熟悉java web开发的同学都清楚,tomcat作为一款非常流行的servlet容器,开源,流行,配置简单,不需要赘述。个人认为,web.xml作为webapp的入口,弄清楚该文件的底层解析过程,进而可以窥探tomcat的底层工作机制,搞明白tomcat对servlert规范的实现机理。
通过本文,可以知道以下部分内容
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webapp部署3种部署方式
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webapp web.xml解析流程
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webapp Context对象信息的生成(不包括对象的生成)
总体来说,webapp部署有三种方式:XML文件描述符、WAR包、文件目录。三种方式部署的总体流程很相似,都是一个web应用分配一个线程来处理,这里统一放到与Host内部的线程池对象中(startStopExecutor),所以有时会看到在默认配置下Tomcat启动后可能有一个叫“-startStop-”的线程还会运行一段时间才结束。但浏览这三种部署方式的实现代码,里面都是构建一个Context对象,并将构建好的Context对象与Host组件关联起来。
1.三种部署方式
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/**
* 部署应用,该方法被org.apache.catalina.startup.HostConfig.start()调用
* 包含3种部署方式,每个部署方式分别使用内部类,分配一个线程处理
*/
protected void deployApps() {
File appBase = appBase();
File configBase = configBase();
String[] filteredAppPaths = filterAppPaths(appBase.list());
// Deploy XML descriptors from configBase
deployDescriptors(configBase, configBase.list());
// Deploy WARs
deployWARs(appBase, filteredAppPaths);
// Deploy expanded folders
deployDirectories(appBase, filteredAppPaths);
}
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3个部署的内部类
org.apache.catalina.startup.HostConfig.DeployWar
org.apache.catalina.startup.HostConfig.DeployDescriptor
org.apache.catalina.startup.HostConfig.DeployDirectory
2.从部署webapp到解析web.xml序列图
找了很多网上资料,对从部署webapp开始,到开始解析web.xml这一段处理过程,没找到相关资料。个人就花时间整理出这个序列图,填补下这方面的空缺。通过该图,可以很清楚的知道,这部分主要完成了2件事:
(1)启动StandardContext,并将context对象添加到StandardHost对象中。
(2)通过触发事件机制,开始Context的解析过程。
3.web.xml解析过程
接第二步骤中序列图,开始分析web.xml的解析过程。从ContextConfig开始。
3.1 org.apache.catalina.startup.ContextConfig.configureStart()
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/**
* Process a "contextConfig" event for this Context.
*/
protected synchronized void configureStart() {
// Called from StandardContext.start()
webConfig();
if (ok) {
validateSecurityRoles();
}
// Configure an authenticator if we need one
if (ok)
authenticatorConfig();
// Make our application available if no problems were encountered
if (ok)
context.setConfigured( true );
else {
log.error(sm.getString( "contextConfig.unavailable" ));
context.setConfigured( false );
}
}
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该方法通过调用webConfig(),具体完成解析工作,此外完成了安全验证相关内容。
3.2 org.apache.catalina.startup.ContextConfig.webConfig()方法
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/**
* Scan the web.xml files that apply to the web application and merge them
* using the rules defined in the spec. For the global web.xml files,
* where there is duplicate configuration, the most specific level wins. ie
* an application's web.xml takes precedence over the host level or global
* web.xml file.
*/
protected void webConfig() {
Set<WebXml> defaults = new HashSet<WebXml>();
defaults.add(getDefaultWebXmlFragment());
WebXml webXml = createWebXml();
// Parse context level web.xml
InputSource contextWebXml = getContextWebXmlSource();
parseWebXml(contextWebXml, webXml, false ); //解析
ServletContext sContext = context.getServletContext();
// Ordering is important here
// Step 1. Identify all the JARs packaged with the application
// If the JARs have a web-fragment.xml it will be parsed at this
// point.
Map<String,WebXml> fragments = processJarsForWebFragments(webXml);
// Step 2. Order the fragments.
Set<WebXml> orderedFragments = null ;
orderedFragments =
WebXml.orderWebFragments(webXml, fragments, sContext);
// Step 3. Look for ServletContainerInitializer implementations
if (ok) {
processServletContainerInitializers();
}
// Step 5. Process JARs for annotations - only need to process
// those fragments we are going to use
if (ok) {
processAnnotations(
orderedFragments, webXml.isMetadataComplete());
}
// Cache, if used, is no longer required so clear it
javaClassCache.clear();
}
// Step 6. Merge web-fragment.xml files into the main web.xml
// Step 7. Apply global defaults
webXml.merge(defaults);
// Step 8. Convert explicitly mentioned jsps to servlets
convertJsps(webXml);
// Step 9. Apply merged web.xml to Context
webXml.configureContext(context);
// Step 9a. Make the merged web.xml available to other
// components,
// Always need to look for static resources
// Step 10. Look for static resources packaged in JARs
processResourceJARs(resourceJars);
// See also StandardContext.resourcesStart() for
// WEB-INF/classes/META-INF/resources configuration
// Step 11. Apply the ServletContainerInitializer config to the
// context
context.addServletContainerInitializer(
entry.getKey(), entry.getValue());
}
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通过源码中的注释,step1 到step11。主要工作包含:1.解析xml,2.合并xml,3.组装Context,4.编译JSP。具体步骤,参考说明
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扫描应用打包的所有Jar来检索Jar包里面的web.xml配置并解析,放入内存。
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对这些已经检索到的web配置进行排序。
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基于SPI机制查找ServletContainerInitializer的实现,写web中间件的同学注意了,了解SPI以及 ServletContainerInitializer机制这对于你来说可能是一个很好的知识点。
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处理/WEB-INF/classes下面的类的注解,某个版本Servlet支持注解方式的配置,可以猜测相关事宜就是在这里干的。
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处理Jar包中的注解类。
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将web配置按照一定规则合并到一起。
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应用全局默认配置,还记得Tomcat包下面的conf文件夹下面有个web.xml配置文件吧。
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将JSP转换为Servlet,这让我想起了若干年前对JSP的理解。
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将web配置应用到Servlet上下文,也即Servlet容器。
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将配置信息保存起来以供其他组件访问,使得其他组件不需要再次重复上面的步骤去获取配置信息了。
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检索Jar包中的静态资源。
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将ServletContainerInitializer配置到上下文。
3.3 org.apache.catalina.startup.ContextConfig.parseWebXml方法
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/**
* Parses the given source and stores the parsed data in the given web.xml
* representation. The byte stream will be closed at the end of the parse
* operation.
*
* @param source Input source containing the XML data to be parsed
* @param dest The object representation of common elements of web.xml and
* web-fragment.xml
* @param fragment Specifies whether the source is web-fragment.xml or
* web.xml
*/
protected void parseWebXml(InputSource source, WebXml dest,
boolean fragment) {
XmlErrorHandler handler = new XmlErrorHandler();
Digester digester;
WebRuleSet ruleSet;
if (fragment) {
digester = webFragmentDigester;
ruleSet = webFragmentRuleSet;
} else {
digester = webDigester;
ruleSet = webRuleSet;
}
digester.push(dest);
digester.setErrorHandler(handler);
digester.parse(source);
}
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使用Digester 对象即系web.xml,并将结果保存到WebXml对象中。
3.4 Digester的解析规则
(1)构造Digester.createWebXmlDigester
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public void createWebXmlDigester( boolean namespaceAware,
boolean validation) {
boolean blockExternal = context.getXmlBlockExternal();
webRuleSet = new WebRuleSet( false );
webDigester = DigesterFactory.newDigester(validation,
namespaceAware, webRuleSet, blockExternal);
webDigester.getParser();
webFragmentRuleSet = new WebRuleSet( true );
webFragmentDigester = DigesterFactory.newDigester(validation,
namespaceAware, webFragmentRuleSet, blockExternal);
webFragmentDigester.getParser();
}
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(2)配置解析规则 WebRuleSet.addRuleInstances
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/**
* <p>Add the set of Rule instances defined in this RuleSet to the
* specified <code>Digester</code> instance, associating them with
* our namespace URI (if any). This method should only be called
* by a Digester instance.</p>
*
* @param digester Digester instance to which the new Rule instances
* should be added.
*/
@Override
public void addRuleInstances(Digester digester) {
digester.addRule(fullPrefix,
new SetPublicIdRule( "setPublicId" ));
digester.addRule(fullPrefix,
new IgnoreAnnotationsRule());
digester.addRule(fullPrefix,
new VersionRule());
// Required for both fragments and non-fragments
digester.addRule(fullPrefix + "/absolute-ordering" , absoluteOrdering);
digester.addRule(fullPrefix + "/ordering" , relativeOrdering);
if (fragment) {
// web-fragment.xml
digester.addRule(fullPrefix + "/name" , name);
digester.addCallMethod(fullPrefix + "/ordering/after/name" ,
"addAfterOrdering" , 0 );
digester.addCallMethod(fullPrefix + "/ordering/after/others" ,
"addAfterOrderingOthers" );
digester.addCallMethod(fullPrefix + "/ordering/before/name" ,
"addBeforeOrdering" , 0 );
digester.addCallMethod(fullPrefix + "/ordering/before/others" ,
"addBeforeOrderingOthers" );
} else {
// web.xml
digester.addCallMethod(fullPrefix + "/absolute-ordering/name" ,
"addAbsoluteOrdering" , 0 );
digester.addCallMethod(fullPrefix + "/absolute-ordering/others" ,
"addAbsoluteOrderingOthers" );
}
digester.addCallMethod(fullPrefix + "/context-param" ,
"addContextParam" , 2 );
digester.addCallParam(fullPrefix + "/context-param/param-name" , 0 );
digester.addCallParam(fullPrefix + "/context-param/param-value" , 1 );
digester.addObjectCreate(fullPrefix + "/filter" ,
"org.apache.catalina.deploy.FilterDef" );
digester.addSetNext(fullPrefix + "/filter" ,
"addFilter" ,
"org.apache.catalina.deploy.FilterDef" );
digester.addCallMethod(fullPrefix + "/filter/description" ,
"setDescription" , 0 );
digester.addCallMethod(fullPrefix + "/filter/display-name" ,
"setDisplayName" , 0 );
digester.addCallMethod(fullPrefix + "/filter/filter-class" ,
"setFilterClass" , 0 );
digester.addCallMethod(fullPrefix + "/filter/filter-name" ,
"setFilterName" , 0 );
digester.addCallMethod(fullPrefix + "/filter/icon/large-icon" ,
"setLargeIcon" , 0 );
digester.addCallMethod(fullPrefix + "/filter/icon/small-icon" ,
"setSmallIcon" , 0 );
digester.addCallMethod(fullPrefix + "/filter/async-supported" ,
"setAsyncSupported" , 0 );
digester.addCallMethod(fullPrefix + "/filter/init-param" ,
"addInitParameter" , 2 );
digester.addCallParam(fullPrefix + "/filter/init-param/param-name" ,
0 );
digester.addCallParam(fullPrefix + "/filter/init-param/param-value" ,
1 );
digester.addObjectCreate(fullPrefix + "/filter-mapping" ,
"org.apache.catalina.deploy.FilterMap" );
digester.addSetNext(fullPrefix + "/filter-mapping" ,
"addFilterMapping" ,
"org.apache.catalina.deploy.FilterMap" );
digester.addCallMethod(fullPrefix + "/filter-mapping/filter-name" ,
"setFilterName" , 0 );
digester.addCallMethod(fullPrefix + "/filter-mapping/servlet-name" ,
"addServletName" , 0 );
digester.addCallMethod(fullPrefix + "/filter-mapping/url-pattern" ,
"addURLPattern" , 0 );
digester.addCallMethod(fullPrefix + "/filter-mapping/dispatcher" ,
"setDispatcher" , 0 );
digester.addCallMethod(fullPrefix + "/listener/listener-class" ,
"addListener" , 0 );
...
}
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在这个方法里,可以看到熟悉的“/servlet/servlet-name”,"/listener/listener-class"等等。稍微懂点Digester解析语法的基础的朋友,立刻可以知道这儿就是解析规则所在,Digester解析web.xml规则都是在此配置的。进一步梳理下,可以弄明白servlet,filter等重要对象的数据载体。
标签 |
数据载体类 |
/filter |
org.apache.catalina.deploy.FilterDef |
/error-page |
org.apache.catalina.deploy.ErrorPage |
/servlet |
org.apache.catalina.deploy.ServletDef |
/filter-mapping | org.apache.catalina.deploy.FilterMap |
/login-config | org.apache.catalina.deploy.LoginConfig |
/session-config | org.apache.catalina.deploy.SessionConfig |
... |
... |
部分私有内部Rule列表
通过分析,可以知道web.xml通过解析之后,配置信息都保存在WebXml对象中了。
WebXml中持有FilterMap,ServletDef,FilterDef等等对象的聚集信息。接下来tomcat就可以按照servlet规范初始化里面的组件了,有空将进一步介绍。
本文转自 randy_shandong 51CTO博客,原文链接:http://blog.51cto.com/dba10g/1775723,如需转载请自行联系原作者