Digester方法详解：
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通读Digester之前先分析下他的结构：
1.1该类继承了方法DefaultHandler2，DefaultHandler2继承了DefaultHandler是和sax解析器配合使用的类。当sax在对字符流进行加工的时候会根据实际情况调用 DefaultHandler中的方法。其使用的设计模式为模板模式。1.2createStartDigester方法中addObjectCreate、addSetNext的参数最终都是继承自抽象类Rule；方法addRuleSet的参数继承自抽象类RuleSetBase，实现接口RuleSet 1.3总结：理解digester前需要了解的接口和抽象类：DefaultHandler、Rule、RuleSet。 DefaultHandler类：
DefaultHandler类中的方法：processingInstruction、ignorableWhitespace、characters、endElement、startElement、endPrefixMapping、startPrefixMapping、endDocument、startDocument、setDocumentLocator、resolveEntity等方法。上面所列出的大多为digester中需要用到的Rule类：主要四个方法 begin 、body、end、finish
RuleSet类：addRuleInstances
关系梳理：
(1)catalina类的load方法中两个主要的createStartDigester和digester.parse方法。我们先理解parse方法；createStartDigester方法到后面再说。
(2) digester.parse()是用sax来解析xml的。
2.1根据其使用的模板设计模式，会在 解析xml节点时调用DefaultHandler类中方法。当调用startDocument时会 调用rule.begin。代码如下：该处的rule使用的是接口，实际调用中，是使用的那个实现类，要看rules.get()拿出的对象是什么。这个在后面学习createStartDigester会再提到。
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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 + "'.");
}
}
}
2.2其余的方法endDocument调用rule.finish、endElement调用rule.body方法等就不在一一列举，代码中很清楚。
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public 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 = (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();
}
(3)createStartDigester方法中会创建一些对象的实例，并调用其中的一些方法。用的方式大多为java中的反射机制。
3.1其中addObjectCreate会创建ObjectCreateRule对象，该对象继承自Rule方法，它实现了rule的begin方法，在其中创建了类的实例。
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public void begin(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);
}
// Instantiate the new object and push it on the context stack
Class clazz = digester.getClassLoader().loadClass(realClassName);
Object instance = clazz.newInstance();
digester.push(instance);
}
3.2 addSetProperties主要是解析xml节点中的属性值，实现了rule接口的begin方法。
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public void begin(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<attNamesLength; n++) {
if (name.equals(attributeNames[n])) {
if (n < propNamesLength) {
// set this to value from list
name = propertyNames[n];
} else {
// set name to null
// we'll check for this later
name = null;
}
break;
}
}
if (digester.log.isDebugEnabled()) {
digester.log.debug("[SetPropertiesRule]{" + digester.match +
"} Setting property '" + name + "' to '" +
value + "'");
}
if (!digester.isFakeAttribute(top, name)
&& !IntrospectionUtils.setProperty(top, name, value)
&& digester.getRulesValidation()) {
digester.log.warn("[SetPropertiesRule]{" + digester.match +
"} Setting property '" + name + "' to '" +
value + "' did not find a matching property.");
}
}
}
3.3 addSetNext方法会创建一个SetNextRule对象，该对象实现了rule的end方法，并在其中调用了指定类中的指定方法。
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public void end() throws Exception {
// Identify the objects to be used
Object child = digester.peek(0);
Object parent = digester.peek(1);
if (digester.log.isDebugEnabled()) {
if (parent == null) {
digester.log.debug("[SetNextRule]{" + digester.match +
"} Call [NULL PARENT]." +
methodName + "(" + child + ")");
} else {
digester.log.debug("[SetNextRule]{" + digester.match +
"} Call " + parent.getClass().getName() + "." +
methodName + "(" + child + ")");
}
}
// Call the specified method
IntrospectionUtils.callMethod1(parent, methodName,
child, paramType, digester.getClassLoader());
}
3.4 addRule方法就是在规则列表中，简单的添加一条规则。不同的是，实现的方法是rules接口中的add方法。添加的规则有：ConnectorCreateRule、SetAllPropertiesRule、SetParentClassLoaderRule。其中ConnectorCreateRule就是创建一个connector。
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public void addRule(String pattern, Rule rule) {
rule.setDigester(this);
getRules().add(pattern, rule);
}
RulesBase中的方法：
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public 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 = (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);
}
}
3.5addRuleSet方法添加的都是继承自RuleSet的对象，重写了方法addRuleInstances的逻辑。从代码中可以清晰的看到，添加的是tomcat中的engin、host等信息。
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// 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/"));
digester.addRuleSet(ClusterRuleSetFactory.getClusterRuleSet("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));
digester.addRuleSet(ClusterRuleSetFactory.getClusterRuleSet("Server/Service/Engine/Cluster/"));
在对象的实现中，addObjectCreate目的是创建对象，setNextSet是调用该对象的指定方法。比如NamingRuleSet对象实现Ejb、Environment、LocalEjb、Resource、Transaction等的初始化动作。EngineRuleSet对象实现Engine和其下的Cluster、Listener、Valve的初始化动作。HostRuleSet对象实现Host和其下的Alias、Cluster、Listener、Valve的初始化。ContextRuleSet对象主要实现Context、Listener、WebappLoader、Manager、Store、Parameter、Resources、Valve等的初始化。
- 总结：
读懂digester,关键是需要理解DefaultHandler、Rule、RuleSet这三个抽象类和接口的关联关系，了解其使用的设计模式，将其串联起来就容易了。我在读代码时，最难理解的是，不知道为何一句parse就能把server.xml中的配置给加载完了，最后通过学习别人的文章后，逐渐弄懂了。以此记录，继续学习。
关于Digester何时调用startElement方法，我目前还没搞懂。但是从调用堆栈可以推测一下。
"main@1" prio=5 tid=0x1 nid=NA runnable java.lang.Thread.State: RUNNABLE
at org.apache.tomcat.util.digester.Digester.startElement(Digester.java:1120)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.startElement(AbstractSAXParser.java:509)
at com.sun.org.apache.xerces.internal.impl.dtd.XMLDTDValidator.startElement(XMLDTDValidator.java:746)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanStartElement(XMLDocumentFragmentScannerImpl.java:1394)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$ContentDriver.scanRootElementHook(XMLDocumentScannerImpl.java:1251)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:3058)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$PrologDriver.next(XMLDocumentScannerImpl.java:820)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:601)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:531)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:887)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:823)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:141)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1213)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:639)
at org.apache.tomcat.util.digester.Digester.parse(Digester.java:1419)
at org.apache.catalina.startup.Catalina.load(Catalina.java:601)
at org.apache.catalina.startup.Catalina.load(Catalina.java:652)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(NativeMethodAccessorImpl.java:-1)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:564)
at org.apache.catalina.startup.Bootstrap.load(Bootstrap.java:309)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:492)