前言
Servlet实际上就是一个java类，只不过可以和浏览器进行一些数据的交换。有Servlet类就有管理Servlet的容器。
Servlet的工作过程大致可以分为以下几个阶段：
- 启动Tomcat容器
- Web应用初始化
- 创建Servlet实例
- 初始化Servlet
- 执行Servlet的service方法
Tomcat的启动过程
Web应用初始化
初始化工作是由ContextConfig类的configureStart方法完成的，该方法的主要任务是完成web.xml配置文件的解析。下面解析的关键代码：
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(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(context.getServletContext());
}
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());
}
}
}在代码已经清晰地说明了Web应用的初始化过程，由于在Tomcat7中增加了对注解(annotation)的支持，所以会对Servlet中的注解进行解析。首先查找jar包中的web-fragment.xml，并对其进行解析，接下来将对/WEB-INF/classes目录下的class进行注解的解析。之后，把web-fragment.xml文件合并到web.xml中，被解析后的web.xml文件的配置项将保存到WebXml对象中，然后把WebXml对象中的属性设置到Context容器中，这个过程是由configureContext方法来完成的。下面是其部分源码——完成Servlet的解析：
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);
}这段代码说明了把Servlet包装成StandardWrapper的过程，并把这个Wrapper实例设置到Context容器中。之所以要把Servlet封装成一个Wrapper，主要为了解耦，因为Wrapper是一个容器而Servlet是一个具体的类。
到目前为止，已经完成了web应用的初始化，其中将web.xml进行了解析并完成了注解的解析，还把配置的Servlet类封装成了一个Wrapper容器，接下来就是根据这个Wrapper创建Servlet实例了。
创建Servlet实例
创建Servlet实例要回到我们分析Wrapper容器中提到的loadServlet方法了，这个方法是由Wrapper容器的标准实现类StandardWrapper完成的，通过loadServlet方法获取servletClass，然后把这个servletClass交给实例管理器(InstanceManager)完成servletClass.class的对象创建。
初始化Servlet
初始化的工作是由StandardWrapper的initServlet方法完成的，这个方法主要就是调用Servlet的init方法，然后把包装了StandardWrapper的StandardWrapperFacade交给Servlet。下面是这个方法的源码：
private synchronized void initServlet(Servlet servlet)
throws ServletException {
if (instanceInitialized && !singleThreadModel) return;
// Call the initialization method of this servlet
try {
instanceSupport.fireInstanceEvent(InstanceEvent.BEFORE_INIT_EVENT,
servlet);
if( Globals.IS_SECURITY_ENABLED) {
boolean success = false;
try {
Object[] args = new Object[] { facade };
SecurityUtil.doAsPrivilege("init",
servlet,
classType,
args);
success = true;
} finally {
if (!success) {
// destroy() will not be called, thus clear the reference now
SecurityUtil.remove(servlet);
}
}
} else {
servlet.init(facade);
}
instanceInitialized = true;
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
servlet);
} catch (UnavailableException f) {
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
servlet, f);
unavailable(f);
throw f;
} catch (ServletException f) {
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
servlet, f);
// If the servlet wanted to be unavailable it would have
// said so, so do not call unavailable(null).
throw f;
} catch (Throwable f) {
ExceptionUtils.handleThrowable(f);
getServletContext().log("StandardWrapper.Throwable", f );
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
servlet, f);
// If the servlet wanted to be unavailable it would have
// said so, so do not call unavailable(null).
throw new ServletException
(sm.getString("standardWrapper.initException", getName()), f);
}
}这样就完成了Servlet的初始化，下面就是执行Servlet的service方法了。
执行service方法
在分析Wrapper方法中提到，StandardWrapper会调用allocate方法从实例池栈中弹出一个Servlet处理请求，这个Servlet实例在完成初始化后，并经过一系列过滤器的过滤后就到达Servlet实例的service方法，这个过程就是过滤器执行的过程。这样Servlet实例就顺利调用到了service方法，之后发生的过程就是我们熟悉的request获取参数并用response进行响应的过程了。
原文博主地址：rhwayfunn