JAV求代码，实现连接数据库，并执行写在XML中的SQL

最好详细一点，新手小白。
就是按照需求，以后修改SQL都再配置文件中修改而不修改代码。

http://www.cnblogs.com/centor/p/6142775.html 这里有

<?xml version="1.0" encoding="UTF-8" ?>

<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">



select * from user where id = #{id}

<!-- 返回List而定义的resultMap，注意column是数据库中定义的列字段，而property是我们自定义的字段，能够解决属性和数据库字段不一致的问题 -->  
<resultMap type="User" id="resultUserList">  
    <id column="id" property="id"></id>  
    <result column="userName" property="userName"/>  
    <result column="userAge" property="userAge"/>  
    <result column="userAddress" property="userAddress"/>  
</resultMap>  
<!-- 通过用户名返回用户列表 -->  
<!-- 记住：在获取List列表的时候，resultMap="resultUserList"千万别写成resultType="resultUserList"，否则报错 -->  
<select id="getUserList" parameterType="string"  resultMap="resultUserList">  
    select * from `user` where userName like #{userName}  
</select>  


<!-- 记住type="Article"新加一个class，要去configuration.xml进行别名定义，否则找不到报错 -->  
<!-- 通过association来关联user，达到多对一的效果，多篇文章对应一个用户 -->  
<!-- column="aid"给数据库列id命了一个别名，区分关联的id,否则只能查询出一条数据 -->  
<resultMap type="Article" id="resultUserArticleList">  
    <id column="aid" property="id"></id>  
    <result column="title" property="title"/>  
    <result column="content" property="content"/>  
    <!-- <association property="user" javaType="User">  
        <id column="id" property="id"></id>  
        <result column="userName" property="userName"/>  
        <result column="userAddress" property="userAddress"/>  
    </association> -->  
    <!-- 上面的association等同于下面的这个，前提条件是之前就已经定义过resultUserList -->  
    <association property="user" javaType="User"  resultMap="resultUserList"></association>  
</resultMap>  
<!-- 获取用户的多个文章 -->  
<select id="getUserAticles" parameterType="int"  resultMap="resultUserArticleList">  
    select user.id,user.userName,user.userAddress,  
   article.id aid,article.title,article.content   
   from user,article   
   where user.id=article.userid and user.id=#{id}  
</select>  




 <!-- useGeneratedKeys设置为"true"表明要MyBatis获取由数据库自动生成的主键；keyProperty="id"指定把获取到的主键值注入到User的id属性 -->  
<insert id="insertUserInfo"  parameterType="User" useGeneratedKeys="true" keyProperty="id">  
    insert into `user`(userName,userAge,userAddress) values (#{userName},#{userAge},#{userAddress})  
</insert>  

<update id="updateUserInfo" parameterType="User">  
    update user set userName=#{userName},userAge=#{userAge},userAddress=#{userAddress} where id=#{id}  
</update>  

<delete id="deleteUserInfo" parameterType="int">  
    delete from `user` where id=#{id}  
</delete>

可以了解一下mybatis的使用，mybatis实现了sql和代码的分离，应该能解决你的问题

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
import javax.xml.parsers.*;
import javax.xml.transform.*;
import org.w3c.dom.*;
import org.xml.sax.*;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;

public class CreateXMLForEnterServelet extends HttpServlet {
private static final String CONTENT_TYPE = "text/html; charset=GBK";

//Initialize global variables
public void init() throws ServletException {
}

//Process the HTTP Get request
public void doGet(HttpServletRequest request, HttpServletResponse response) throws
        ServletException, IOException {
    response.setContentType(CONTENT_TYPE);
    PrintWriter out = response.getWriter();
    //获取传递过来的值
    String databaseType = request.getParameter("select");
    String userName = request.getParameter("userName");
    String userPassword = request.getParameter("userPassword");
    String dbName = request.getParameter("dbName");
    String dbSouse = request.getParameter("dbSouces");
    int port = Integer.parseInt(request.getParameter("portNum"));
    Document doc;
    Element root;
    try
    {
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = dbf.newDocumentBuilder();
        String path = request.getRealPath("/");
        File file = new File( path + "/" + "DBInfo.xml");
        if(!file.exists())
        {
            doc = db.newDocument();
            root = doc.createElement("DBInfo");
            doc.appendChild(root);
        }
        else
        {
            doc = db.parse(file);
            root = doc.getDocumentElement();
        }

//添加子节点
Element rowNode = doc.createElement(databaseType);
root.appendChild(rowNode);
Node node = doc.createElement("Driver");
Node nodestr = doc.createElement("ConnectString");
Node serverName = doc.createElement("ServerName");
Node nodeName = doc.createElement("UserName");
Node nodePWD = doc.createElement("PassWord");
Node nodePort = doc.createElement("Port");
Node nodeDB = doc.createElement("DataBase");

        rowNode.appendChild(node);
        rowNode.appendChild(nodestr);
        rowNode.appendChild(serverName);
        rowNode.appendChild(nodeDB);
        rowNode.appendChild(nodeName);
        rowNode.appendChild(nodePWD);
        rowNode.appendChild(nodePort);

        String nodeValue;//Driver
        String nodeConStr;//连接字符串
        if(databaseType.equals("ORACLE"))
        {
           nodeValue = "oracle.jdbc.driver.OracleDriver";
           nodeConStr = "jdbc:oracle:thin";
        }
        else
        {
            nodeValue = "com.microsoft.jdbc.sqlserver.SQLServerDriver";
            nodeConStr = "jdbc:microsoft:sqlserver";
        }
        Text text = doc.createTextNode(nodeValue);
        Text textstr = doc.createTextNode(nodeConStr);
        Text textName = doc.createTextNode(userName);
        Text textPWD = doc.createTextNode(userPassword);
        Text textServer = doc.createTextNode(dbName);
        Text textPort = doc.createTextNode(String.valueOf(port));
        Text textDB = doc.createTextNode(dbSouse);

        node.appendChild(text);
        nodestr.appendChild(textstr);
        nodeName.appendChild(textName);
        nodePWD.appendChild(textPWD);
        serverName.appendChild(textServer);
        nodePort.appendChild(textPort);
        nodeDB.appendChild(textDB);

        DOMSource doms = new DOMSource(doc);

        StreamResult sr = null;

        TransformerFactory tff = TransformerFactory.newInstance();
        Transformer tf = tff.newTransformer();
        sr = new StreamResult(file);
        tf.transform(doms,sr);
        out.println("<h1>Create XMLFile Is Ok!!</h1>");
        request.getRequestDispatcher("index.jsp").forward(request,response);
    }
    catch(Exception ex)
    {
        out.println("<h1>生成 XML 配置文件时出现错误：</h1><br/><h4>"+ex+"</h4>");
        out.println("<br/><br/>");
        out.print("<a href='index.jsp'>【点这里回到首页】</a>");

    }

    out.close();

}

//Process the HTTP Post request
public void doPost(HttpServletRequest request, HttpServletResponse response) throws
        ServletException, IOException {
    doGet(request, response);
}

//Clean up resources
public void destroy() {
}

}

推荐这种东西看mybatis官网教程。
http://www.mybatis.org/mybatis-3/zh/getting-started.html

mybatis官网教程里有

这个很简单，建议拿一个项目看一下

你可以直接百度 SSM框架教学，会有很多的视频，新手应该多敲代码，不然基本学不下去，遇到问题应该多思考，在这并不是你偷懒就能成为一名高级工程师的，个人建议你可以去慕课网、腾讯课堂先把基础学习好，我也是这样过来的都知道学习的枯燥~