLog.isDebugEnabled()) { if (getCount() > 1) { saxLog.debug("endDocument(): " + getCount() + " elements left"); } else { saxLog.debug("endDocument()"); } } while (getCount() > 1) { pop(); } // Fire "finish" events for all defined rules Iterator rules = getRules().rules().iterator(); while (rules.hasNext()) { Rule rule = rules.next(); try { rule.finish(); } catch (Exception e) { log.error("Finish event threw exception", e); throw createSAXException(e); } catch (Error e) { log.error("Finish event threw error", e); throw e; } } // Perform final cleanup clear();}@Overridepublic void endElement(String namespaceURI, String localName, String qName) throws SAXException { boolean debug = log.isDebugEnabled(); if (debug) { if (saxLog.isDebugEnabled()) { saxLog.debug("endElement(" + namespaceURI + "," + localName + "," + qName + ")"); } log.debug(" match='" + match + "'"); log.debug(" bodyText='" + bodyText + "'"); } // Parse system properties bodyText = updateBodyText(bodyText); // the actual element name is either in localName or qName, depending // on whether the parser is namespace aware String name = localName; if ((name == null) || (name.length() < 1)) { name = qName; } // Fire "body" events for all relevant rules List rules = matches.pop(); if ((rules != null) && (rules.size() > 0)) { String bodyText = this.bodyText.toString(); for (int i = 0; i < rules.size(); i++) { try { Rule rule = rules.get(i); if (debug) { log.debug(" Fire body() for " + rule); } rule.body(namespaceURI, name, bodyText); } catch (Exception e) { log.error("Body event threw exception", e); throw createSAXException(e); } catch (Error e) { log.error("Body event threw error", e); throw e; } } } else { if (debug) { log.debug(" No rules found matching '" + match + "'."); } if (rulesValidation) { log.warn(" No rules found matching '" + match + "'."); } } // Recover the body text from the surrounding element bodyText = bodyTexts.pop(); // Fire "end" events for all relevant rules in reverse order if (rules != null) { for (int i = 0; i < rules.size(); i++) { int j = (rules.size() - i) - 1; try { Rule rule = rules.get(j); if (debug) { log.debug(" Fire end() for " + rule); } rule.end(namespaceURI, name); } catch (Exception e) { log.error("End event threw exception", e); throw createSAXException(e); } catch (Error e) { log.error("End event threw error", e); throw e; } } } // Recover the previous match expression int slash = match.lastIndexOf('/'); if (slash >= 0) { match = match.substring(0, slash); } else { match = ""; }} 当我们创建好Digester后，会调用addObjectCreate、addSetProperties、addSetNext方法陆续添加很多Rule，这些方法的实现如代码：
public void addObjectCreate(String pattern, String className, String attributeName) { addRule(pattern, new ObjectCreateRule(className, attributeName));} public void addSetProperties(String pattern) { addRule(pattern, new SetPropertiesRule());}public void addSetNext(String pattern, String methodName, String paramType) { addRule(pattern, new SetNextRule(methodName, paramType));} 这三个方法分别创建ObjectCreateRule、SetPropertiesRule及SetNextRule，为了便于理解，我们举例说明(Server标签)：
digester.addObjectCreate("Server","org.apache.catalina.core.StandardServer","className");digester.addSetProperties("Server");digester.addSetNext("Server","setServer", "org.apache.catalina.Server"); 我们知道最终会创建ObjectCreateRule、SetPropertiesRule及SetNextRule，并且调用addRule方法。
public void addRule(String pattern, Rule rule) { rule.setDigester(this); getRules().add(pattern, rule);} 从代码可以看出，addRule方法首先调用getRules方法获取RulesBase，然后调用RulesBase的add方法。代码如下：
//getRules()获取RulesBasepublic Rules getRules() { if (this.rules == null) { this.rules = new RulesBase(); this.rules.setDigester(this); } return (this.rules);}//RulesBase的add方法@Overridepublic void add(String pattern, Rule rule) { // to help users who accidently add '/' to the end of their patterns int patternLength = pattern.length(); if (patternLength>1 && pattern.endsWith("/")) { pattern = pattern.substring(0, patternLength-1); } List list = cache.get(pattern); if (list == null) { list = new ArrayList (); cache.put(pattern, list); } list.add(rule); rules.add(rule); if (this.digester != null) { rule.setDigester(this.digester); } if (this.namespaceURI != null) { rule.setNamespaceURI(this.namespaceURI); }} 其中，cache的数据结构为HashMap<String,List<Rule>>，每个键值维护一个List<Rule>，由此可知，对Server标签来说，对应的Rule列表为ObjectCreateRule、SetPropertiesRule及SetNextRule。
Digester解析XML的入口是其parse方法，其处理步骤如下：
1.创建XMLReader ；
2.使用XMLReader解析XML。
public Object parse(InputSource input) throws IOException, SAXException { configure(); getXMLReader().parse(input); return (root);}//configure()protected void configure() { // Do not configure more than once if (configured) { return; } log = LogFactory.getLog("org.apache.tomcat.util.digester.Digester"); saxLog = LogFactory.getLog("org.apache.tomcat.util.digester.Digester.sax"); // Perform lazy configuration as needed initialize(); // call hook method for subclasses that want to be initialized once only // Nothing else required by default // Set the configuration flag to avoid repeating configured = true;} getXMLReader方法调用getParser创建SAXParser ，然后调用SAXParser 的getXMLReader方法创建XMLReader.
public XMLReader getXMLReader() throws SAXException { if (reader == null){ reader = getParser().getXMLReader(); } reader.setDTDHandler(this); reader.setContentHandler(this); if (entityResolver == null){ reader.setEntityResolver(this); } else { reader.setEntityResolver(entityResolver); } reader.setProperty( "http://xml.org/sax/properties/lexical-handler", this); reader.setErrorHandler(this); return reader;} getParser方法调用getFactory方法创建SAXParserFactory，然后调用SAXParserFactory的newSAXParser方法创建SAXParser
public SAXParser getParser() { // Return the parser we already created (if any) if (parser != null) { return (parser); } // Create a new parser try { parser = getFactory().newSAXParser(); } catch (Exception e) { log.error("Digester.getParser: ", e); return (null); } return (parser);} getFactory方法使用SAX的API生成SAXParserFactory实例.
public SAXParserFactory getFactory() throws SAXNotRecognizedException, SAXNotSupportedException,ParserConfigurationException { if (factory == null) { factory = SAXParserFactory.newInstance(); factory.setNamespaceAware(namespaceAware); // Preserve xmlns attributes if (namespaceAware) { factory.setFeature( "http://xml.org/sax/features/namespace-prefixes", true); } factory.setValidating(validating); if (validating) { // Enable DTD validation factory.setFeature( "http://xml.org/sax/features/validation", true); // Enable schema validation factory.setFeature( "http://apache.org/xml/features/validation/schema", true); } } return (factory);} XMLReader解析XML时，会生成事件，回调Digester的startDocument方法，解析的第一个元素是Server，此时回调Digester的startElement方法，入参Attributes list即Server上的属性，如port、shutdown等，入参qName即为Server.
@Overridepublic void startElement(String namespaceURI, String localName, String qName, Attributes list) throws SAXException { boolean debug = log.isDebugEnabled(); if (saxLog.isDebugEnabled()) { saxLog.debug("startElement(" + namespaceURI + "," + localName + "," + qName + ")"); } // Parse system properties list = updateAttributes(list); // Save the body text accumulated for our surrounding element bodyTexts.push(bodyText); bodyText = new StringBuilder(); // the actual element name is either in localName or qName, depending // on whether the parser is namespace aware String name = localName; if ((name == null) || (name.length() < 1)) { name = qName; } // Compute the current matching rule StringBuilder sb = new StringBuilder(match); if (match.length() > 0) { sb.append('/'); } sb.append(name); match = sb.toString(); if (debug) { log.debug(" New match='" + match + "'"); } // Fire "begin" events for all relevant rules List rules = getRules().match(namespaceURI, match); matches.push(rules); if ((rules != null) && (rules.size() > 0)) { for (int i = 0; i < rules.size(); i++) { try { Rule rule = rules.get(i); if (debug) { log.debug(" Fire begin() for " + rule); } rule.begin(namespaceURI, name, list); } catch (Exception e) { log.error("Begin event threw exception", e); throw createSAXException(e); } catch (Error e) { log.error("Begin event threw error", e); throw e; } } } else { if (debug) { log.debug(" No rules found matching '" + match + "'."); } }} startElement方法的处理步骤如下：
1.match刚开始为空字符串，拼接Server后变为Server。
2.调用RulesBase的match方法，返回cache中按照键值Server匹配的ObjectCreateRule、SetPropertiesRule及SetNextRule。
3.循环列表依次遍历ObjectCreateRule、SetPropertiesRule及SetNextRule，并调用它们的begin方法。
ObjectCreateRule的begin方法将生成Server的实例（默认为"org.apache.catalina.core.StandardServer"，用户可以通过给Server标签指定className使用其它Server实现），最后将Server的实例压入Digester的栈中.
@Overridepublic void begin(String namespace, String name, Attributes attributes) throws Exception { // Identify the name of the class to instantiate String realClassName = className; if (attributeName != null) { String value = attributes.getValue(attributeName); if (value != null) { realClassName = value; } } if (digester.log.isDebugEnabled()) { digester.log.debug("[ObjectCreateRule]{" + digester.match + "}New " + realClassName); } if (realClassName == null) { throw new NullPointerException("No class name specified for " + namespace + " " + name); } // Instantiate the new object and push it on the context stack Class clazz = digester.getClassLoader().loadClass(realClassName); Object instance = clazz.newInstance(); digester.push(instance);} SetPropertiesRule的begin方法首先将刚才压入栈中的Server实例出栈，然后给Server实例设置各个属性值，如port、shutdown等:
@Overridepublic void begin(String namespace, String theName, Attributes attributes) throws Exception { // Populate the corresponding properties of the top object Object top = digester.peek(); if (digester.log.isDebugEnabled()) { if (top != null) { digester.log.debug("[SetPropertiesRule]{" + digester.match + "} Set " + top.getClass().getName() + " properties"); } else { digester.log.debug("[SetPropertiesRule]{" + digester.match + "} Set NULL properties"); } } // set up variables for custom names mappings int attNamesLength = 0; if (attributeNames != null) { attNamesLength = attributeNames.length; } int propNamesLength = 0; if (propertyNames != null) { propNamesLength = propertyNames.length; } for (int i = 0; i < attributes.getLength(); i++) { String name = attributes.getLocalName(i); if ("".equals(name)) { name = attributes.getQName(i); } String value = attributes.getValue(i); // we'll now check for custom mappings for (int n = 0; n SetNextRule的begin不做什么动作。当遇到Server的结束标签时，还会依次调用ObjectCreateRule、SetPropertiesRule及SetNextRule的end方法，不再赘述。所有元素的解析都与Server标签同理，最终将server.xml文件中设置的元素及其属性值，构造出tomcat中的容器，如：Server、Service、Connector等.