Jsp学习——servlet详解

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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。

[html] view plaincopyprint?

<preclass="html"name="code"> <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></pre>
<pre></pre>
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，
<pre class="html"name="code"><formaction = "login"id = "login"method = "post"></pre>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。
<preclass="html"name="code"><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></pre>
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《<spanclass="link_title"><atitle="JSP学习——JSP基础知识（二）"href="http://blog.csdn.net/yangywyangyw/article/details/6672119">JSP学习——JSP基础知识（二）</a>
<h3></h3>
》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：
<preclass="html"name="code"> <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></pre>
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：
<preclass="html"name="code"> ServletConfigconfig = 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);</pre>
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：
<preclass="html"name="code"><%@ pagelanguage="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>
<formaction = "login"id = "login"method = "post">
user name:<inputtype = "text"name = "username"><hr>
user password:<inputtype = "text"name = "userpassword"><hr>
<inputtype = "submit"value = "submit">
</form>
</body>
</html>
</pre>
error.jsp页面代码如下：
<preclass="html"name="code">login failed!</pre>success.jsp页面代码如下：
<preclass="html"name="code">login successful!</pre>LoginServlet类代码如下：
<preclass="java"name="code">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();
}
}
}
}
</pre>DbDao类代码如下（使用Singleton设计模式）：
<preclass="java"name="code">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);
}
}</pre>web.xml配置文件信息如下：
<preclass="html"name="code"><?xmlversion="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>
</pre>

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