一个常见的JDBC封装导致的问题

大家看一个数据库封装类
操作一次没问题，操作两次就报错
com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: No operations allowed after connection closed.


Debug调试，发现connection不能真正关掉，使用connection完后，调用close（）方法，下次getConnection（）时候
instance不为null

public class ConnectionUtil {
	private final static String  url="jdbc:mysql://localhost:3306/qq?useUnicode=true&characterEncoding=UTF-8";
	private final static String  username="root";
	private final static String  password="root";
	private  static Connection   instance=null;
	
	private ConnectionUtil(){
		if(instance==null){
			try {
				Class.forName("org.gjt.mm.mysql.Driver");
				instance=DriverManager.getConnection(url, username, password);
			} catch (ClassNotFoundException e) {
				e.printStackTrace();
			} catch (SQLException e) {
				e.printStackTrace();
			}
			
		}
	}
	
	public static Connection getConnection(){
		if(instance==null){
			new ConnectionUtil();
		}
		return instance;
		
	}
	
	
	public static void close(Connection con,PreparedStatement ps,ResultSet rs){
		if(con!=null){
			try {
				con.close();
			} catch (SQLException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}finally{
				con=null;
			}
		}
		if(ps!=null){
			try {
				ps.close();
			} catch (SQLException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}finally{
				ps=null;
			}
		}
		if(rs!=null){
			try {
				rs.close();
			} catch (SQLException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}finally{
				rs=null;
			}
		}
		
	}
	
	public static void close(Connection con,PreparedStatement ps){
		if(con!=null){
			try {
				con.close();
			} catch (SQLException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}finally{
				con=null;
			}
		}
		if(ps!=null){
			try {
				ps.close();
			} catch (SQLException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}finally{
				ps=null;
			}
		}
		
		
	}
	
	public static void close(Connection con){
		if(con!=null){
			try {
				con.close();
			} catch (SQLException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}finally{
				con=null;
			}
		}
	}

}

结贴了。。。。有其他结贴方式吗？