Detailed explanation of servlet and tomcat in Java

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What is Servlet
In order to allow the two different software systems of Web server and Web application to cooperate, it is necessary to A set of standard interfaces, Servlet is the most important interface.

Provisions:

The Web server can access any class in the Web application that implements the Servlet interface.

The program code used in Web applications to be dynamically called by the Web server is located in the implementation class of the Servlet interface.

SUN Company (now acquired by Oracle...) has developed a series of standard Java interfaces (collectively referred to as Java Servlet API) for Web applications to collaborate with Web servers.

SUN also specifies some details for publishing Web servers and running Web applications. SUN Corporation refers to this series of standard Java interfaces and specifications as Servlet specifications.

Servlet is a small plug-in that runs on the server.

What is a Servlet container?

In the Servlet specification, a Web server that can publish and run JavaWeb applications is called a Servlet container , his main special name is the dynamic execution of program code in the Servlet implementation class in JavaWeb applications.

What is Tomcat

Tomcat is a Servlet container and a lightweight web server.

Apache Server, Microsoft IIS, and Apache Tomcat are all web servers.

When Tomcat serves as a Web server, it is mainly responsible for implementing HTTP transmission and other tasks.

When Tomcat serves as a Servlet container, it is mainly responsible for parsing Request, generating ServletRequest and ServletResponse, passing them to the corresponding Servlet (calling the service() method), and then returning the corresponding results of the Servlet.

Tomcat composition structure

##Server represents the entire Servlet container component and is the top-level element of Tomcat. It can contain one or more Services;

Service includes an Engine and one or more Connectors;

Connector represents the actual interaction with the client program Component, responsible for receiving customer requests and returning response results to customers;

Engine, processing customer requests received by all Connectors in the same Service;

Host, in Engine can contain multiple Hosts, each Host defines a virtual host, which can contain one or more Web applications;

Context, a Host can contain multiple Contexts, each Context Represents a single web application running on a virtual host.

These fields are configured in conf/server.xml. The following is a section of apache tomcat 6.0.36 default server.xml:

<?xml version=&#39;1.0&#39; encoding=&#39;utf-8&#39;?> 
<!-- 
 Licensed to the Apache Software Foundation (ASF) under one or more 
 contributor license agreements. See the NOTICE file distributed with 
 this work for additional information regarding copyright ownership. 
 The ASF licenses this file to You under the Apache License, Version 2.0 
 (the "License"); you may not use this file except in compliance with 
 the License. You may obtain a copy of the License at 
 
   http://www.apache.org/licenses/LICENSE-2.0 
 
 Unless required by applicable law or agreed to in writing, software 
 distributed under the License is distributed on an "AS IS" BASIS, 
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 
 See the License for the specific language governing permissions and 
 limitations under the License. 
--> 
<!-- Note: A "Server" is not itself a "Container", so you may not 
   define subcomponents such as "Valves" at this level. 
  Documentation at /docs/config/server.html 
 --> 
<Server port="8005" shutdown="SHUTDOWN"> 
 
 <!--APR library loader. Documentation at /docs/apr.html --> 
 <Listener className="org.apache.catalina.core.AprLifecycleListener" SSLEngine="on" /> 
 <!--Initialize Jasper prior to webapps are loaded. Documentation at /docs/jasper-howto.html --> 
 <Listener className="org.apache.catalina.core.JasperListener" /> 
 <!-- Prevent memory leaks due to use of particular java/javax APIs--> 
 <Listener className="org.apache.catalina.core.JreMemoryLeakPreventionListener" /> 
 <!-- JMX Support for the Tomcat server. Documentation at /docs/non-existent.html --> 
 <Listener className="org.apache.catalina.mbeans.ServerLifecycleListener" /> 
 <Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener" /> 
 
 <!-- Global JNDI resources 
    Documentation at /docs/jndi-resources-howtohtml 
 --> 
 <GlobalNamingResources> 
  <!-- Editable user database that can also be used by 
     UserDatabaseRealm to authenticate users 
  --> 
  <Resource name="UserDatabase" auth="Container" 
       type="org.apache.catalina.UserDatabase" 
       description="User database that can be updated and saved" 
       factory="org.apache.catalina.users.MemoryUserDatabaseFactory" 
       pathname="conf/tomcat-users.xml" /> 
 </GlobalNamingResources> 
 
 <!-- A "Service" is a collection of one or more "Connectors" that share 
    a single "Container" Note: A "Service" is not itself a "Container",  
    so you may not define subcomponents such as "Valves" at this level. 
    Documentation at /docs/config/service.html 
  --> 
 <Service name="Catalina"> 
  
  <!--The connectors can use a shared executor, you can define one or more named thread pools--> 
  <!-- 
  <Executor name="tomcatThreadPool" namePrefix="catalina-exec-"  
    maxThreads="150" minSpareThreads="4"/> 
  --> 
   
   
  <!-- A "Connector" represents an endpoint by which requests are received 
     and responses are returned. Documentation at : 
     Java HTTP Connector: /docs/config/http.html (blocking & non-blocking) 
    Java AJP Connector: /docs/config/ajp.html 
    APR (HTTP/AJP) Connector: /docs/apr.html 
     Define a non-SSL HTTP/1 Connector on port 8080 
  --> 
  <Connector port="8080" protocol="HTTP/1.1"  
        connectionTimeout="20000"  
        redirectPort="8443" /> 
  <!-- A "Connector" using the shared thread pool--> 
  <!-- 
  <Connector executor="tomcatThreadPool" 
        port="8080" protocol="HTTP/1.1"  
        connectionTimeout="20000"  
        redirectPort="8443" /> 
  -->       
  <!-- Define a SSL HTTP/1.1 Connector on port 8443 
     This connector uses the JSSE configuration, when using APR, the  
     connector should be using the OpenSSL style configuration 
     described in the APR documentation --> 
  <!-- 
  <Connector port="8443" protocol="HTTP/1" SSLEnabled="true" 
        maxThreads="150" scheme="https" secure="true" 
        clientAuth="false" sslProtocol="TLS" /> 
  --> 
 
  <!-- Define an AJP 1.3 Connector on port 8009 --> 
  <Connector port="8009" protocol="AJP/1.3" redirectPort="8443" /> 
 
 
  <!-- An Engine represents the entry point (within Catalina) that processes 
     every request The Engine implementation for Tomcat stand alone 
     analyzes the HTTP headers included with the request, and passes them 
    on to the appropriate Host (virtual host). 
    Documentation at /docs/config/engine.html --> 
 
  <!-- You should set jvmRoute to support load-balancing via AJP ie : 
  <Engine name="Catalina" defaultHost="localhost" jvmRoute="jvm1">      
  -->  
  <Engine name="Catalina" defaultHost="localhost"> 
 
   <!--For clustering, please take a look at documentation at: 
     /docs/cluster-howto.html (simple how to) 
     /docs/config/cluster.html (reference documentation) --> 
   <!-- 
   <Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"/> 
   -->     
 
   <!-- The request dumper valve dumps useful debugging information about 
      the request and response data received and sent by Tomcat. 
      Documentation at: /docs/config/valve.html --> 
   <!-- 
   <Valve className="org.apache.catalina.valves.RequestDumperValve"/> 
   --> 
 
   <!-- This Realm uses the UserDatabase configured in the global JNDI 
      resources under the key "UserDatabase". Any edits 
      that are performed against this UserDatabase are immediately 
      available for use by the Realm. --> 
   <Realm className="org.apache.catalina.realm.UserDatabaseRealm" 
       resourceName="UserDatabase"/> 
 
   <!-- Define the default virtual host 
      Note: XML Schema validation will not work with Xerces 2.2. 
    --> 
   <Host name="localhost" appBase="webapps" 
      unpackWARs="true" autoDeploy="true" 
      xmlValidation="false" xmlNamespaceAware="false"> 
 
    <!-- SingleSignOn valve, share authentication between web applications 
       Documentation at: /docs/config/valve.html --> 
    <!-- 
    <Valve className="org.apache.catalina.authenticator.SingleSignOn" /> 
    --> 
 
    <!-- Access log processes all example. 
       Documentation at: /docs/config/valve.html --> 
    <!-- 
    <Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"  
        prefix="localhost_access_log." suffix=".txt" pattern="common" resolveHosts="false"/> 
    --> 
 
   </Host> 
  </Engine> 
 </Service> 
</Server>

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