Tomcat装载Servlet的三种情况

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java

Tomcat Servlet

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>

<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>

<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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