Tomcat7中web应用加载原理（二）web.xml解析
程序员文章站
2022-03-07 10:24:25
...
前一篇文章讲了org.apache.catalina.startup.HostConfig的lifecycleEvent方法中所做的事情。最后看到在Tomcat启动时或启动后（后台线程定时扫描）会调用HostConfig类的deployApps方法：
/**
* Deploy applications for any directories or WAR files that are found
* in our "application root" directory.
*/
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);
}
可以看到这里部署应用有三种方式：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（即调用host.addChild(context)这句，具体代码在HostConfig类的deployDescriptor(ContextName cn, File contextXml)、deployDirectory(ContextName cn, File dir)、deployWAR(ContextName cn, File war)三个方法中，这里不再贴出代码来详细分析）。
前一篇文章只分析到这步，可以看出与一个web应用相对应的一个Context对象已经构建出来了，但如果容器只执行到这里根本无法响应一个浏览器的一次请求。就web服务器的实现来看一次请求过来除了需要根据内部Context构建找到这次请求访问的web应用具体所对应的Context对象，还需要包含web应用中具体的哪个Servlet来处理这次请求，中间是否还需要执行相应的过滤器(filter)、监听器(listener)等，做过java的web开发的都知道，这些信息是配置在一个web应用的WEB-INF\web.xml文件的（servlet3中已经支持将这些配置信息放到Java文件的注解中，但万变不离其宗，总归要在web应用的某个地方说明，并在容器启动时加载，这样才能真正提供web服务，响应请求）。
看到这里可以猜到Tomcat容器加载web应用时必定会有对于每个应用的web.xml文件的解析过程，本文就来看看这个解析过程。
在本文开头提到的三种部署应用的实现代码中有一些共通的代码，这里摘出来说明一下：
Class<?> clazz = Class.forName(host.getConfigClass());
LifecycleListener listener =
(LifecycleListener) clazz.newInstance();
context.addLifecycleListener(listener);
host.addChild(context);
第一段是在所有Context对象构建时会添加一个监听器，这里监听器的类名是StandardHost类的实例变量configClass，其默认值就是org.apache.catalina.startup.ContextConfig。第二段是将当前构建的Context对象添加到父容器Host对象中。
先看下StandardHost的addChild方法的实现：
public void addChild(Container child) {
child.addLifecycleListener(new MemoryLeakTrackingListener());
if (!(child instanceof Context))
throw new IllegalArgumentException
(sm.getString("standardHost.notContext"));
super.addChild(child);
}
可以看到这段代码最后调用了父类的addChild方法：
public void addChild(Container child) {
if (Globals.IS_SECURITY_ENABLED) {
PrivilegedAction<Void> dp =
new PrivilegedAddChild(child);
AccessController.doPrivileged(dp);
} else {
addChildInternal(child);
}
}
这里看下addChildInternal方法的实现：
private void addChildInternal(Container child) {
if( log.isDebugEnabled() )
log.debug("Add child " + child + " " + this);
synchronized(children) {
if (children.get(child.getName()) != null)
throw new IllegalArgumentException("addChild: Child name '" +
child.getName() +
"' is not unique");
child.setParent(this); // May throw IAE
children.put(child.getName(), child);
}
// Start child
// Don't do this inside sync block - start can be a slow process and
// locking the children object can cause problems elsewhere
if ((getState().isAvailable() ||
LifecycleState.STARTING_PREP.equals(getState())) &&
startChildren) {
try {
child.start();
} catch (LifecycleException e) {
log.error("ContainerBase.addChild: start: ", e);
throw new IllegalStateException
("ContainerBase.addChild: start: " + e);
}
}
fireContainerEvent(ADD_CHILD_EVENT, child);
}
可以看到会调用子容器的start方法，就是指调用StandardContext的start方法。
即给host对象添加子容器时将会调用子容器的start方法，按照前面文章的分析，调用StandardContext的start方法最终会调用org.apache.catalina.core.StandardContext类的startInternal方法（该方法代码较长，建议自己阅读，不再贴出），这里将会发布一系列事件，按调用前后顺序这些事件包括：BEFORE_INIT_EVENT、AFTER_INIT_EVENT、BEFORE_START_EVENT、CONFIGURE_START_EVENT、START_EVENT、AFTER_START_EVENT。
前面提到在构建Context对象时都会注册一个监听器org.apache.catalina.startup.ContextConfig，看下这个类的lifecycleEvent方法中（为什么会执行这个方法可以看本博文章的分析）监听了哪些事件：
/**
* Process events for an associated Context.
*
* @param event The lifecycle event that has occurred
*/
@Override
public void lifecycleEvent(LifecycleEvent event) {
// Identify the context we are associated with
try {
context = (Context) event.getLifecycle();
} catch (ClassCastException e) {
log.error(sm.getString("contextConfig.cce", event.getLifecycle()), e);
return;
}
// Process the event that has occurred
if (event.getType().equals(Lifecycle.CONFIGURE_START_EVENT)) {
configureStart();
} else if (event.getType().equals(Lifecycle.BEFORE_START_EVENT)) {
beforeStart();
} else if (event.getType().equals(Lifecycle.AFTER_START_EVENT)) {
// Restore docBase for management tools
if (originalDocBase != null) {
context.setDocBase(originalDocBase);
}
} else if (event.getType().equals(Lifecycle.CONFIGURE_STOP_EVENT)) {
configureStop();
} else if (event.getType().equals(Lifecycle.AFTER_INIT_EVENT)) {
init();
} else if (event.getType().equals(Lifecycle.AFTER_DESTROY_EVENT)) {
destroy();
}
}
与Context的start方法调用相关的事件监听前后顺序为：AFTER_INIT_EVENT（执行init方法）、BEFORE_START_EVENT（执行beforeStart方法）、CONFIGURE_START_EVENT（执行configureStart方法）。
在configureStart方法将直接调用webConfig方法，正是在这个方法中将会解析web.xml文件：
/**
* 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() {
/*
* Anything and everything can override the global and host defaults.
* This is implemented in two parts
* - Handle as a web fragment that gets added after everything else so
* everything else takes priority
* - Mark Servlets as overridable so SCI configuration can replace
* configuration from the defaults
*/
/*
* The rules for annotation scanning are not as clear-cut as one might
* think. Tomcat implements the following process:
* - As per SRV.1.6.2, Tomcat will scan for annotations regardless of
* which Servlet spec version is declared in web.xml. The EG has
* confirmed this is the expected behaviour.
* - As per http://java.net/jira/browse/SERVLET_SPEC-36, if the main
* web.xml is marked as metadata-complete, JARs are still processed
* for SCIs.
* - If metadata-complete=true and an absolute ordering is specified,
* JARs excluded from the ordering are also excluded from the SCI
* processing.
* - If an SCI has a @HandlesType annotation then all classes (except
* those in JARs excluded from an absolute ordering) need to be
* scanned to check if they match.
*/
Set<WebXml> defaults = new HashSet<WebXml>();
defaults.add(getDefaultWebXmlFragment());
WebXml webXml = createWebXml();
// Parse context level web.xml
InputSource contextWebXml = getContextWebXmlSource();
parseWebXml(contextWebXml, webXml, false);
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();
// Step 2. Order the fragments.
Set<WebXml> orderedFragments = null;
orderedFragments =
WebXml.orderWebFragments(webXml, fragments, sContext);
// Step 3. Look for ServletContainerInitializer implementations
if (ok) {
processServletContainerInitializers(orderedFragments);
}
if (!webXml.isMetadataComplete() || typeInitializerMap.size() > 0) {
// Step 4. Process /WEB-INF/classes for annotations
if (ok) {
// Hack required by Eclipse's "serve modules without
// publishing" feature since this backs WEB-INF/classes by
// multiple locations rather than one.
NamingEnumeration<Binding> listBindings = null;
try {
try {
listBindings = context.getResources().listBindings(
"/WEB-INF/classes");
} catch (NameNotFoundException ignore) {
// Safe to ignore
}
while (listBindings != null &&
listBindings.hasMoreElements()) {
Binding binding = listBindings.nextElement();
if (binding.getObject() instanceof FileDirContext) {
File webInfClassDir = new File(
((FileDirContext) binding.getObject()).getDocBase());
processAnnotationsFile(webInfClassDir, webXml,
webXml.isMetadataComplete());
} else {
String resource =
"/WEB-INF/classes/" + binding.getName();
try {
URL url = sContext.getResource(resource);
processAnnotationsUrl(url, webXml,
webXml.isMetadataComplete());
} catch (MalformedURLException e) {
log.error(sm.getString(
"contextConfig.webinfClassesUrl",
resource), e);
}
}
}
} catch (NamingException e) {
log.error(sm.getString(
"contextConfig.webinfClassesUrl",
"/WEB-INF/classes"), e);
}
}
// 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();
}
if (!webXml.isMetadataComplete()) {
// Step 6. Merge web-fragment.xml files into the main web.xml
// file.
if (ok) {
ok = webXml.merge(orderedFragments);
}
// Step 7. Apply global defaults
// Have to merge defaults before JSP conversion since defaults
// provide JSP servlet definition.
webXml.merge(defaults);
// Step 8. Convert explicitly mentioned jsps to servlets
if (ok) {
convertJsps(webXml);
}
// Step 9. Apply merged web.xml to Context
if (ok) {
webXml.configureContext(context);
}
} else {
webXml.merge(defaults);
convertJsps(webXml);
webXml.configureContext(context);
}
// Step 9a. Make the merged web.xml available to other
// components, specifically Jasper, to save those components
// from having to re-generate it.
// TODO Use a ServletContainerInitializer for Jasper
String mergedWebXml = webXml.toXml();
sContext.setAttribute(
org.apache.tomcat.util.scan.Constants.MERGED_WEB_XML,
mergedWebXml);
if (context.getLogEffectiveWebXml()) {
log.info("web.xml:\n" + mergedWebXml);
}
// Always need to look for static resources
// Step 10. Look for static resources packaged in JARs
if (ok) {
// Spec does not define an order.
// Use ordered JARs followed by remaining JARs
Set<WebXml> resourceJars = new LinkedHashSet<WebXml>();
if (orderedFragments != null) {
for (WebXml fragment : orderedFragments) {
resourceJars.add(fragment);
}
}
for (WebXml fragment : fragments.values()) {
if (!resourceJars.contains(fragment)) {
resourceJars.add(fragment);
}
}
processResourceJARs(resourceJars);
// See also StandardContext.resourcesStart() for
// WEB-INF/classes/META-INF/resources configuration
}
// Step 11. Apply the ServletContainerInitializer config to the
// context
if (ok) {
for (Map.Entry<ServletContainerInitializer,
Set<Class<?>>> entry :
initializerClassMap.entrySet()) {
if (entry.getValue().isEmpty()) {
context.addServletContainerInitializer(
entry.getKey(), null);
} else {
context.addServletContainerInitializer(
entry.getKey(), entry.getValue());
}
}
}
}
这个方法里面做的事情，在英文注释中说的很清楚了，概括起来包括合并Tomcat全局web.xml、当前应用中的web.xml、web-fragment.xml和web应用的注解中的配置信息，并将解析出的各种配置信息（如servlet配置、filter配置等）关联到Context对象中（在上面的代码第140行：webXml.configureContext(context)）。
看下configureContext方法：
/**
* Configure a {@link Context} using the stored web.xml representation.
*
* @param context The context to be configured
*/
public void configureContext(Context context) {
// As far as possible, process in alphabetical order so it is easy to
// check everything is present
// Some validation depends on correct public ID
context.setPublicId(publicId);
// Everything else in order
context.setEffectiveMajorVersion(getMajorVersion());
context.setEffectiveMinorVersion(getMinorVersion());
for (Entry<String, String> entry : contextParams.entrySet()) {
context.addParameter(entry.getKey(), entry.getValue());
}
context.setDisplayName(displayName);
context.setDistributable(distributable);
for (ContextLocalEjb ejbLocalRef : ejbLocalRefs.values()) {
context.getNamingResources().addLocalEjb(ejbLocalRef);
}
for (ContextEjb ejbRef : ejbRefs.values()) {
context.getNamingResources().addEjb(ejbRef);
}
for (ContextEnvironment environment : envEntries.values()) {
context.getNamingResources().addEnvironment(environment);
}
for (ErrorPage errorPage : errorPages.values()) {
context.addErrorPage(errorPage);
}
for (FilterDef filter : filters.values()) {
if (filter.getAsyncSupported() == null) {
filter.setAsyncSupported("false");
}
context.addFilterDef(filter);
}
for (FilterMap filterMap : filterMaps) {
context.addFilterMap(filterMap);
}
for (JspPropertyGroup jspPropertyGroup : jspPropertyGroups) {
JspPropertyGroupDescriptor descriptor =
new ApplicationJspPropertyGroupDescriptor(jspPropertyGroup);
context.getJspConfigDescriptor().getJspPropertyGroups().add(
descriptor);
}
for (String listener : listeners) {
context.addApplicationListener(
new ApplicationListener(listener, false));
}
for (Entry<String, String> entry : localeEncodingMappings.entrySet()) {
context.addLocaleEncodingMappingParameter(entry.getKey(),
entry.getValue());
}
// Prevents IAE
if (loginConfig != null) {
context.setLoginConfig(loginConfig);
}
for (MessageDestinationRef mdr : messageDestinationRefs.values()) {
context.getNamingResources().addMessageDestinationRef(mdr);
}
// messageDestinations were ignored in Tomcat 6, so ignore here
context.setIgnoreAnnotations(metadataComplete);
for (Entry<String, String> entry : mimeMappings.entrySet()) {
context.addMimeMapping(entry.getKey(), entry.getValue());
}
// Name is just used for ordering
for (ContextResourceEnvRef resource : resourceEnvRefs.values()) {
context.getNamingResources().addResourceEnvRef(resource);
}
for (ContextResource resource : resourceRefs.values()) {
context.getNamingResources().addResource(resource);
}
for (SecurityConstraint constraint : securityConstraints) {
context.addConstraint(constraint);
}
for (String role : securityRoles) {
context.addSecurityRole(role);
}
for (ContextService service : serviceRefs.values()) {
context.getNamingResources().addService(service);
}
for (ServletDef servlet : servlets.values()) {
Wrapper wrapper = context.createWrapper();
// Description is ignored
// Display name is ignored
// Icons are ignored
// jsp-file gets passed to the JSP Servlet as an init-param
if (servlet.getLoadOnStartup() != null) {
wrapper.setLoadOnStartup(servlet.getLoadOnStartup().intValue());
}
if (servlet.getEnabled() != null) {
wrapper.setEnabled(servlet.getEnabled().booleanValue());
}
wrapper.setName(servlet.getServletName());
Map<String,String> params = servlet.getParameterMap();
for (Entry<String, String> entry : params.entrySet()) {
wrapper.addInitParameter(entry.getKey(), entry.getValue());
}
wrapper.setRunAs(servlet.getRunAs());
Set<SecurityRoleRef> roleRefs = servlet.getSecurityRoleRefs();
for (SecurityRoleRef roleRef : roleRefs) {
wrapper.addSecurityReference(
roleRef.getName(), roleRef.getLink());
}
wrapper.setServletClass(servlet.getServletClass());
MultipartDef multipartdef = servlet.getMultipartDef();
if (multipartdef != null) {
if (multipartdef.getMaxFileSize() != null &&
multipartdef.getMaxRequestSize()!= null &&
multipartdef.getFileSizeThreshold() != null) {
wrapper.setMultipartConfigElement(new MultipartConfigElement(
multipartdef.getLocation(),
Long.parseLong(multipartdef.getMaxFileSize()),
Long.parseLong(multipartdef.getMaxRequestSize()),
Integer.parseInt(
multipartdef.getFileSizeThreshold())));
} else {
wrapper.setMultipartConfigElement(new MultipartConfigElement(
multipartdef.getLocation()));
}
}
if (servlet.getAsyncSupported() != null) {
wrapper.setAsyncSupported(
servlet.getAsyncSupported().booleanValue());
}
wrapper.setOverridable(servlet.isOverridable());
context.addChild(wrapper);
}
for (Entry<String, String> entry : servletMappings.entrySet()) {
context.addServletMapping(entry.getKey(), entry.getValue());
}
if (sessionConfig != null) {
if (sessionConfig.getSessionTimeout() != null) {
context.setSessionTimeout(
sessionConfig.getSessionTimeout().intValue());
}
SessionCookieConfig scc =
context.getServletContext().getSessionCookieConfig();
scc.setName(sessionConfig.getCookieName());
scc.setDomain(sessionConfig.getCookieDomain());
scc.setPath(sessionConfig.getCookiePath());
scc.setComment(sessionConfig.getCookieComment());
if (sessionConfig.getCookieHttpOnly() != null) {
scc.setHttpOnly(sessionConfig.getCookieHttpOnly().booleanValue());
}
if (sessionConfig.getCookieSecure() != null) {
scc.setSecure(sessionConfig.getCookieSecure().booleanValue());
}
if (sessionConfig.getCookieMaxAge() != null) {
scc.setMaxAge(sessionConfig.getCookieMaxAge().intValue());
}
if (sessionConfig.getSessionTrackingModes().size() > 0) {
context.getServletContext().setSessionTrackingModes(
sessionConfig.getSessionTrackingModes());
}
}
for (Entry<String, String> entry : taglibs.entrySet()) {
TaglibDescriptor descriptor = new ApplicationTaglibDescriptor(
entry.getValue(), entry.getKey());
context.getJspConfigDescriptor().getTaglibs().add(descriptor);
}
// Context doesn't use version directly
for (String welcomeFile : welcomeFiles) {
/*
* The following will result in a welcome file of "" so don't add
* that to the context
* <welcome-file-list>
* <welcome-file/>
* </welcome-file-list>
*/
if (welcomeFile != null && welcomeFile.length() > 0) {
context.addWelcomeFile(welcomeFile);
}
}
// Do this last as it depends on servlets
for (JspPropertyGroup jspPropertyGroup : jspPropertyGroups) {
String jspServletName = context.findServletMapping("*.jsp");
if (jspServletName == null) {
jspServletName = "jsp";
}
if (context.findChild(jspServletName) != null) {
for (String urlPattern : jspPropertyGroup.getUrlPatterns()) {
context.addServletMapping(urlPattern, jspServletName, true);
}
} else {
if(log.isDebugEnabled()) {
for (String urlPattern : jspPropertyGroup.getUrlPatterns()) {
log.debug("Skiping " + urlPattern + " , no servlet " +
jspServletName);
}
}
}
}
for (Entry<String, String> entry : postConstructMethods.entrySet()) {
context.addPostConstructMethod(entry.getKey(), entry.getValue());
}
for (Entry<String, String> entry : preDestroyMethods.entrySet()) {
context.addPreDestroyMethod(entry.getKey(), entry.getValue());
}
}
可以看到里面对context调用了各种set、add方法，从而将web.xml中的各种配置信息与表示一个web应用的context对象关联起来。