我们已经知道，在tomcat中有四种类型的servlet容器，分别为Engine、Host、Context 和Wrapper，本文接下来对tomcat中Wrapper接口的标准实现进行说明。
对于每个引入的HTTP请求，连接器都会调用与其关联的servlet容器的invoke()方法；然后，servlet容器会调用所有子容器的invoke()方法
这里面的流程通常是servlet容器调用其管道对象的invoke()方法，其管道对象的invoke()方法最后调用其基础阀，管道对象的基础阀里面会调用子容器的invoke()方法；子容器的invoke()方法的调用序列与之相同。
在tomcat中，servlet类可以实现javax.servlet.SingleThreadModel接口，这样的servlet类也称为SingleThreadModel（STM）servlet类；根据Servlet规范，实现此接口的目的是保证servlet实例一次只处理一个请求。
StandardWrapper对象的主要任务是载入它所代表的servlet类，并进行实例化；不过StandardWrapper类并不调用servlet的service方法，该任务由StandardWrapperValve对象（StandardWrapper实例的管道对象中的基础阀）完成；StandardWrapperValve对象通过调用StandardWrapper实例的allocate()方法获取servlet实例，在获取实例后，StandardWrapperValve实例就会调用servlet实例的service()方法
对于STM 类型的servlet类与非STM 类型的servlet类，StandardWrapper实例的载入方式是不一样的；对于非STM 类型的servlet类，StandardWrapper实例只会载入一次，对于随后的请求都会返回servlet的同一个实例，它假设该servlet类的service()方法在多线程环境中是线程安全的。
而对于STM 类型的servlet类，StandardWrapper实例必须保证每一时刻只能有一个线程在执行STM servlet类的service()方法；StandardWrapper实例通过将STM 类型的servlet实例保存在一个java.util.Stack类型的栈中
public Servlet allocate() throws ServletException {
if (debug >= 1)
log("Allocating an instance");
// If we are currently unloading this servlet, throw an exception
if (unloading)
throw new ServletException
(sm.getString("standardWrapper.unloading", getName()));
// If not SingleThreadedModel, return the same instance every time
if (!singleThreadModel) {
// Load and initialize our instance if necessary
if (instance == null) {
synchronized (this) {
if (instance == null) {
try {
instance = loadServlet();
} catch (ServletException e) {
throw e;
} catch (Throwable e) {
throw new ServletException
(sm.getString("standardWrapper.allocate"), e);
}
}
}
}
if (!singleThreadModel) {
if (debug >= 2)
log(" Returning non-STM instance");
countAllocated++;
return (instance);
}
}
synchronized (instancePool) {
while (countAllocated >= nInstances) {
// Allocate a new instance if possible, or else wait
if (nInstances < maxInstances) {
try {
instancePool.push(loadServlet());
nInstances++;
} catch (ServletException e) {
throw e;
} catch (Throwable e) {
throw new ServletException
(sm.getString("standardWrapper.allocate"), e);
}
} else {
try {
instancePool.wait();
} catch (InterruptedException e) {
;
}
}
}
if (debug >= 2)
log(" Returning allocated STM instance");
countAllocated++;
return (Servlet) instancePool.pop();
}
}
而在loadServlet()方法中，首先获取要加载的servlet的完整类名及类加载器，然后通过类加载器加载该servlet类，然后实例化该servlet类，最后调用该servlet实例的init()方法初始化（传入javax.servlet.ServletConfig对象）
public synchronized Servlet loadServlet() throws ServletException {
// Nothing to do if we already have an instance or an instance pool
if (!singleThreadModel && (instance != null))
return instance;
PrintStream out = System.out;
SystemLogHandler.startCapture();
Servlet servlet = null;
try {
// If this "servlet" is really a JSP file, get the right class.
// HOLD YOUR NOSE - this is a kludge that avoids having to do special
// case Catalina-specific code in Jasper - it also requires that the
// servlet path be replaced by the <jsp-file> element content in
// order to be completely effective
String actualClass = servletClass;
if ((actualClass == null) && (jspFile != null)) {
Wrapper jspWrapper = (Wrapper)
((Context) getParent()).findChild(Constants.JSP_SERVLET_NAME);
if (jspWrapper != null)
actualClass = jspWrapper.getServletClass();
}
// Complain if no servlet class has been specified
if (actualClass == null) {
unavailable(null);
throw new ServletException
(sm.getString("standardWrapper.notClass", getName()));
}
// Acquire an instance of the class loader to be used
Loader loader = getLoader();
if (loader == null) {
unavailable(null);
throw new ServletException
(sm.getString("standardWrapper.missingLoader", getName()));
}
ClassLoader classLoader = loader.getClassLoader();
// Special case class loader for a container provided servlet
if (isContainerProvidedServlet(actualClass)) {
classLoader = this.getClass().getClassLoader();
log(sm.getString
("standardWrapper.containerServlet", getName()));
}
// Load the specified servlet class from the appropriate class loader
Class classClass = null;
try {
if (classLoader != null) {
System.out.println("Using classLoader.loadClass");
classClass = classLoader.loadClass(actualClass);
} else {
System.out.println("Using forName");
classClass = Class.forName(actualClass);
}
} catch (ClassNotFoundException e) {
unavailable(null);
throw new ServletException
(sm.getString("standardWrapper.missingClass", actualClass),
e);
}
if (classClass == null) {
unavailable(null);
throw new ServletException
(sm.getString("standardWrapper.missingClass", actualClass));
}
// Instantiate and initialize an instance of the servlet class itself
try {
servlet = (Servlet) classClass.newInstance();
} catch (ClassCastException e) {
unavailable(null);
// Restore the context ClassLoader
throw new ServletException
(sm.getString("standardWrapper.notServlet", actualClass), e);
} catch (Throwable e) {
unavailable(null);
// Restore the context ClassLoader
throw new ServletException
(sm.getString("standardWrapper.instantiate", actualClass), e);
}
// Check if loading the servlet in this web application should be
// allowed
if (!isServletAllowed(servlet)) {
throw new SecurityException
(sm.getString("standardWrapper.privilegedServlet",
actualClass));
}
// Special handling for ContainerServlet instances
if ((servlet instanceof ContainerServlet) &&
isContainerProvidedServlet(actualClass)) {
System.out.println("calling setWrapper");
((ContainerServlet) servlet).setWrapper(this);
System.out.println("after calling setWrapper");
}
// Call the initialization method of this servlet
try {
instanceSupport.fireInstanceEvent(InstanceEvent.BEFORE_INIT_EVENT,
servlet);
servlet.init(facade);
// Invoke jspInit on JSP pages
if ((loadOnStartup > 0) && (jspFile != null)) {
// Invoking jspInit
HttpRequestBase req = new HttpRequestBase();
HttpResponseBase res = new HttpResponseBase();
req.setServletPath(jspFile);
req.setQueryString("jsp_precompile=true");
servlet.service(req, res);
}
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) {
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);
}
// Register our newly initialized instance
singleThreadModel = servlet instanceof SingleThreadModel;
if (singleThreadModel) {
if (instancePool == null)
instancePool = new Stack();
}
fireContainerEvent("load", this);
} finally {
String log = SystemLogHandler.stopCapture();
if (log != null && log.length() > 0) {
if (getServletContext() != null) {
getServletContext().log(log);
} else {
out.println(log);
}
}
}
return servlet;
}
StandardWrapper类的loadServlet()方法在载入servlet类后，会调用该servlet实例的init()方法，该方法需要传入一个javax.servlet.ServletConfig实例作为参数；那么， StandardWrapper对象是如何获取ServletConfig对象的呢
答案就在StandardWrapper类本身，该类不仅实现了Wrapper接口，还实现了javax.servlet.ServletConfig接口，下面是相关方法实现
/**
* Return the initialization parameter value for the specified name,
* if any; otherwise return <code>null</code>.
*
* @param name Name of the initialization parameter to retrieve
*/
public String getInitParameter(String name) { return (findInitParameter(name)); } /**
* Return the set of initialization parameter names defined for this
* servlet. If none are defined, an empty Enumeration is returned.
*/
public Enumeration getInitParameterNames() { synchronized (parameters) {
return (new Enumerator(parameters.keySet()));
} } /**
* Return the servlet context with which this servlet is associated.
*/
public ServletContext getServletContext() { if (parent == null)
return (null);
else if (!(parent instanceof Context))
return (null);
else
return (((Context) parent).getServletContext()); } /**
* Return the name of this servlet.
*/
public String getServletName() { return (getName()); }
StandardWrapper实例在它的loadServlet()方法里面调用它所载入的servlet类的实例的init()方法，该方法需要一个javax.servlet.ServletConfig类型参数，理论上StandardWrapper对象可以将自身传入init()方法，不过为了类型安全，StandardWrapper类将自身实例包装成StandardWrapperFacade类的一个实例，然后再传给init()方法
public final class StandardWrapperFacade
implements ServletConfig { /**
* Create a new facede around a StandardWrapper.
*/
public StandardWrapperFacade(StandardWrapper config) { super();
this.config = (ServletConfig) config; } /**
* Wrapped config.
*/
private ServletConfig config = null; public String getServletName() {
return config.getServletName();
} public ServletContext getServletContext() {
ServletContext theContext = config.getServletContext();
if ((theContext != null) &&
(theContext instanceof ApplicationContext))
theContext = ((ApplicationContext) theContext).getFacade();
return (theContext);
} public String getInitParameter(String name) {
return config.getInitParameter(name);
} public Enumeration getInitParameterNames() {
return config.getInitParameterNames();
} }
StandardWrapperValve类是StandardWrapper实例中的基础阀，要完成两个操作
（1）执行与该servlet相关联的全部过滤器
（2）调用servlet实例的service()方法
public void invoke(Request request, Response response,
ValveContext valveContext)
throws IOException, ServletException {
// Initialize local variables we may need
boolean unavailable = false;
Throwable throwable = null;
StandardWrapper wrapper = (StandardWrapper) getContainer();
ServletRequest sreq = request.getRequest();
ServletResponse sres = response.getResponse();
Servlet servlet = null;
HttpServletRequest hreq = null;
if (sreq instanceof HttpServletRequest)
hreq = (HttpServletRequest) sreq;
HttpServletResponse hres = null;
if (sres instanceof HttpServletResponse)
hres = (HttpServletResponse) sres; // Check for the application being marked unavailable
if (!((Context) wrapper.getParent()).getAvailable()) {
hres.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
sm.getString("standardContext.isUnavailable"));
unavailable = true;
} // Check for the servlet being marked unavailable
if (!unavailable && wrapper.isUnavailable()) {
log(sm.getString("standardWrapper.isUnavailable",
wrapper.getName()));
if (hres == null) {
; // NOTE - Not much we can do generically
} else {
long available = wrapper.getAvailable();
if ((available > 0L) && (available < Long.MAX_VALUE))
hres.setDateHeader("Retry-After", available);
hres.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
sm.getString("standardWrapper.isUnavailable",
wrapper.getName()));
}
unavailable = true;
} // Allocate a servlet instance to process this request
try {
if (!unavailable) {
servlet = wrapper.allocate();
}
} catch (ServletException e) {
log(sm.getString("standardWrapper.allocateException",
wrapper.getName()), e);
throwable = e;
exception(request, response, e);
servlet = null;
} catch (Throwable e) {
log(sm.getString("standardWrapper.allocateException",
wrapper.getName()), e);
throwable = e;
exception(request, response, e);
servlet = null;
} // Acknowlege the request
try {
response.sendAcknowledgement();
} catch (IOException e) {
sreq.removeAttribute(Globals.JSP_FILE_ATTR);
log(sm.getString("standardWrapper.acknowledgeException",
wrapper.getName()), e);
throwable = e;
exception(request, response, e);
} catch (Throwable e) {
log(sm.getString("standardWrapper.acknowledgeException",
wrapper.getName()), e);
throwable = e;
exception(request, response, e);
servlet = null;
} // Create the filter chain for this request
ApplicationFilterChain filterChain =
createFilterChain(request, servlet); // Call the filter chain for this request
// NOTE: This also calls the servlet's service() method
try {
String jspFile = wrapper.getJspFile();
if (jspFile != null)
sreq.setAttribute(Globals.JSP_FILE_ATTR, jspFile);
else
sreq.removeAttribute(Globals.JSP_FILE_ATTR);
if ((servlet != null) && (filterChain != null)) {
filterChain.doFilter(sreq, sres);
}
sreq.removeAttribute(Globals.JSP_FILE_ATTR);
} catch (IOException e) {
sreq.removeAttribute(Globals.JSP_FILE_ATTR);
log(sm.getString("standardWrapper.serviceException",
wrapper.getName()), e);
throwable = e;
exception(request, response, e);
} catch (UnavailableException e) {
sreq.removeAttribute(Globals.JSP_FILE_ATTR);
log(sm.getString("standardWrapper.serviceException",
wrapper.getName()), e);
// throwable = e;
// exception(request, response, e);
wrapper.unavailable(e);
long available = wrapper.getAvailable();
if ((available > 0L) && (available < Long.MAX_VALUE))
hres.setDateHeader("Retry-After", available);
hres.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
sm.getString("standardWrapper.isUnavailable",
wrapper.getName()));
// Do not save exception in 'throwable', because we
// do not want to do exception(request, response, e) processing
} catch (ServletException e) {
sreq.removeAttribute(Globals.JSP_FILE_ATTR);
log(sm.getString("standardWrapper.serviceException",
wrapper.getName()), e);
throwable = e;
exception(request, response, e);
} catch (Throwable e) {
sreq.removeAttribute(Globals.JSP_FILE_ATTR);
log(sm.getString("standardWrapper.serviceException",
wrapper.getName()), e);
throwable = e;
exception(request, response, e);
} // Release the filter chain (if any) for this request
try {
if (filterChain != null)
filterChain.release();
} catch (Throwable e) {
log(sm.getString("standardWrapper.releaseFilters",
wrapper.getName()), e);
if (throwable == null) {
throwable = e;
exception(request, response, e);
}
} // Deallocate the allocated servlet instance
try {
if (servlet != null) {
wrapper.deallocate(servlet);
}
} catch (Throwable e) {
log(sm.getString("standardWrapper.deallocateException",
wrapper.getName()), e);
if (throwable == null) {
throwable = e;
exception(request, response, e);
}
} // If this servlet has been marked permanently unavailable,
// unload it and release this instance
try {
if ((servlet != null) &&
(wrapper.getAvailable() == Long.MAX_VALUE)) {
wrapper.unload();
}
} catch (Throwable e) {
log(sm.getString("standardWrapper.unloadException",
wrapper.getName()), e);
if (throwable == null) {
throwable = e;
exception(request, response, e);
}
} }
上述创建过滤器链的过程本人后来作了补充：How Tomcat Works（十四）补充
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