本文介绍了Tomcat8中如何解析server.xml文件，重点讲解了 Digester 和 SAX 解析机制。通过类图和源码分析，展示了ObjectCreateRule规则的实现，包括开始和结束元素标签的处理。同时，文章讨论了模式和规则的概念，以及它们在解析过程中的匹配和执行。解析过程包括检查临时目录、创建JNDI上下文、构建模式与规则、解析XML文件以及执行匹配的规则。最后，对模式和规则进行了总结，强调它们在将XML配置转化为Java对象中的作用。
关于sax介绍参考：http://blog.youkuaiyun.com/liu_jian140126/article/details/53035939
1.类图
ObjectCreateRule（规则方法，用此规则作为实例）
- ObjectCreateRule(String className)：要创建的对象的Java类名
- ObjectCreateRule(String className,
String attributeName) ：attributeName属性名称，如果存在，则包含一个覆盖要创建的类名 - begin(String namespace, String name, Attributes attributes)：
处理开始元素标签执行的动作 - end(String namespace, String name)：处理结束元素标签执行的动作。
2.源码分析
代码清单1:
protected void initDirs() {
String temp = System.getProperty("java.io.tmpdir");
if (temp == null || (!(new File(temp)).isDirectory())) {
log.error(sm.getString("embedded.notmp", temp));
}
}代码清单2：
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 );
}
}
}代码清单3：
/**
* 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<Class<?>, List<String>> fakeAttributes = new HashMap<>();
ArrayList<String> 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.NamingResourcesImpl");
digester.addSetProperties("Server/GlobalNamingResources");
digester.addSetNext("Server/GlobalNamingResources",
"setGlobalNamingResources",
"org.apache.catalina.deploy.NamingResourcesImpl");
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);
}代码清单4：
/**
* Parse the content of the specified input source using this Digester.
* Returns the root element from the object stack (if any).
*
* @param input Input source containing the XML data to be parsed
*
* @exception IOException if an input/output error occurs
* @exception SAXException if a parsing exception occurs
*/
public Object parse(InputSource input) throws IOException, SAXException {
configure();
getXMLReader().parse(input);
return (root);
}代码清单5：（以ObjectCreateRule规则为例）
/**
* 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
*/
@Override
public void startElement(String namespaceURI, String localName,
String qName, Attributes list)
throws SAXException {
boolean debug = log.isDebugEnabled();
if("GlobalNamingResources".equals(qName)) {
System.out.println();
}
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<Rule> 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 + "'.");
}
}
}代码清单6（以ObjectCreateRule规则为例）
/**
* Process the beginning of this element.
*
* @param namespace the namespace URI of the matching element, or an
* empty string if the parser is not namespace aware or the element has
* no namespace
* @param name the local name if the parser is namespace aware, or just
* the element name otherwise
* @param attributes The attribute list for this element
*/
@Override
public 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);
}
step1：判断tomcat临时目录是否存在
step2：指定创建tomcat（JNDI）上下文的工厂类、查询创建对应的JNDI上下文
step3：创建tomcat/conf/server.xml文件中对应的模式与规则
step4：解析server.xml文件
step5：Digester采用sax解析，遇到元素开始标签执行
- 更新元素属性列表
- 取得模式
- 获取指定模式的规则列表
setp6：执行模式匹配的规则（动作）
疑问：
1、什么是模式
2、什么是规则
3、模式与规则怎么匹配的
4、解析server.xml文件怎么获取模式，匹配规则列表，执行动作的？
3. 概念
Digester本来仅仅是Jakarta Struts中的一个工具，用于处理struts-config.xml配置文件。显然，将XML文件转换成相应的Java对象是一项很通用的功能，
4.模式
模式可以比喻成一个KEY，当Digester解析xml文件，它会跟踪xml每个元素，比如Server元素的模式是Server ，Listener元素模式是Server/Listener
<Server port="8005" shutdown="SHUTDOWN">
<!--APR library loader. Documentation at /docs/apr.html -->
<Listener className="org.apache.catalina.core.AprLifecycleListener" SSLEngine="on" />
<!--Initialize Jasper prior to webapps are loaded. Documentation at /docs/jasper-howto.html -->
<Listener className="org.apache.catalina.core.JasperListener" />
<!-- Prevent memory leaks due to use of particular java/javax APIs-->
<Listener className="org.apache.catalina.core.JreMemoryLeakPreventionListener" />
<!-- JMX Support for the Tomcat server. Documentation at /docs/non-existent.html -->
<Listener className="org.apache.catalina.mbeans.ServerLifecycleListener" />
<Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener" />
</Server>5. 规则
规则是指要执行的动作，动作可以是实例化一个对象，设置对象的属性，设置对象属性是引用的值。我们结合代码理解。一个模式可以对应多条规则，
ObjectCreateRule规则三个方法：
- addObjectCreate()：实例化一个java对象（创建对象的规则）
- addSetProperties()：设置java对象的属性（设置属性的规则）
- addSetNext()：设置java对象属性是引用的值
总结：
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