tomcat Architecture

1. Server Start up
2. Request Process Flow
Server Start up

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Tomcat 5 Startup Sequence

Sequence 1. Start from Command Line
Class: org.apache.catalina.startup.Bootstrap
What it does:
 a) Set up classloaders 
  commonLoader (common)-> System Loader
  sharedLoader (shared)-> commonLoader -> System Loader
  catalinaLoader(server) -> commonLoader -> System Loader
 b) Load startup class (reflection)
  org.apache.catalina.startup.Catalina
  setParentClassloader -> sharedLoader
  Thread.contextClassloader -> catalinaLoader
 c) Bootstrap.daemon.init() complete
 
Sequence 2. Process command line argument (start, startd, stop, stopd)
Class: org.apache.catalina.startup.Bootstrap (assume command->start)
What it does: 
 a) Catalina.setAwait(true);
 b) Catalina.load()
  b1) initDirs() -> set properties like 
                    catalina.home
                    catalina.base == catalina.home (most cases)
  b2) initNaming
   setProperty(javax.naming.Context.INITIAL_CONTEXT_FACTORY,
        org.apache.naming.java.javaURLContextFactory ->default)
  b3) createStartDigester() 
   Configures a digester for the main server.xml elements like
   org.apache.catalina.core.StandardServer (can change of course :)
   org.apache.catalina.deploy.NamingResources
    Stores naming resources in the J2EE JNDI tree
   org.apache.catalina.LifecycleListener
    implements events for start/stop of major components
   org.apache.catalina.core.StandardService
    The single entry for a set of connectors,
    so that a container can listen to multiple connectors
    ie, single entry
   org.apache.coyote.tomcat5.CoyoteConnector
    Connectors to listen for incoming requests only
   It also adds the following rulesets to the digester
    NamingRuleSet
    EngineRuleSet
    HostRuleSet
    ContextRuleSet
  b4) Load the server.xml and parse it using the digester
      Parsing the server.xml using the digester is an automatic
      XML-object mapping tool, that will create the objects defined in server.xml
      Startup of the actual container has not started yet.
  b5) Assigns System.out and System.err to the SystemLogHandler class
  b6) Calls initialize on all components, this makes each object register itself with the 
      JMX agent.
      During the process call the Connectors also initialize the adapters.
      The adapters are the components that do the request pre-processing.
      Typical adapters are HTTP1.1 (default if no protocol is specified,
      org.apache.coyote.http11.Http11Protocol)
      AJP1.3 for mod_jk etc.

 c) Catalina.start()
  c1) Starts the NamingContext and binds all JNDI references into it
  c2) Starts the services under <Server> which are:
   StandardService -> starts Engine (ContainerBase ->Logger,Loader,Realm,Cluster etc)
  c3) StandardHost (started by the service)
    Configures a ErrorReportValvem to do proper HTML output for different HTTP 
    errors codes
    Starts the Valves in the pipeline (at least the ErrorReportValve)
    Configures the StandardHostValve, 
     this valves ties the Webapp Class loader to the thread context
     it also finds the session for the request
     and invokes the context pipeline
    Starts the HostConfig component
     This component deploys all the webapps
      (webapps & conf/Catalina/localhost/*.xml)
     Webapps are installed using the deployer (StandardHostDeployer)
     The deployer will create a Digester for your context, this digester
     will then invoke ContextConfig.start()
      The ContextConfig.start() will process the default web.xml (conf/web.xml)
      and then process the applications web.xml (WEB-INF/web.xml)
      
  c4) During the lifetime of the container (StandardEngine) there is a background thread that 
      keeps checking if the context has changed. If a context changes (timestamp of war file, 
      context xml file, web.xml) then a reload is issued (stop/remove/deploy/start)
      
 d) Tomcat receives a request on an HTTP port
     d1) The request is received by a separate thread which is waiting in the PoolTcpEndPoint 
          class. It is waiting for a request in a regular ServerSocket.accept() method.
          When a request is received, this thread wakes up.
     d2) The PoolTcpEndPoint assigns the a TcpConnection to handle the request. 
         It also supplies a JMX object name to the catalina container (not used I believe)
     d3) The processor to handle the request in this case is Coyote Http11Processor, 
         and the process method is invoked.
         This same processor is also continuing to check the input stream of the socket
         until the keep alive point is reached or the connection is disconnected.
     d4) The HTTP request is parsed using an internal buffer class (Coyote Http11 Internal Buffer)
         The buffer class parses the request line, the headers, etc and store the result in a 
         Coyote request (not an HTTP request) This request contains all the HTTP info, such
         as servername, port, scheme, etc.
     d5) The processor contains a reference to an Adapter, in this case it is the 
         Coyote Tomcat 5 Adapter. Once the request has been parsed, the Http11 processor
         invokes service() on the adapter. In the service method, the Request contains a 
         CoyoteRequest and CoyoteRespons (null for the first time)
         The CoyoteRequest(Response) implements HttpRequest(Response) and HttpServletRequest(Response)
         The adapter parses and associates everything with the request, cookies, the context through a 
         Mapper, etc
     d6) When the parsing is finished, the CoyoteAdapter invokes its container (StandardEngine)
         and invokes the invoke(request,response) method.
         This initiates the HTTP request into the Catalina container starting at the engine level
     d7) The StandardEngine.invoke() simply invokes the container pipeline.invoke()
     d8) By default the engine only has one valve the StandardEngineValve, this valve simply
         invokes the invoke() method on the Host pipeline (StandardHost.getPipeLine())
     d9) the StandardHost has two valves by default, the StandardHostValve and the ErrorReportValve
     d10) The standard host valve associates the correct class loader with the current thread
          It also retrieves the Manager and the session associated with the request (if there is one)
          If there is a session access() is called to keep the session alive
     d11) After that the StandardHostValve invokes the pipeline on the context associated
          with the request.
     d12) The first valve that gets invoked by the Context pipeline is the FormAuthenticator
          valve. Then the StandardContextValve gets invoke.
          The StandardContextValve invokes any context listeners associated with the context.
          Next it invokes the pipeline on the Wrapper component (StandardWrapperValve)
     d13) During the invocation of the StandardWrapperValve, the JSP wrapper (Jasper) gets invoked
          This results in the actual compilation of the JSP.
          And then invokes the actual servlet.
 e) Invocation of the servlet class