Java數據庫學習之SQL語句動態拼接

public class UserDaoImpl implements UserDao {

	@Override
	public List<User> getUserByPage(PageInfo pif,User user){
		List<User> lu = new ArrayList<User>();
		Connection conn = DButil.getconn();
//		注意拼接式字符串之間要有空格 userinfo u "
		String sql = "SELECT u.uname,u.uaddr from userinfo u ";
//		!"".equals(user.getUname()) 如果前段搜索用戶名不填寫，傳到后端的是空值
		boolean contactAnd = false;
		String myWhere = "where";
		String myAnd = "and";
		List<String> params = new ArrayList<>();
		if(user.getUname()!=null&&!"".equals(user.getUname())) {
			if(contactAnd) {
//		" 空格 uname = ?空格 "
				sql += myAnd + " uname = ? ";
			}else {
				sql += myWhere + " uname = ? ";
				contactAnd = true;
			}
			params.add(user.getUname());
		}
		if(user.getUaddr()!=null&&!"".equals(user.getUaddr())) {
			if(contactAnd) {
				sql += myAnd + " uaddr like ? ";
			}else {
				sql += myWhere + " uaddr like ? ";
				contactAnd = true;
			}
			params.add("%"+user.getUaddr()+"%");
		}
//		拼接分頁查詢的limit
		sql+= " Limit "+(pif.getPage()-1)*pif.getPageSize() +","+pif.getPageSize();
		try {
			PreparedStatement ps = conn.prepareStatement(sql);
//			占位符個數不確定，所以不能直接.所以需要對是否有占位符有幾個進行判斷
//			如果有拼接占位符號？，則在循環中對占位符進行賦值
			for(int i = 0;i<params.size();i++) {
				ps.setString(i+1,params.get(i));
			}
			ResultSet rs = ps.executeQuery();
			while(rs.next()) {
				User user1 = new User();
				user1.setUname(rs.getString("uname"));
				user1.setUaddr(rs.getString("uaddr"));
				lu.add(user1);
			}
			System.out.println(lu);
		} catch (SQLException e) {
			e.printStackTrace();
		}
		return lu;
	}