Tomcat装载Servlet的三种情况

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一 时刻1

Servlet容器启动时自动装载某些Servlet,实现它只需要在web.xml文件中的<Servlet></Servlet>之间添加如下代码

<loadon-startup>1</loadon-startup>

数字越小表示优先级别越高。

1 关键代码

<?xml version="1.0" encoding="UTF-8"?>

<web-app version="2.5"

        xmlns="http://java.sun.com/xml/ns/javaee";

        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";

        xsi:schemaLocation="http://java.sun.com/xml/ns/javaee

http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd";>

  <display-name></display-name>

  <servlet>

    <description>This is the description of my J2EE component</description>

    <display-name>This is the display name of my J2EE component</display-name>

    <servlet-name>TestServlet1</servlet-name>

    <servlet-class>servlet.TestServlet1</servlet-class>

 

    <load-on-startup>2</load-on-startup>

 

  </servlet>

  <servlet>

    <description>This is the description of my J2EE component</description>

    <display-name>This is the display name of my J2EE component</display-name>

    <servlet-name>TestServlet2</servlet-name>

    <servlet-class>servlet.TestServlet2</servlet-class>

 

    <load-on-startup>1</load-on-startup>

 

  </servlet>

 

 

  <servlet-mapping>

    <servlet-name>TestServlet1</servlet-name>

    <url-pattern>/servlet/TestServlet1</url-pattern>

  </servlet-mapping>

  <servlet-mapping>

    <servlet-name>TestServlet2</servlet-name>

    <url-pattern>/servlet/TestServlet2</url-pattern>

  </servlet-mapping>       

  <welcome-file-list>

    <welcome-file>index.jsp</welcome-file>

  </welcome-file-list>

</web-app>

2 测试

TestServlet2构造方法被执行....

TestServlet2的初始化方法被执行....

TestServlet1构造方法被执行....

TestServlet1的初始化方法被执行....

 

二 时刻2

在Servlet容器启动后,客户首次向Servlet发送请求。

1 关键代码

package servlet;

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class TestServlet1 extends HttpServlet {

   

    

    

    /**

     * Constructor of the object.

     */

    public TestServlet1() {

        System.out.println("TestServlet1构造方法被执行....");

    }

    /**

     * Destruction of the servlet. <br>

     */

    public void destroy() {

        System.out.println("TestServlet1销毁方法被执行....");

    }

    /**

     * The doGet method of the servlet. <br>

     *

     * This method is called when a form has its tag value method equals to get.

     *

     * @param request the request send by the client to the server

     * @param response the response send by the server to the client

     * @throws ServletException if an error occurred

     * @throws IOException if an error occurred

     */

    public void doGet(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

        System.out.println("TestServlet1的doGet()方法被执行...");

        response.setContentType("text/html;charset=utf-8");

        PrintWriter out = response.getWriter();

        out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");

        out.println("<HTML>");

        out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");

        out.println("  <BODY>");

        out.println("<h1>大家好,我是TestServlet1!</h1>");

        out.println("  </BODY>");

        out.println("</HTML>");

        out.flush();

        out.close();

    }

    /**

     * The doPost method of the servlet. <br>

     *

     * This method is called when a form has its tag value method equals to post.

     *

     * @param request the request send by the client to the server

     * @param response the response send by the server to the client

     * @throws ServletException if an error occurred

     * @throws IOException if an error occurred

     */

    public void doPost(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

        System.out.println("TestServlet1的doPost()方法被执行...");

        doGet(request,response); //让doPost()执行与doGet()相同的操作。

    }

    /**

     * Initialization of the servlet. <br>

     *

     * @throws ServletException if an error occurs

     */

    public void init() throws ServletException {

        // Put your code here

        System.out.println("TestServlet1的初始化方法被执行....");

    }

}

 

2 测试

TestServlet1构造方法被执行....

TestServlet1的初始化方法被执行....

TestServlet1的doGet()方法被执行...

九月 24, 2017 2:51:01 下午 org.apache.catalina.core.StandardServer await

信息: A valid shutdown command was received via the shutdown port. Stopping the Server instance.

九月 24, 2017 2:51:01 下午 org.apache.coyote.AbstractProtocol pause

信息: Pausing ProtocolHandler ["http-apr-8888"]

九月 24, 2017 2:51:01 下午 org.apache.coyote.AbstractProtocol pause

信息: Pausing ProtocolHandler ["ajp-apr-8009"]

九月 24, 2017 2:51:01 下午 org.apache.catalina.core.StandardService stopInternal

信息: Stopping service Catalina

九月 24, 2017 2:51:02 下午 org.apache.catalina.core.ApplicationContext log

信息: SessionListener: contextDestroyed()

九月 24, 2017 2:51:02 下午 org.apache.catalina.core.ApplicationContext log

信息: ContextListener: contextDestroyed()

TestServlet1销毁方法被执行....

 

三 时刻3

Servlet类文件被更新后,重新装载Servlet。

修改文件后,生成类文件时间发生变化



 

 

四 说明

Servlet被装载后,Servlet容器创建一个Servlet实例并且调用Servlet的init()方法进行初始化。在Servlet的整个生命周期,init()方法只被调用一次。

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