一、servlet的生命周期
在《JSP学习 —— 开篇:JSP,servlet容器,Tomcat,servlet容器之间的关系》一文中,我已经对servlet的生命周期进行了简单阐述,现在,就来进行详细阐述:servlet其实就是一个Java类,它负责响应和处理页面消息。它的实例化方式有两种:一种是当web应用启动一开始,tomcat就对servlet进行初始化;另一种是当客户端请求一个servlet时,对应的servlet进行创建;创建时调用HttpServlet的Init(或用户自己重写的Init)函数进行初始化,之后调用service函数进行消息响应,最后调用destory方法进行收尾处理。对于其何时创建,如何创建,都不是由我们来控制的,而是由容器根据配置文件自己进行创建。
事实上,在标准MVC结构中,servlet只充当Control部分,它从来不对页面进行输出处理,也不进行业务逻辑处理,只是将页面消息进行对应的后台处理后(期间会和Model打交道),进行简单的页面跳转。
对于servlet类的配置,其实只要注意两点就足够了:一是指定servlet类名,二是指定该类要处理的动作。
<servlet> <servlet-name>myServlet</servlet-name> <servlet-class>com.cancique.dao.LoginServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>myServlet</servlet-name> <url-pattern>/login</url-pattern> </servlet-mapping>对于sevlet的名字,暂时我们可以随意指定,对于只有一个servlet类的Web应用而言,是没必要注意的。要注意而且需要特别注意的时servlet-class和url-pattern,servlet-class指定servlet的类名,类名必须包含完整的包前缀,而url指定要处理的动作名,如:我在这里打算处理一个表单请求的action,
<form action = "login" id = "login" method = "post">要处理的action必须和url-pattern相同,否则servlet不能处理我们提交的请求,运行时会出错。
二、load-on-startup Servlet
一般来说,容器会将绝大多数servlet类的初始化都将在页面进行请求之后,然后根据请求找到对应的servlet再初始化,那么如果我们想将一个servlet类在WEB应用启动后尽早的执行初始化,那么只有靠load-on-startup配置参数了。这个配置参数同servlet的配置参数一起配置,它的参数值是个整型值,赋给它的值越小,它就越先初始化。
<servlet> <servlet-name>myServlet</servlet-name> <servlet-class>com.cancique.dao.LoginServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>myServlet</servlet-name> <url-pattern>/login</url-pattern> </servlet-mapping>三、访问servlet的配置参数
我们在配置servlet时,还可以配置一些运行时用到的参数,这种参数可以通过ServletConfig类来解析,看起来和我前面讲到的《JSP学习——JSP基础知识(二)
》里面提到的application对象解析配置文件中数据库连接信息的参数很类似,不同的是application可以在任何地方进行解析,而ServletConfig对象只能在servlet中进行解析。比如:如果我们在servlet中需要得到数据库的连接信息以初始化DAO对象,那么可以在web.xml的servlet配置信息中增加一些配置参数:<servlet> <servlet-name>myServlet</servlet-name> <servlet-class>com.cancique.dao.LoginServlet</servlet-class> <init-param> <param-name>driver</param-name> <param-value>com.mysql.jdbc.Driver</param-value> </init-param> <init-param> <param-name>url</param-name> <param-value>jdbc:mysql://localhost:3306/test_db</param-value> </init-param> <init-param> <param-name>user</param-name> <param-value>root</param-value> </init-param> <init-param> <param-name>pass</param-name> <param-value>900622</param-value> </init-param> </servlet> <servlet-mapping> <servlet-name>myServlet</servlet-name> <url-pattern>/login</url-pattern> </servlet-mapping>在需要用到这些参数的servlet类中,我们可以通过ServletConfig对象来解析:
ServletConfig config = getServletConfig(); String driver = config.getInitParameter("driver"); String url = config.getInitParameter("url"); String user = config.getInitParameter("user"); String pass = config.getInitParameter("pass"); DbDao dao = DbDao.instance(driver,url,user,pass);四、综合应用:servlet作为MVC架构中Control层的实例——一个简单的登录验证
index.jsp页面代码如下:
<%@ page language="java" import="java.util.*" errorPage="error.jsp"%> <%@ page pageEncoding="gb2312"%> <%@page import="java.sql.SQLException"%> <%@page import="java.net.ConnectException"%> <%@page import="java.sql.DriverManager"%> <%@page import="java.sql.*" %> <html> <body> <form action = "login" id = "login" method = "post"> user name:<input type = "text" name = "username"><hr> user password:<input type = "text" name = "userpassword"><hr> <input type = "submit" value = "submit"><br> </form> </body> </html>
error.jsp页面代码如下:login failed!success.jsp页面代码如下:login successful!LoginServlet类代码如下:package com.cancique.dao; import java.io.IOException; import java.sql.*; import javax.servlet.RequestDispatcher; import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpSession; public class LoginServlet extends HttpServlet{ public void service(HttpServletRequest request, HttpServletResponse response) { RequestDispatcher rd; //请求转发 String username = request.getParameter("username"); String userpassword = request.getParameter("userpassword"); String errMsg = null; try { ServletConfig config = getServletConfig(); String driver = config.getInitParameter("driver"); String url = config.getInitParameter("url"); String user = config.getInitParameter("user"); String pass = config.getInitParameter("pass"); DbDao dao = DbDao.instance(driver,url,user,pass); String sql = "select userpassword from user_information where username = '" + username + "'"; ResultSet resultSet = dao.query(sql); if(resultSet.next()) { if(resultSet.getString("userpassword").equals(userpassword)) { HttpSession session = request.getSession(true); session.setAttribute("username", username); rd = request.getRequestDispatcher("/success.jsp"); rd.forward(request, response); } else { errMsg +="密码错误!"; } } else { errMsg += "用户名错误!"; } }catch(Exception e) { rd = request.getRequestDispatcher("/error.jsp"); request.setAttribute("exception", "业务异常"); try { rd.forward(request, response); } catch (ServletException e1) { e1.printStackTrace(); } catch (IOException e1) { e1.printStackTrace(); } } } }DbDao类代码如下(使用Singleton设计模式):package com.cancique.dao; import java.sql.*; public class DbDao { private static DbDao dao; private Connection conn; private String driver; private String url; private String username; private String pass; private DbDao() {} private DbDao(String driver,String url, String username,String pass) { this.driver = driver; this.url = url; this.username = username; this.pass = pass; } public static DbDao getDao() { return dao; } public static void setDao(DbDao dao) { DbDao.dao = dao; } public String getDriver() { return driver; } public void setDriver(String driver) { this.driver = driver; } public String getUrl() { return url; } public void setUrl(String url) { this.url = url; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public String getPass() { return pass; } public void setPass(String pass) { this.pass = pass; } public void getConnection() throws ClassNotFoundException, SQLException { if(conn == null) { Class.forName(this.driver); conn = DriverManager.getConnection(url, username, pass); } } public static DbDao instance(String driver,String url, String username,String pass) { if(dao == null) { dao = new DbDao(driver,url,username,pass); } return dao; } public boolean insert(String sql) throws ClassNotFoundException, SQLException { getConnection(); Statement statement = conn.createStatement(); if(statement.executeUpdate(sql) != -1) { return false; } return true; } public ResultSet query(String sql) throws ClassNotFoundException, SQLException { getConnection(); Statement statement = conn.createStatement(); return statement.executeQuery(sql); } public void delete(String sql) throws ClassNotFoundException, SQLException { getConnection(); Statement statement = conn.createStatement(); statement.executeUpdate(sql); } public void update(String sql) throws ClassNotFoundException, SQLException { getConnection(); Statement statement = conn.createStatement(); statement.executeUpdate(sql); } }web.xml配置文件信息如下:<?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"> <welcome-file-list> <welcome-file>index.jsp</welcome-file> </welcome-file-list> <servlet> <servlet-name>myServlet</servlet-name> <servlet-class>com.cancique.dao.LoginServlet</servlet-class> <init-param> <param-name>driver</param-name> <param-value>com.mysql.jdbc.Driver</param-value> </init-param> <init-param> <param-name>url</param-name> <param-value>jdbc:mysql://localhost:3306/test_db</param-value> </init-param> <init-param> <param-name>user</param-name> <param-value>root</param-value> </init-param> <init-param> <param-name>pass</param-name> <param-value>900622</param-value> </init-param> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>myServlet</servlet-name> <url-pattern>/login</url-pattern> </servlet-mapping> </web-app>