我写的MYSQL数据库连接BEAN老是出错.请那个好心的人帮我写一份好吗.

Question

package db;
import java.sql.*;
public class mysql_conn{
String conn_str="jdbc:mysql://localhost/test";
String username="root";
String password="";
private Connection conn=null;
private Statement stmt=null;
ResultSet rs=null;
//#################
public ResultSet query(String sql) throws ClassNotFoundException,SQLException
{
try{
Class.forName("org.gjt.mm.mysql.Driver");
conn=java.sql.DriverManager.getConnection(conn_str,username,password);
stmt=conn.createStatement();
rs=stmt.executeQuery(sql);}
catch(SQLException ex) {System.err.println("query():"+ex.getMessage());}
return rs;
}
//###################
public void update(String sql) throws ClassNotFoundException,SQLException
{
try{
Class.forName("org.gjt.mm.mysql.Driver");
conn=java.sql.DriverManager.getConnection(conn_str,username,password);
stmt=conn.createStatement();
stmt.executeUpdate(sql);}
catch(SQLException ex) {System.err.println("update():"+ex.getMessage());}
}
//######################
public void closestmt()
{try{stmt.close();}
catch(SQLException ex) {System.err.println("closestmt():"+ex.getMessage());}
}
//######################
public void closeconn()
{try{conn.close();}
catch(SQLException ex) {System.err.println("closeconn():"+ex.getMessage());}
}
//####################
}