Servlet生命週期的三個核心方法分別是 init() , service() 和 destroy()。每個Servlet都會實現這些方法，並且在特定的運行時間調用它們。
1)在Servlet生命週期的初始化階段，web容器通過調用init()方法來初始化Servlet實例，並且可以傳遞一個實現 javax.servlet.ServletConfig 接口的對象給它。這個配置對象（configuration object）使Servlet能夠讀取在web應用的web.xml文件裏定義的名值（name-value）初始參數。這個方法在Servlet實例的生命週期裏只調用一次。
2)初始化後，Servlet實例就可以處理客戶端請求了。web容器調用Servlet的service()方法來處理每一個請求。service() 方法定義了能夠處理的請求類型並且調用適當方法來處理這些請求。編寫Servlet的開發者必須爲這些方法提供實現。如果發出一個Servlet沒實現的請求，那麼父類的方法就會被調用並且通常會給請求方（requester）返回一個錯誤信息。
通常，我們不需要重寫（override）這個方法。
3)最後，web容器調用destroy()方法來終結Servlet。如果你想在Servlet的生命週期內關閉或者銷燬一些文件系統或者網絡資源，你可以調用這個方法來實現。destroy() 方法和init()方法一樣，在Servlet的生命週期裏只能調用一次。
如下例子
package com.wxp.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 MyFirstServlet extends HttpServlet {
/**
* Constructor of the object.
*/
public MyFirstServlet() {
super();
}
/**
* Destruction of the servlet. <br>
*/
public void destroy() {
super.destroy(); // Just puts "destroy" string in log
// Put your code here
}
/**
* 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 {
response.setContentType("text/html;charset=utf-8");
PrintWriter out = response.getWriter();
try {
out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
out.println("<HTML>");
out.println(" <HEAD><TITLE>MyFirstServlet</TITLE></HEAD>");
out.println(" <BODY>");
out.print("Servlet MyFirstServlet at");
out.print(request.getContextPath());
out.println(" </BODY>");
out.println("</HTML>");
} catch (Exception e) {
e.printStackTrace();
}finally{
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 {
this.doGet(request, response);
}
/**
* Initialization of the servlet. <br>
*
* @throws ServletException if an error occurs
*/
public void init() throws ServletException {
// Put your code here
}
public String getServletInfo(){
return "MyFirstServlet";
}
}