Servlet - Java Web Core Component

在Java Web的世界里，Tomcat 等Web Server被称作servlet容器。这一称谓的由来就是因为，Tomcat 实际上运行JSP 页面和Servlet。servlet实际上包含了诸多的业务逻辑，后来经过分层思想的演化，业务逻辑会被独立成dao,service等层，但在servlet中还是会调用service层的逻辑。这时候servlet实际上是页面和逻辑层的桥梁，差不多是MVC中的控制器（C）的意思。

正是因为这种原因，structs2这个很好的MVC框架才诞生出来，这时候通过Action，ActionSupport，等接口屏蔽了servlet的doGet,doPost等方法。为什么需要屏蔽这些方法呢？我们分别看看doGet和doPost里面做了些什么。

public void doGet(ServletRequest rep, ServletResponse req)throws ServletException, IOException {System.out.println("doGet");req.setContentType("text/html");PrintWriter out = req.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.print("    This is ");out.print(this.getClass());out.println(", using the GET method");out.println("  </BODY>");out.println("</HTML>");out.flush();out.close();
}

public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {System.out.println("doPost");response.setContentType("text/html");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.print("    This is ");out.print(this.getClass());out.println(", using the POST method");out.println("  </BODY>");out.println("</HTML>");out.flush();out.close();
}

是的,您没有看错，确实在拼html标签。动态的部分来自业务逻辑代码返回的，而静态部分需要在这里拼接字符串，效率很低，而且很容易出错。

下面我们来看看实现servlet的两种方式：实现Servlet接口和继承httpServlet

import java.io.IOException;
import java.io.PrintWriter;import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;public class MyServlet2 implements Servlet{public MyServlet2(){System.out.println("MyServlet2");}public void destroy() {// TODO Auto-generated method stubSystem.out.println("destroy");}public ServletConfig getServletConfig() {// TODO Auto-generated method stubSystem.out.println("ServletConfig");return null;}public String getServletInfo() {System.out.println("getServletInfo");// TODO Auto-generated method stubreturn null;}public void init(ServletConfig arg0) throws ServletException {// TODO Auto-generated method stubSystem.out.println("init");}public void service(ServletRequest rep, ServletResponse req)throws ServletException, IOException {// TODO Auto-generated method stubdoGet(rep,req);System.out.println("service");}public void doGet(ServletRequest rep, ServletResponse req)throws ServletException, IOException {System.out.println("doGet");req.setContentType("text/html");PrintWriter out = req.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.print("    This is ");out.print(this.getClass());out.println(", using the GET method");out.println("  </BODY>");out.println("</HTML>");out.flush();out.close();}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {System.out.println("doPost");response.setContentType("text/html");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.print("    This is ");out.print(this.getClass());out.println(", using the POST method");out.println("  </BODY>");out.println("</HTML>");out.flush();out.close();}}

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 MyServlet extends HttpServlet {/*** Constructor of the object.*/public MyServlet() {super();System.out.println("constructor.");}/*** Destruction of the servlet. <br>*/public void destroy() {System.out.println("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 {System.out.println("doGet");response.setContentType("text/html");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.print("    This is ");out.print(this.getClass());out.println(", using the GET method");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("doPost");response.setContentType("text/html");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.print("    This is ");out.print(this.getClass());out.println(", using the POST method");out.println("  </BODY>");out.println("</HTML>");out.flush();out.close();}/*** Initialization of the servlet. <br>** @throws ServletException if an error occurs*/public void init() throws ServletException {// Put your code hereSystem.out.println("init");}}

这两个servlet算是最简单的两个servlet了，因为只包含doGet和doPost方法，实际开发过程中，肯定比这个复杂，但是大量地拼接字符串不是好的做法。

我们实际使用structs的Action,实现Action接口或者继承ActionSupport

package com.wicresoft.action;import com.opensymphony.xwork2.Action;public class IndexAction2 implements Action {public String execute() throws Exception {System.out.println("IndexAction2 executed.");// TODO Auto-generated method stubreturn SUCCESS;}
}

package com.wicresoft.action;import com.opensymphony.xwork2.ActionSupport;public class IndexAction3 extends ActionSupport{@Overridepublic String execute() throws Exception {System.out.println("IndexAction3 executed.");// TODO Auto-generated method stub//return super.execute();return "success";}public String UserAdd(){return SUCCESS;}public String Add(){return SUCCESS;}}

值得注意的是，这两种方式和实现servlet的两种方式遥相呼应。

Servlet生命周期分为三个阶段：
1，初始化阶段调用init()方法
2，响应客户请求阶段　　调用service()方法
3，终止阶段　　调用destroy()方法

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的整个生命周期内，init()方法只被调用一次。

在servlet的实例被创建后，如果有请求过来，servlet容器会开新的线程来处理请求，而不再创建新的servlet的实例，也是因此,init()方法只会被调用一次！

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