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对于servlet初步理解

2016-05-06 12:33 363 查看 
Servlet在MVC框架中经常担当控制器(Controller)的角色,其作用类似于调度员,下面我们就根据一个简单JavaEE的示例来理解Servlet的机制和原理.




树结构如上,其中

login.jsp作为测试用户登录页面.

welcome.jsp作为登录成功页面

DaDao.java作为数据库操作类

LoginServlet.java就是我们所说的Servlet类

我们来看具体内容:

login.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; UTF-8">
<title>连接数据库范例</title>
</head>
<body>
<span style="color: blue; font-weight: bold;"> <%
if (request.getAttribute("err") != null) {
out.println(request.getAttribute("err") + "<br/>");
}
%>
</span> 请输入用户名密码:
<form id="login" method="post" action="login">
用户名: <input type="text" name="username"><br /> 密码:<input
type="password" name="password"><br /> <input type="submit"
value="登录"><br />
</form>
</body>
</html>


以上页面只负责传递两个参数,一个是用户名,另一个是用户密码,将参数传递给LoginServlet的实例中,再通过验证数据库进行结果判断,然后Servlet将结果视图反馈给用户.

welcome.jsp

<%@ page contentType="text/html; charset=GBK" language="java" errorPage="" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title> 欢迎页面 </title>
<meta name="website" content="http://www.crazyit.org" />
</head>
<body>
<h3>欢迎登录</h3>
<%=session.getAttribute("name")%>,欢迎登录！
</body>
</html>


此页面是用户密码验证成功后返回的视图.

DbDao.java


package glq;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

public class DbDao {

private Connection conn;
private String driver;
private String url;
private String username;
private String password;
public DbDao() {
}
public DbDao(String driver, String url, String username, String password) {
this.driver = driver;
this.url = url;
this.username = username;
this.password = password;
}
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 getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}

public Connection getConnection() throws Exception {
if(conn==null){
Class.forName(this.driver);
conn=DriverManager.getConnection(url, username, password);
}
return conn;
}

public boolean insert(String sql,Object... args) throws Exception, Throwable{
PreparedStatement ps=getConnection().prepareStatement(sql);
for(int i=0;i<args.length;i++){
ps.setObject(i+1, args[i]);
}
if(ps.executeUpdate()!=1){
return false;
}
return true;
}

public ResultSet query(String sql,Object... args) throws Exception{
PreparedStatement ps=getConnection().prepareStatement(sql);
for(int i=0;i<args.length;i++){
ps.setObject(i+1, args[i]);
}
return ps.executeQuery();
}

public void modify(String sql,Object... args) throws Exception{
PreparedStatement ps=getConnection().prepareStatement(sql);
for(int i=0;i<args.length;i++){
ps.setObject(i+1, args[i]);
}
ps.executeUpdate();
ps.close();
}
public void closeConn() throws Exception{
if(conn!=null &&!conn.isClosed()){
conn.close();
}
}
}

DaDao类对数据库操作进行了简单的封装,插入了连接数据库,查询记录,修改记录,插曲记录等方法.


LoginServlet.java


package glq;

import java.io.IOException;
import java.sql.ResultSet;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
@WebServlet(name="login" ,urlPatterns={"/login"})
public class LoginServlet extends HttpServlet {

/**
*
*/
private static final long serialVersionUID = 1L;

@Override
protected void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
String errMsg = "";
RequestDispatcher rd;
String username = req.getParameter("username");
String password = req.getParameter("password");

try {
//创建数据库构造函数
//注意,连接时要在数据库中创建名为servlet的数据库
DbDao dd = new DbDao("com.mysql.jdbc.Driver", "jdbc:mysql://localhost:3306/servlet", "root", "123456");
//执行查询方法
//注意,要在servlet数据库中添加user_inf表,并在表中添加两个属//性:name,pass
ResultSet rs = dd.query("select pass from user_inf where name=?", username);
//如果存在该用户
if (rs.next()) {
//判定用户密码是否相同
if (rs.getString("pass").equals(password)) {
//获得session
HttpSession session = req.getSession();
//注册name值为username
session.setAttribute("name", username);
//将参数和页面转向welcome.jsp
rd = req.getRequestDispatcher("/welcome.jsp");
rd.forward(req, res);
} else {
//如果密码匹配失败
errMsg += "密码不对,请重新输入";
}
} else {
//如果没找到用户
errMsg += "用户不存在";
}
} catch (Exception e) {
e.printStackTrace();
}
//当错误信息存在时,将错误信息errMsg作为参数传递给request中的err变//量,使其在登录页面显示,并因为登录失败跳转至登录页面
if (errMsg != null && !errMsg.equals("")) {
rd = req.getRequestDispatcher("/login.jsp");
req.setAttribute("err", errMsg);
rd.forward(req, res);
}

}
}


在测试时,需要在WEB-INF的lib文件中加入mysql-connector-java-5.1.38-bin.jar包,此包为数据库工具包.

至于tomcat的构建,以及web项目的建立,需要自行检索.
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