//注册时判断用户名是否存在
	public static boolean UserExists(String username) {
		//可以建立一个连接池保存一定数量的连接，当有对象需要数据库连接时，直接将这个连接返回给该对象，
		ConnectionPool pool = ConnectionPool.getInstance();
		Connection connection = pool.getConnection();
		PreparedStatement ps = null;
		ResultSet rs = null;
		
		String qr = "SELECT username FROM User "+ "WHERE username = ?";
		try {
			ps = connection.prepareStatement(qr);
			ps.setString(1, username);
			rs = ps.executeQuery();
			return rs.next();
		} catch (SQLException e) {
			System.out.println(e);
			return false;
		} finally {
			DBUtil.closeResultSet(rs);
			DBUtil.closePreparedStatement(ps);
			pool.freeConnection(connection);
		}
		
	}
	//将注册用户信息保存至数据库
	public static int insert(User user) {
		ConnectionPool pool = ConnectionPool.getInstance();
		Connection connection = pool.getConnection();
		PreparedStatement ps = null;
		
		String qr = "INSERT INTO User (username, password)"+"VALUES (?, ?)";
		try { 
			
			ps = connection.prepareStatement(qr);
			ps.setString(1, user.getusername());
			ps.setString(2, user.getpassword());
			
			return ps.executeUpdate();
		} catch (SQLException e) {
			System.out.println(e);
			return 0;
		} finally {
			DBUtil.closePreparedStatement(ps);
			pool.freeConnection(connection);
		} 
		
	}