突然想到一个问题，在tomcat容器中访问index.jsp时，容器是怎么做的呢

elixiat

今天看到tomcat conf中的web.xml的这样一段配置：

<servlet>
<servlet-name>jsp</servlet-name>
<servlet-class>org.apache.jasper.servlet.JspServlet</servlet-class>
<init-param>
<param-name>fork</param-name>
<param-value>false</param-value>
</init-param>
<init-param>
<param-name>xpoweredBy</param-name>
<param-value>false</param-value>
</init-param>
<load-on-startup>3</load-on-startup>
</servlet>
<!-- The mapping for the JSP servlet -->
<servlet-mapping>
<servlet-name>jsp</servlet-name>
<url-pattern>*.jsp</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>jsp</servlet-name>
<url-pattern>*.jspx</url-pattern>
</servlet-mapping>

这个servlet什么用呢？他拦截所有后缀时.jsp或.jspx后缀的请求，原来如此，以前是以为访问一个jspy页面，还以为是直接找到这个文件，原来是经过这个servlet来转发请求的。
看看这个servlet代码：

package org.apache.jasper.servlet;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.lang.reflect.Constructor;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.PeriodicEventListener;
import org.apache.jasper.Constants;
import org.apache.jasper.EmbeddedServletOptions;
import org.apache.jasper.Options;
import org.apache.jasper.compiler.JspRuntimeContext;
import org.apache.jasper.compiler.Localizer;
import org.apache.jasper.security.SecurityUtil;
import org.apache.juli.logging.Log;
import org.apache.juli.logging.LogFactory;
/**
* The JSP engine (a.k.a Jasper).
*
* The servlet container is responsible for providing a
* URLClassLoader for the web application context Jasper
* is being used in. Jasper will try get the Tomcat
* ServletContext attribute for its ServletContext class
* loader, if that fails, it uses the parent class loader.
* In either case, it must be a URLClassLoader.
*
* @author Anil K. Vijendran
* @author Harish Prabandham
* @author Remy Maucherat
* @author Kin-man Chung
* @author Glenn Nielsen
*/
public class JspServlet extends HttpServlet implements PeriodicEventListener {
// Logger
private Log log = LogFactory.getLog(JspServlet.class);
private ServletContext context;
private ServletConfig config;
private Options options;
private JspRuntimeContext rctxt;

/*
* Initializes this JspServlet.
*/
public void init(ServletConfig config) throws ServletException {
super.init(config);
this.config = config;
this.context = config.getServletContext();
// Initialize the JSP Runtime Context
// Check for a custom Options implementation
String engineOptionsName =
config.getInitParameter("engineOptionsClass");
if (engineOptionsName != null) {
// Instantiate the indicated Options implementation
try {
ClassLoader loader = Thread.currentThread()
.getContextClassLoader();
Class engineOptionsClass = loader.loadClass(engineOptionsName);
Class[] ctorSig = { ServletConfig.class, ServletContext.class };
Constructor ctor = engineOptionsClass.getConstructor(ctorSig);
Object[] args = { config, context };
options = (Options) ctor.newInstance(args);
} catch (Throwable e) {
// Need to localize this.
log.warn("Failed to load engineOptionsClass", e);
// Use the default Options implementation
options = new EmbeddedServletOptions(config, context);
}
} else {
// Use the default Options implementation
options = new EmbeddedServletOptions(config, context);
}
rctxt = new JspRuntimeContext(context, options);
if (log.isDebugEnabled()) {
log.debug(Localizer.getMessage("jsp.message.scratch.dir.is",
options.getScratchDir().toString()));
log.debug(Localizer.getMessage("jsp.message.dont.modify.servlets"));
}
}

/**
* Returns the number of JSPs for which JspServletWrappers exist, i.e.,
* the number of JSPs that have been loaded into the webapp with which
* this JspServlet is associated.
*
* <p>This info may be used for monitoring purposes.
*
* @return The number of JSPs that have been loaded into the webapp with
* which this JspServlet is associated
*/
public int getJspCount() {
return this.rctxt.getJspCount();
}

/**
* Resets the JSP reload counter.
*
* @param count Value to which to reset the JSP reload counter
*/
public void setJspReloadCount(int count) {
this.rctxt.setJspReloadCount(count);
}

/**
* Gets the number of JSPs that have been reloaded.
*
* <p>This info may be used for monitoring purposes.
*
* @return The number of JSPs (in the webapp with which this JspServlet is
* associated) that have been reloaded
*/
public int getJspReloadCount() {
return this.rctxt.getJspReloadCount();
}

/**
* <p>Look for a <em>precompilation request</em> as described in
* Section 8.4.2 of the JSP 1.2 Specification.  <strong>WARNING</strong> -
* we cannot use <code>request.getParameter()</code> for this, because
* that will trigger parsing all of the request parameters, and not give
* a servlet the opportunity to call
* <code>request.setCharacterEncoding()</code> first.</p>
*
* @param request The servlet requset we are processing
*
* @exception ServletException if an invalid parameter value for the
*  <code>jsp_precompile</code> parameter name is specified
*/
boolean preCompile(HttpServletRequest request) throws ServletException {
String queryString = request.getQueryString();
if (queryString == null) {
return (false);
}
int start = queryString.indexOf(Constants.PRECOMPILE);
if (start < 0) {
return (false);
}
queryString =
queryString.substring(start + Constants.PRECOMPILE.length());
if (queryString.length() == 0) {
return (true);             // ?jsp_precompile
}
if (queryString.startsWith("&")) {
return (true);             // ?jsp_precompile&foo=bar...
}
if (!queryString.startsWith("=")) {
return (false);            // part of some other name or value
}
int limit = queryString.length();
int ampersand = queryString.indexOf("&");
if (ampersand > 0) {
limit = ampersand;
}
String value = queryString.substring(1, limit);
if (value.equals("true")) {
return (true);             // ?jsp_precompile=true
} else if (value.equals("false")) {
// Spec says if jsp_precompile=false, the request should not
// be delivered to the JSP page; the easiest way to implement
// this is to set the flag to true, and precompile the page anyway.
// This still conforms to the spec, since it says the
// precompilation request can be ignored.
return (true);             // ?jsp_precompile=false
} else {
throw new ServletException("Cannot have request parameter " +
Constants.PRECOMPILE + " set to " +
value);
}
}

public void service (HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
String jspUri = null;
String jspFile = (String) request.getAttribute(Constants.JSP_FILE);
if (jspFile != null) {
// JSP is specified via <jsp-file> in <servlet> declaration
jspUri = jspFile;
} else {
/*
* Check to see if the requested JSP has been the target of a
* RequestDispatcher.include()
*/
jspUri = (String) request.getAttribute(Constants.INC_SERVLET_PATH);
if (jspUri != null) {
/*
* Requested JSP has been target of
* RequestDispatcher.include(). Its path is assembled from the
* relevant javax.servlet.include.* request attributes
*/
String pathInfo = (String) request.getAttribute(
"javax.servlet.include.path_info");
if (pathInfo != null) {
jspUri += pathInfo;
}
} else {
/*
* Requested JSP has not been the target of a
* RequestDispatcher.include(). Reconstruct its path from the
* request's getServletPath() and getPathInfo()
*/
jspUri = request.getServletPath();
String pathInfo = request.getPathInfo();
if (pathInfo != null) {
jspUri += pathInfo;
}
}
}
if (log.isDebugEnabled()) {   
log.debug("JspEngine --> " + jspUri);
log.debug("\t     ServletPath: " + request.getServletPath());
log.debug("\t        PathInfo: " + request.getPathInfo());
log.debug("\t        RealPath: " + context.getRealPath(jspUri));
log.debug("\t      RequestURI: " + request.getRequestURI());
log.debug("\t     QueryString: " + request.getQueryString());
}
try {
boolean precompile = preCompile(request);
serviceJspFile(request, response, jspUri, null, precompile);
} catch (RuntimeException e) {
throw e;
} catch (ServletException e) {
throw e;
} catch (IOException e) {
throw e;
} catch (Throwable e) {
throw new ServletException(e);
}
}
public void destroy() {
if (log.isDebugEnabled()) {
log.debug("JspServlet.destroy()");
}
rctxt.destroy();
}

public void periodicEvent() {
rctxt.checkCompile();
}
// -------------------------------------------------------- Private Methods
private void serviceJspFile(HttpServletRequest request,
HttpServletResponse response, String jspUri,
Throwable exception, boolean precompile)
throws ServletException, IOException {
JspServletWrapper wrapper = rctxt.getWrapper(jspUri);
if (wrapper == null) {
synchronized(this) {
wrapper = rctxt.getWrapper(jspUri);
if (wrapper == null) {
// Check if the requested JSP page exists, to avoid
// creating unnecessary directories and files.
if (null == context.getResource(jspUri)) {
handleMissingResource(request, response, jspUri);
return;
}
boolean isErrorPage = exception != null;
wrapper = new JspServletWrapper(config, options, jspUri,
isErrorPage, rctxt);
rctxt.addWrapper(jspUri,wrapper);
}
}
}
try {
wrapper.service(request, response, precompile);
} catch (FileNotFoundException fnfe) {
handleMissingResource(request, response, jspUri);
}
}

private void handleMissingResource(HttpServletRequest request,
HttpServletResponse response, String jspUri)
throws ServletException, IOException {
String includeRequestUri =
(String)request.getAttribute("javax.servlet.include.request_uri");
if (includeRequestUri != null) {
// This file was included. Throw an exception as
// a response.sendError() will be ignored
String msg =
Localizer.getMessage("jsp.error.file.not.found",jspUri);
// Strictly, filtering this is an application
// responsibility but just in case...
throw new ServletException(SecurityUtil.filter(msg));
} else {
try {
response.sendError(HttpServletResponse.SC_NOT_FOUND,
request.getRequestURI());
} catch (IllegalStateException ise) {
log.error(Localizer.getMessage("jsp.error.file.not.found",
jspUri));
}
}
return;
}

}