Java数据库学习之SQL语句动态拼接

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间要有空格 userinfo u "
		String sql = "SELECT u.uname,u.uaddr from userinfo u ";
//		!"".equals(user.getUname()) 如果前段搜索用户名不填写，传到后端的是空值
		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()+"%");
		}
//		拼接分页查询的limit
		sql+= " Limit "+(pif.getPage()-1)*pif.getPageSize() +","+pif.getPageSize();
		try {
			PreparedStatement ps = conn.prepareStatement(sql);
//			占位符个数不确定，所以不能直接.所以需要对是否有占位符有几个进行判断
//			如果有拼接占位符号？，则在循环中对占位符进行赋值
			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;
	}