tomcat解析(六)Digester(二)startElement

在用SAX对XML进行解析时,需要设置相应的ContentHandler实现，该对象将用于响应各种xml解析的事件，如开始标签时会调用其StartElement方法，遇到标签体时会调用其characters方法，而结束标签会调用其eneElement方法，我们可以看到Digester便是继承了DefaultHandler类，而该类则实现了ContentHandler接口，因此当我们调用digester.parse对相应的xml文件进行解析时，Digester类的这几个方法便会执行，本文先粗略地看一下startElement的实现，如下:
 /** * Process notification of the start of an XML element being reached. * * @param namespaceURI The Namespace URI, or the empty string if the element * has no Namespace URI or if Namespace processing is not being performed. * @param localName The local name (without prefix), or the empty * string if Namespace processing is not being performed. * @param qName The qualified name (with prefix), or the empty * string if qualified names are not available./ * @param list The attributes attached to the element. If there are * no attributes, it shall be an empty Attributes object. * @exception SAXException if a parsing error is to be reported */ public 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); if (debug) { log.debug(" Pushing body text '" + bodyText.toString() + "'"); } bodyText = new StringBuffer(); // 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 StringBuffer sb = new StringBuffer(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 = (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方法有四个参数，分别为命名空间，两个标签路径名加一个标签属性对象，下面我们先简单地介绍一下startElement方法里的内容
1.更新属性中的系统值引用
updateAttributes(list)主要是用于将属性值中以${}标示的系统属性值转换为真正的值,使用的属性均从System.getProperty获取
2.初始化标签体内容
bodyText是Digester挂有的一个用于存放标签体内容的StringBuffer对象，当解析开始标签时应该还没有标签体内容的，因此这里给了一个空的StringBuffer对象，后又将bodyText放入到bodyTexts(一个ArrayStack)里
3.获取路径名
此处将标签的路径字符赋给name,XML格式如:<Servers><Server><Server></Servers>,当解析到<Server>标签时路径则为Servers/Server,在得到路径后又将该路径赋予match变量;
4.得到与标签体路径名匹配的Rule List，触发其begin方法

这部分内容是tomcat解析xml的重要部分，将等到characters及endElement方法讲解完后一起进行说明