马士兵 Servlet & JSP

  [学习笔记]马士兵 Servlet & JSP(1.Servlet)  2010-04-04 21:52:48

分类： Java

1.HTTP协议基础测试（获取页面源代码）

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.Socket;
import java.net.UnknownHostException;

/**
 * HTTP协议基础测试
 * 本程序将Tomcat首页的页面源代码拿下来
 * 用此方法，我们可以将我们访问到的页面的内容都拿下来
 * @author ykn
 *
 */
public class HttpTest {

    public static void main(String[] args) {
        
        Socket s = null;
        PrintWriter pw = null;
        BufferedReader br = null;
            
        try {
            // 建立连接端口，s指向本地机器tomcat服务器端口上
            s = new Socket("127.0.0.1",8888);
            // 对于本程序而言是输出，则相当于是准备向tomcat服务器端口写请求
            pw = new PrintWriter(new OutputStreamWriter(s.getOutputStream()));
            // 请求访问页面（此处等同于访问 http://localhost:8888/）    
            pw.println("GET / HTTP/1.1");
            pw.println("Host:  www.bjsxt.com");
            pw.println("Content-Type:text/html");
            pw.println("");
            // 上一句表示请求内容结束
            pw.flush();
            // 上面这一段用于本程序向Tomcat服务器发出访问请求（get）
            
            
            // 服务器端作出响应，对于本程序而言是输入
            br = new BufferedReader(new InputStreamReader(s.getInputStream()));
            String str = "";
            // 将服务器端的响应信息打印输出（即将 http://localhost:8888/页面代码源文件中的内容输出）
            // 用此方法，我们可以将我们访问到的页面的内容都拿下来
            while((str = br.readLine()) != null) {
                System.out.println(str);
            }
            
        } catch (UnknownHostException e) {
            System.out.println("未知的主机异常。。。");
            e.printStackTrace();
        } catch (IOException e) {
            System.out.println("IO异常。。。");
            e.printStackTrace();
        } finally {
            try {
                if (br != null) {
                    br.close();
                    br = null;
                }
                if (pw != null) {
                    pw.close();
                    pw = null;
                }
                if (s != null) {
                    s.close();
                    s = null;
                }
            } catch (IOException e) {
                System.out.println("IO异常。。。");
                e.printStackTrace();
            }            
        }
    }

}

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2.最简单的servlet示例
说明：
①将HelloWorldServlet .class拷贝到test\WEB-INF\classes目录下（test是项目名）
②在web.xml中添加对应的servlet处理语句：
    <servlet>
        <servlet-name>HW</servlet-name>
        <servlet-class>HelloWorldServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>HW</servlet-name>
        <url-pattern>/abc</url-pattern>
    </servlet-mapping>
③在URL地址栏中以 http://localhost:8888/test/abc形式访问
④页面上显示结果：Hello World!

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;

/**
 * 简单的servlet示例
 * @author jukey
 *
 */
public class HelloWorldServlet extends HttpServlet {

    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=gb2312");
        PrintWriter out = response.getWriter();
        out.println("Hello World!");
    }
    
}

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3.Servlet的生命周期测试

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet的生命周期
 * @author jukey
 *
 */
public class TestLifeCycleServlet extends HttpServlet {

    // 处理请求
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("doGet");
    }

    // 实例化
    // 注意：构造方法不能有返回值类型，连void也不行（否则就变成普通方法了）
    public TestLifeCycleServlet() {
        System.out.println("constructed");
    }
    
    // 退出服务
    public void destroy() {
        System.out.println("destory");
    }

    // 初始化
    public void init() throws ServletException {
        System.out.println("init");
    }

}

/*
测试结果（在tomcat服务器控制台窗口）：
constructed
init
doGet
*/

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4.读取指定的参数

①Threeparams.htm

<form id="form1" name="form1" method="post" action="servlet/Threeparams">
    <table width="343" border="1">
        <tr>
            <td width="92">param1</td>
            <td width="94"><input name="param1" type="text" id="param1"/></td>    
        </tr>
        <tr>
            <td width="92">param2</td>
            <td width="94"><input name="param2" type="text" id="param2"/></td>    
        </tr>
        <tr>
            <td width="92">param3</td>
            <td width="94"><input name="param3" type="text" id="param3"/></td>    
        </tr>
        <tr>
            <td>&nbsp;</td>
            <td><input type="submit" value="提交"/></td>    
        </tr>
    </table>
</form>

②ThreeParams.java

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;

/**
 * 读取指定的参数
 * @author jukey
 *
 */
public class ThreeParams extends HttpServlet {

    // 参数在传递过程中，在URL地址栏中显示出来
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=gb2312");
        PrintWriter pw = response.getWriter();
        pw.println(request.getParameter("param1"));
        pw.println("<br>");
        pw.println(request.getParameter("param2"));
        pw.println("<br>");
        pw.println(request.getParameter("param3"));
        pw.println("<br>");
    }

    // 参数在传递过程中，在URL地址栏中隐藏了
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("doPost");
        // 调用doGet方法
        doGet(request,response);
    }

}

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5.读取所有的参数

①Showparameters.htm

<html>
    <head>
        <title>A sample show parameters</title>
    </head>
    <body>A sample show parameters
    
        <form method="post" action="servlet/ShowParameters">
            Item Number:<input type="text" name="itemNum"><br>
            Quantity:<input type="text" name="quantity"><br>
            Price each:<input type="text" name="price" value="$"><br>
            First Name:<input type="text" name="firstName"><br>
            Last Name:<input type="text" name="lastName"><br>
            Middle Initial:<input type="text" name="initial"><br>
            
            Shipping Address:<textarea name="address" row="3" cols="40"></textarea><br>
            Credit Card:<br>
            <!--单选框示例-->
            &nbsp;&nbsp;<input type="radio" name="cardType" value="Visa">Visa<br>
            &nbsp;&nbsp;<input type="radio" name="cardType" value="Master Card">Master Card<br>
            &nbsp;&nbsp;<input type="radio" name="cardType" value="Amex">Amex<br>
            &nbsp;&nbsp;<input type="radio" name="cardType" value="Discover">Discover<br>
            &nbsp;&nbsp;<input type="radio" name="cardType" value="Java SmartCard">Java SmartCard<br>
            <!--密码输入框-->
            Credit Card password:<input type="password" name="cardNum"><br>
            Repeat Credit Card password:<input type="password" name="cardNum"><br><br>
            
            <center><input type="submit" value="Submit Order"></center>
            
        </form>
    </body>
</html>

②ShowParameters.java

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * 读取所有的参数
 * @author jukey
 *
 */
public class ShowParameters extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=gb2312");
        PrintWriter out = response.getWriter();
        
        String title = "Reading All Request Parameters";
        out.println("init");
        out.println("<html><head><title>读取所有参数</title></head>"
                + title
                + "\n" + "<table border=1 align=center>\n"
                + "<TH>Parameter Name<TH>Parameter Value(s)");
        
        // Returns an Enumeration of String objects containing the names of the parameters contained in this request. 
        Enumeration paramNames = request.getParameterNames();
        // Tests if this enumeration contains more elements. 
        while(paramNames.hasMoreElements()) {
            // Returns the next element of this enumeration if this enumeration object has at least one more element to provide. 
            String paraName = (String)paramNames.nextElement();
            out.println("<tr><td>" + paraName + "\n<td>");
            // Returns an array of String objects containing all of the values the given request parameter has, or null if the parameter does not exist. 
            // 注意参数paraName（变量）不能加双引号，否则就是找参数名叫paraName的对应值了。
            String[] paramValues = request.getParameterValues(paraName);
            // 这个参数只有一个值
            if(paramValues.length == 1) {
                String paramValue = paramValues[0];
                if(paramValue.length() == 0) {
                    out.println("<I>no value</I>");
                } else {
                    out.println(paramValue);
                }
            }else {
                // 这个参数有好几条值
                out.println("<UL>");
                for(int i = 0; i < paramValues.length; i++) {
                    out.println("<LI>" + paramValues[i]);
                }
                out.println("</UL>");
            }
        }
        out.println("</table>\n<body><html>");
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("doPost");
        doGet(request,response);
    }

}

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6.Cookies的设置和读取

①设置Cookie到客户端

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * 设置Cookie到客户端
 * @author jukey
 *
 */
public class SetCookies extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 向客户端写入Cookie，共6个
        for(int i = 0; i < 3; i++) {
            // 3个没有设置时间的Cookie，属于本窗口及其子窗口
            Cookie cookie = new Cookie("Session-Cookie-" + i, "Cookie-Value-S" + i);
            response.addCookie(cookie);
            // 以下3个Cookie设置了时间（3600秒，1小时），属于文本，别的窗口也可以访问到这些Cookie
            cookie = new Cookie("Persistent-Cookie-" + i, "Cookie-Value-P" + i);
            cookie.setMaxAge(3600);
            response.addCookie(cookie);
        }
        
        response.setContentType("text/html;charset=gb2312");
        PrintWriter out = response.getWriter();
        String title = "Setting Cookies";
        out.println("<html><head><title>设置Cookie</title></head>"
                + "<body>" + title + "<br>"
                + "There are six cookies associates with this page.<br>"
                + "to see them,visit the <a href=\"ShowCookies\">\n"
                + "<code>ShowCookies</code> servlet</a>"
                + "</body></html>");
    }

}

②读取客户端的Cookies

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * 读取客户端的Cookies
 * @author jukey
 *
 */
public class ShowCookies extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        response.setContentType("text/html;charset=gb2312");
        PrintWriter pw = response.getWriter();
        String title = "Active Cookies";
        pw.println("init");
        pw.println("<html><head><title>读取客户端</title></head>"
                + title
                + "\n" + "<table border=1 align=center>\n"
                + "<TH>Cookie Name<TH>Cookie Value" + "<br>");
        
        // 读取客户端的所有Cookie
        Cookie[] cookies = request.getCookies();
        if(cookies != null) {
            Cookie cookie;
            for(int i = 0; i < cookies.length; i++) {
                cookie = cookies[i];
                pw.println("<tr>\n" + "<td>" + cookie.getName() +"</td>\n"
                        + "<td>" + cookie.getValue() +"</td></tr>\n");
            }
        }
        pw.println("</table>\n<body><html>");
    }

}

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7.Session的测试

①演示Servlet API中的session管理机制（常用方法）

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Date;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * 演示Servlet API中的session管理机制（常用方法）
 * @author jukey
 *
 */
public class SessionInfoServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        // Returns the current session associated with this request, or if the request does not have a session, creates one. 
        HttpSession mySession = request.getSession(true);
        
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        String title = "Session Info Servlet";
        out.println("<html>");
        out.println("<head>");
        out.println("<title>Session Info Servlet</title>");
        out.println("</head>");
        out.println("<body>");
        out.println("<h3>Session Infomation</h3>");
        
        // Returns true if the client does not yet know about the session or if the client chooses not to join the session. 
        out.println("New Session:" + mySession.isNew() + "<br>");
        // Returns a string containing the unique identifier assigned to this session.
        out.println("Session Id:" + mySession.getId() + "<br>");
        // Returns the time when this session was created, measured in milliseconds since midnight January 1, 1970 GMT. 
        out.println("Session Create Time:" + new Date(mySession.getCreationTime()) + "<br>");
        out.println("Session Last Access Time:" + new Date(mySession.getLastAccessedTime()) + "<br>");
        
        out.println("<h3>Request Infomation</h3>");
        // Returns the session ID specified by the client.
        out.println("Session Id From Request:" + request.getRequestedSessionId() + "<br>");
        // Checks whether the requested session ID came in as a cookie. 
        out.println("Session Id Via Cookie:" + request.isRequestedSessionIdFromCookie() + "<br>");
        // Checks whether the requested session ID came in as part of the request URL. 
        out.println("Session Id Via URL:" + request.isRequestedSessionIdFromURL() + "<br>");
        // Checks whether the requested session ID is still valid. 
        out.println("Valid Session Id:" + request.isRequestedSessionIdValid() + "<br>");
        
        out.println("<a href=" + response.encodeURL("SessionInfoServlet") + ">refresh</a>");
        out.println("</body></html>");
        out.close();
    }

}

②Session追踪

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Date;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Session追踪
 * 如果浏览器支持Cookie，创建Session的时候会把SessionId保存在Cookie中
 * 否则必须自己编程使用URL重写的方式实现Session：response.encodeURL()
 * @author jukey
 *
 */

public class ShowSession extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        response.setContentType("text/html;charset=gb2312");
        PrintWriter out = response.getWriter();
        String str = "Session Tracking Example";
        String heading;
        
        // 如果会话已经存在，则返回一个HttpSession；否则创建一个新的
        HttpSession session = request.getSession(true);
        // 从当前session中读取属性accessCount的值
        Integer accessCount = (Integer)session.getAttribute("accessCount");
        if(accessCount == null) {
            accessCount = new Integer(0);
            heading = "Welcome newUser";
        } else {
            heading = "Welcome Back";
            accessCount = new Integer(accessCount.intValue() + 1);
        }
        // 向当前session中插入键（key，属性）值（value）对
        // Binds an object to this session, using the name specified.
        session.setAttribute("accessCount", accessCount);
        
        out.println("<html><head><title>Session追踪</title></head>"
                + "<body>" + heading + "<br>"
                + "<h2>Information on Your Session</h2><br>"
                + "\n" + "<table border=1 align=center>\n"
                + "<TH>Info Type<TH>Value" + "<br>"
                + "<tr>\n" + "<td>ID</td>\n"
                + "<td>" + session.getId() +"</td></tr>\n"
                + "<tr>\n" + "<td>CreatTime</td>\n"
                + "<td>" + new Date(session.getCreationTime()) +"</td></tr>\n"
                + "<tr>\n" + "<td>LastAccessTime</td>\n"
                + "<td>" + new Date(session.getLastAccessedTime()) +"</td></tr>\n"
                + "<tr>\n" + "<td>Number of Access</td>\n"
                + "<td>" + accessCount +"</td></tr>\n"
                + "</body></html>");
    }

}

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8.Application测试

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Application测试
 * 用于保存整个web应用的生命周期内都可以访问的数据
 * 可供多个不同窗口访问，可作为某一页面被总共访问次数的计数器（比如网站首页的访问量）
 * @author jukey
 *
 */
public class TestServletContext extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        response.setContentType("text/html;charset=gb2312");
        PrintWriter out = response.getWriter();
        
        // Returns a reference to the ServletContext in which this servlet is running. 
        ServletContext application = this.getServletContext();
        // 从当前application中读取属性accessCount的值
        Integer accessCount = (Integer)application.getAttribute("accessCount");
        if(accessCount == null) {
            accessCount = new Integer(0);
        } else {
            accessCount = new Integer(accessCount.intValue() + 1);
        }
        // 向当前application中插入键（key，属性）值（value）对
        application.setAttribute("accessCount", accessCount);
        
        out.println("<html><head><title>ServletContext测试</title></head><br>"
                + "<body><td>" + accessCount +"</td>\n"
                + "</body></html>");
        
    }

}

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9.Servlet类本身位于包中的情况

package com.bjsxt.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;

/**
 * Servlet类本身位于包中的情况
 * classes中要class文件及其各级目录一起放置
 * web中如下设置：<servlet-class>com.bjsxt.servlet.HelloWorldServlet2</servlet-class>
 * @author jukey
 *
 */
public class HelloWorldServlet2 extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        response.setContentType("text/html;charset=gb2312");
        PrintWriter out = response.getWriter();
        out.println("Hello World!");
        
    }

}

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10.在Servlet中直接连接数据库，并查询显示信息

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * 在Servlet中直接连接数据库，并查询显示信息
 * 每个application都应该有自己的驱动包，放在各自项目的WEB-INF\lib\目录下
 * @author jukey
 *
 */
public class ShowRs extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        Connection conn = null;
        Statement stmt = null;
        ResultSet rs = null;
        
        response.setContentType("text/html;charset=gb2312");
        PrintWriter out = response.getWriter();
        out.println("<table border=1>");
        out.println("<tr><td>Content:</td></tr>");

        try {
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection("jdbc:mysql://localhost/bbs?user=root&password=mysql");
            stmt = conn.createStatement();
            String sql = "select * from article";
            rs = stmt.executeQuery(sql);
            while(rs.next()) {
                out.println("<tr>");
                out.println("<td>" + rs.getString("cont") + "</td>");
                out.println("</tr>");
            }
        } catch (SQLException ex) {
            ex.printStackTrace();
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
             if (rs != null) {
                 try {
                     rs.close();
                     rs = null;
                 } catch (SQLException sqlEx) { 
                     sqlEx.printStackTrace();
                 }
             }
             if (stmt != null) {
                 try {
                     stmt.close();
                     stmt = null;
                 } catch (SQLException sqlEx) { 
                     sqlEx.printStackTrace();
                 }
             }
             if (conn != null) {
                 try {
                     conn.close();
                     conn = null;
                 } catch (SQLException sqlEx) { 
                     sqlEx.printStackTrace();
                 }
             }
        }
    }

}

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11.Servlet中使用专门的数据库操作类DB

①Servlet中使用专门的数据库操作类DB，这样，本程序就比前一个显得简单清楚

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet中使用专门的数据库操作类DB，这样，本程序就比前一个显得简单清楚
 * DB类，专门跟数据库打交道，分工明确，各司其职
 * @author jukey
 *
 */
public class ShowRsUseBean extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        response.setContentType("text/html;charset=gb2312");
        PrintWriter out = response.getWriter();
        out.println("<table border=1>");
        out.println("<tr><td>Content:</td></tr>");
        
        // 直接调用DB中的静态方法来为本类服务
        Connection conn = DB.getConnection();
        Statement stmt = DB.getStatement(conn);
        String sql = "select * from article";
        ResultSet rs = DB.getResultSet(stmt, sql);
        
        try {
            while(rs.next()) {
                out.println("<tr>");
                out.println("<td>" + rs.getString("cont") + "</td>");
                out.println("</tr>");
            }
        } catch (SQLException e) {
            System.out.println("执行SQL遍历过程中出现了错误。。。");
            e.printStackTrace();
        } finally {
            // 直接调用DB中的静态方法来关闭一系列被打开的资源
            DB.close(rs);
            DB.close(stmt);
            DB.close(conn);
        }
    }

}

②专门跟数据库打交道的类

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

/**
 * 专门跟数据库打交道的类
 * @author ykn
 *
 */
public class DB {

    // 获取连接
    public static Connection getConnection() {
        Connection conn = null;
        try {
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection("jdbc:mysql://localhost/bbs?user=root&password=mysql");
        } catch (ClassNotFoundException e) {
            System.out.println("驱动程序未找到，请加入mysql.jdbc的驱动包。。。");
            e.printStackTrace();
        } catch (SQLException e) {
            System.out.println("执行数据库连接过程中出现了错误。。。");
            e.printStackTrace();
        }
        return conn;
    }
    
    // 获取表达式语句
    public static Statement getStatement(Connection conn) {
        Statement stmt = null;
        try {
            if (conn != null) {
                stmt = conn.createStatement();
            }
        } catch (SQLException e) {
            System.out.println("执行获取表达式语句过程中出现了错误。。。");
            e.printStackTrace();
        }        
        return stmt;
    }
    
    // 获取查询的结果集
    public static ResultSet getResultSet(Statement stmt, String sql) {
        ResultSet rs = null;
        try {
            if (stmt != null) {
                rs = stmt.executeQuery(sql);
            }
        } catch (SQLException e) {
            System.out.println("执行查询过程中出现了错误。。。");
            e.printStackTrace();
        }
        return rs;
    }
    
    // 关闭连接
    public static void close(Connection conn) {
        try {
            if (conn != null) {
                conn.close();
                conn = null;
            }
        } catch (SQLException e) {
            System.out.println("执行关闭数据库连接过程中出现了错误。。。");
            e.printStackTrace();
        }        
    }
    
    // 关闭表达式语句
    public static void close(Statement stmt) {
        try {
            if (stmt != null) {
                stmt.close();
                stmt = null;
            }
        } catch (SQLException e) {
            System.out.println("执行关闭表达式语句过程中出现了错误。。。");
            e.printStackTrace();
        }        
    }
    
    // 关闭结果集
    public static void close(ResultSet rs) {
        try {
            if (rs != null) {
                rs.close();
                rs = null;
            }
        } catch (SQLException e) {
            System.out.println("执行关闭结果集过程中出现了错误。。。");
            e.printStackTrace();
        }        
    }
}