import java.sql.*;
import java.util.*;
public class DbWrapper
{
    // 定義連接池對象為靜態變量，將一直存在，直到工作目錄關閉。
    private static DataSource ds = null;

    // 1.用連接池的方式獲得連接
    // 如果不是做多數據庫程序，推薦使用此方法
    // 相關內容：在tomcat管理界面配置連接池
    public static Connection openConnection() throws Exception
    {
        // 只需要初始化1次
        if ( ds == null )
        {
            Context initContext = new InitialContext();
            Context envContext = (Context) initContext.lookup("java:/comp/env");
            DataSource ds = (DataSource) envContext.lookup("jdbc/MyDataSource");
        }
        return ds.getConnection();
    }
    
    // 2.用jdbc驅動獲得連接
    // 相關內容：JSP數據庫連接大全
    public static Connection openConnection(
        String driver, 
        String url, 
        String username, 
        String password) 
            throws Exception
    {
        Class.forName(driver).newInstance();
        return DriverManager.getConnection(url, username, password);
    }
    
    public static void closeConnection(Connection conn) throws Exception
    {
        if ( conn != null )
        {
            conn.close();
        }
    }

    public static int executeUpdate(String sql) throws Exception
    {
       int count = 0;
        
        Connection conn = null;
        Statement  stmt = null;
        
        try
        {
            conn = openConnection();
            stmt = conn.createStatement();
        
            count = stmt.executeUpdate(sql);
        }
        catch ( Exception e )
        {
            throw e;
        }
        finally
        {
            closeConnection(conn);
        }
        
        return count;
    }
    
    public static List executeQuery(String sql) throws Exception
    {
        List list = new ArrayList();
        
        Connection conn = null;
        Statement  stmt = null;
        ResultSet  rs   = null;
        
        try
        {
            conn = openConnection();
            stmt = conn.createStatement();
            rs   = stmt.executeQuery(sql);
            
            ResultSetMetaData rsmd = rs.getMetaData();
        
            while ( rs.next() )
            {
                Map map = new HashMap();
                
                for ( int i = 1; i <= rsmd.getColumnCount(); i++ )
                {
                    map.put(rsmd.getColumnName(i), rs.getObject(i));
                }
                
                list.add(map);
            }
        }
        catch ( Exception e )
        {
            e.printStackTrace();
        }
        finally
        {
            if ( rs != null ) rs.close();
            closeConnection(conn);
        }
        
        return list;
    }
}

// 1.對于insert, update, delete語句
int count = DbWrapper.executeUpdate(sql);

// 2.對于selete語句
java.util.List list = DbWrapper.executeQuery(sql);

// 方法一:按名字取值，注意大小寫是嚴格區分的
for ( int i = 0; i < list.size(); i++ )
{
    java.util.Map map = (java.util.Map)list.get(i);
    out.println(mag.get("column_name").toString());
}
 
// 方法二：遍歷取值
for ( int i = 0; i < list.size(); i++ )
{
    java.util.Map map = (java.util.Map)list.get(i);

    for (java.util.Iterator it = map.keySet().iterator(); it.hasNext();) 
    {
        String column_name = it.next().toString());

        // 取值時注意null判斷
        out.println(column_name + " = " + map.get(column_name)  == null ? "" : map.get(column_name).toString());
    }
}