本文详细剖析了Tomcat中Servlet的创建与执行流程，包括StandardWrapperValve的invoke方法、servlet实例化过程及过滤器链执行机制。
我们知道当tomcat收到一个请求会经过一系列的调用过程猜到servlet的执行。那么我们从这中间的standardWrapperValve的invoke开始。
public final void invoke(Request request, Response response)
throws IOException, ServletException {
........此处省略部分代码..............
servlet = wrapper.allocate();
........此处省略部分代码..............
ApplicationFilterFactory factory =
ApplicationFilterFactory.getInstance();
ApplicationFilterChain filterChain =
factory.createFilterChain(request, wrapper, servlet);
........此处省略部分代码..............
if (comet) {
filterChain.doFilterEvent(request.getEvent());
request.setComet(true);
} else {
filterChain.doFilter(request.getRequest(),
response.getResponse());
}
}
........此处省略部分代码..............
if (comet) {
request.setComet(true);
filterChain.doFilterEvent(request.getEvent());
} else {
filterChain.doFilter
(request.getRequest(), response.getResponse());
}
}
........此处省略部分代码..............
}可以看出invoke的作用有两个：一个是创建servlet，另外一个是执行过滤器链，其实当执行完过滤器后会去调用servlet的service方法(在ApplicationFilterChain中的doFilter)。
public Servlet allocate() throws ServletException {
// If we are currently unloading this servlet, throw an exception
if (unloading)
throw new ServletException
(sm.getString("standardWrapper.unloading", getName()));
boolean newInstance = false;
// 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 {
if (log.isDebugEnabled())
log.debug("Allocating non-STM instance");
instance = loadServlet();
// For non-STM, increment here to prevent a race
// condition with unload. Bug 43683, test case #3
if (!singleThreadModel) {
newInstance = true;
countAllocated.incrementAndGet();
}
} catch (ServletException e) {
throw e;
} catch (Throwable e) {
throw new ServletException
(sm.getString("standardWrapper.allocate"), e);
}
}
}
}
if (!singleThreadModel) {
if (log.isTraceEnabled())
log.trace(" Returning non-STM instance");
// For new instances, count will have been incremented at the
// time of creation
if (!newInstance) {
countAllocated.incrementAndGet();
}
return (instance);
}
}
synchronized (instancePool) {
while (countAllocated.get() >= 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 (log.isTraceEnabled())
log.trace(" Returning allocated STM instance");
countAllocated.incrementAndGet();
return (Servlet) instancePool.pop();
}
}在分析代码之前，先来说说SingleThreadModel接口吧。实现此接口的目的是保证servlet一次只能有一个请求，不会有两个线程同是使用servlet的service方法，该接口不能避免同步而产生的问题，如访问静态类变量或该servlet以外的类或变量。我们把实现了该接口的servlet叫做STM组件。
在allocate中对两种情况都做了实现。如果一个servlet没有实现接口SingleThreadModel，那么该servlet被加载一次，以后每次的请求都返回相同的实例即可。而实现该接口的情况就不同了，如果StandardWrapper只维持了一个STM servlet的实例，那么就需要对这个servlet实例做同步操作，当然为了性能考虑也可以维护一个servlet池。在tomcat中很明显可以看出它是维护了一个实例池。知道了这些，那么理解allocate方法就不难了吧。
虽然StandardWrapperValve从StandardWrapper中的allocate方法获得了servlet实例，但是真正创建该servlet实例的并不是allocate方法，而是loadServlet方法。我们从allocate方法中也可以看出这点。
instance = loadServlet();instancePool.push(loadServlet());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;
if (swallowOutput) {
SystemLogHandler.startCapture();
}
Servlet servlet;
try {
long t1=System.currentTimeMillis();
// 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();
// Merge init parameters
String paramNames[] = jspWrapper.findInitParameters();
for (int i = 0; i < paramNames.length; i++) {
if (parameters.get(paramNames[i]) == null) {
parameters.put
(paramNames[i],
jspWrapper.findInitParameter(paramNames[i]));
}
}
}
}
// 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) &&
! ((Context)getParent()).getPrivileged() ) {
// If it is a priviledged context - using its own
// class loader will work, since it's a child of the container
// loader
classLoader = this.getClass().getClassLoader();
}
// Load the specified servlet class from the appropriate class loader
Class classClass = null;
try {
if (SecurityUtil.isPackageProtectionEnabled()){
final ClassLoader fclassLoader = classLoader;
final String factualClass = actualClass;
try{
classClass = (Class)AccessController.doPrivileged(
new PrivilegedExceptionAction(){
public Object run() throws Exception{
if (fclassLoader != null) {
return fclassLoader.loadClass(factualClass);
} else {
return Class.forName(factualClass);
}
}
});
} catch(PrivilegedActionException pax){
Exception ex = pax.getException();
if (ex instanceof ClassNotFoundException){
throw (ClassNotFoundException)ex;
} else {
getServletContext().log( "Error loading "
+ fclassLoader + " " + factualClass, ex );
}
}
} else {
if (classLoader != null) {
classClass = classLoader.loadClass(actualClass);
} else {
classClass = Class.forName(actualClass);
}
}
} catch (ClassNotFoundException e) {
unavailable(null);
getServletContext().log( "Error loading " + classLoader + " " + actualClass, e );
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();
// Annotation processing
if (!((Context) getParent()).getIgnoreAnnotations()) {
if (getParent() instanceof StandardContext) {
((StandardContext)getParent()).getAnnotationProcessor().processAnnotations(servlet);
((StandardContext)getParent()).getAnnotationProcessor().postConstruct(servlet);
}
}
} catch (ClassCastException e) {
unavailable(null);
// Restore the context ClassLoader
throw new ServletException
(sm.getString("standardWrapper.notServlet", actualClass), e);
} catch (Throwable e) {
unavailable(null);
// Added extra log statement for Bugzilla 36630:
// http://issues.apache.org/bugzilla/show_bug.cgi?id=36630
if(log.isDebugEnabled()) {
log.debug(sm.getString("standardWrapper.instantiate", actualClass), e);
}
// 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) ||
((Context)getParent()).getPrivileged() )) {
((ContainerServlet) servlet).setWrapper(this);
}
classLoadTime=(int) (System.currentTimeMillis() -t1);
// 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);
}
// Invoke jspInit on JSP pages
if ((loadOnStartup >= 0) && (jspFile != null)) {
// Invoking jspInit
DummyRequest req = new DummyRequest();
req.setServletPath(jspFile);
req.setQueryString(Constants.PRECOMPILE + "=true");
DummyResponse res = new DummyResponse();
if( Globals.IS_SECURITY_ENABLED) {
Object[] args = new Object[]{req, res};
SecurityUtil.doAsPrivilege("service",
servlet,
classTypeUsedInService,
args);
args = null;
} else {
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) {
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);
}
// Register our newly initialized instance
singleThreadModel = servlet instanceof SingleThreadModel;
if (singleThreadModel) {
if (instancePool == null)
instancePool = new Stack();
}
fireContainerEvent("load", this);
loadTime=System.currentTimeMillis() -t1;
} finally {
if (swallowOutput) {
String log = SystemLogHandler.stopCapture();
if (log != null && log.length() > 0) {
if (getServletContext() != null) {
getServletContext().log(log);
} else {
out.println(log);
}
}
}
}
return servlet;
} ClassLoader classLoader = loader.getClassLoader();就是为了得到这样一种特殊的servlet的类加载器。接下来就可以创建该servlet的一个实例啦。 servlet = (Servlet) classClass.newInstance();
if (!isServletAllowed(servlet)) {
throw new SecurityException
(sm.getString("standardWrapper.privilegedServlet",
actualClass));
}
如果你足够细心的话，你会发现我没有对其中的很重要的一行代码进行解释：那就是
servlet.init(facade);protected StandardWrapperFacade facade =
new StandardWrapperFacade(this);StandardWrapperFacade其实是非常简单的。只有几个方法而已，而且还是通过它封装的StandardWrapper来实现的。为了防止开发人员用到不必要的public方法tomcat采用了这个封装类。
以上就是如何得到一个servlet实例的具体情况。也许你已经忘了standardWrapperValve的invoke方法中的第二个作用了：执行过滤器链。
filterChain.doFilter(request.getRequest(),
response.getResponse());先来看看ApplicationFilterChain的内部基础结构：
private ApplicationFilterConfig[] filters =
new ApplicationFilterConfig[0];void addFilter(ApplicationFilterConfig filterConfig) {
if (n == filters.length) {
ApplicationFilterConfig[] newFilters =
new ApplicationFilterConfig[n + INCREMENT];
System.arraycopy(filters, 0, newFilters, 0, n);
filters = newFilters;
}
filters[n++] = filterConfig;
}毫无疑问过滤器是ApplicationFilterConfig类型的，
ApplicationFilterConfig(Context context, FilterDef filterDef)public void doFilter(ServletRequest request, ServletResponse response)
throws IOException, ServletException {
if( Globals.IS_SECURITY_ENABLED ) {
final ServletRequest req = request;
final ServletResponse res = response;
try {
java.security.AccessController.doPrivileged(
new java.security.PrivilegedExceptionAction() {
public Object run()
throws ServletException, IOException {
internalDoFilter(req,res);
return null;
}
}
);
} catch( PrivilegedActionException pe) {
Exception e = pe.getException();
if (e instanceof ServletException)
throw (ServletException) e;
else if (e instanceof IOException)
throw (IOException) e;
else if (e instanceof RuntimeException)
throw (RuntimeException) e;
else
throw new ServletException(e.getMessage(), e);
}
} else {
internalDoFilter(request,response);
}
}........此处省略部分代码..............
private void internalDoFilter(ServletRequest request,
ServletResponse response)
throws IOException, ServletException {
// Call the next filter if there is one
if (pos < n) {
ApplicationFilterConfig filterConfig = filters[pos++];
Filter filter = null;
try {
filter = filterConfig.getFilter();
support.fireInstanceEvent(InstanceEvent.BEFORE_FILTER_EVENT,
filter, request, response);
if( Globals.IS_SECURITY_ENABLED ) {
final ServletRequest req = request;
final ServletResponse res = response;
Principal principal =
((HttpServletRequest) req).getUserPrincipal();
Object[] args = new Object[]{req, res, this};
SecurityUtil.doAsPrivilege
("doFilter", filter, classType, args, principal);
args = null;
} else {
filter.doFilter(request, response, this);
}
support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
filter, request, response);
} ........此处省略部分代码..............
return;
}
// We fell off the end of the chain -- call the servlet instance
try {
if (Globals.STRICT_SERVLET_COMPLIANCE) {
lastServicedRequest.set(request);
lastServicedResponse.set(response);
}
support.fireInstanceEvent(InstanceEvent.BEFORE_SERVICE_EVENT,
servlet, request, response);
if ((request instanceof HttpServletRequest) &&
(response instanceof HttpServletResponse)) {
if( Globals.IS_SECURITY_ENABLED ) {
final ServletRequest req = request;
final ServletResponse res = response;
Principal principal =
((HttpServletRequest) req).getUserPrincipal();
Object[] args = new Object[]{req, res};
SecurityUtil.doAsPrivilege("service",
servlet,
classTypeUsedInService,
args,
principal);
args = null;
} else {
servlet.service((HttpServletRequest) request,
(HttpServletResponse) response);
}
} else {
servlet.service(request, response);
}
support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
servlet, request, response);
} ........此处省略部分代码..............
}过滤器链在调用完了过滤器的doFilter之后就会调用servlet的service方法。在每个过滤器的doFilter中我们可以这样实现：
public void doFilter(ServletRequest request, ServletResponse response
,FilterChain chain)
{
//...do something.....
chain.doFilter(request,response,chain);
}
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