本文详细解析了Tomcat启动过程中的文件加载与解析机制，包括如何通过反射调用Catalina类的load方法加载server.xml配置文件，以及使用Digester进行XML文件解析的具体流程。
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<Class<?>, List<String>> fakeAttributes =
new HashMap<Class<?>, List<String>>();
ArrayList<String> attrs = new ArrayList<String>();
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：
@Override
public 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();
}
@Override
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);
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 + "'.");
}
}
}
@Override
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<Rule> 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();
}
@Override
public 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<Rule> 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RulesBase
public Rules getRules() {
if (this.rules == null) {
this.rules = new RulesBase();
this.rules.setDigester(this);
}
return (this.rules);
}
//RulesBase的add方法
@Override
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<Rule> list = cache.get(pattern);
if (list == null) {
list = new ArrayList<Rule>();
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.
@Override
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);
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 + "'.");
}
}
}
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.
@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);
}
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:
@Override
public 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<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.");
}
}
}
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.
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