[英]How to fix NullPointerException
我正在写Java servlet
,它应该通过user_id获得DVD。 但是我对NullPointerException
有问题。 有人知道如何解决吗? 当我尝试通过if (nickname != null )
修复它时, request.setAttribute("dvds", dvds);
属性dvds出现问题request.setAttribute("dvds", dvds);
谢谢
List<Dvd> dvds;
try {
String nickname = request.getParameter("nickname");
User user = userDao.getByLogin(nickname);
Long userId = user.getId();
dvds = this.dvdDao.getDvdsByUserId(userId);
} catch (SQLException e) {
throw new ServletException("Unable to get dvds", e);
}
request.setAttribute("dvds", dvds);
RequestDispatcher dispatcher = request.getRequestDispatcher("WEB-INF/loans.jsp");
dispatcher.forward(request, response);
}
}
public List<Dvd> getDvdsByUserId(Long user_id) throws SQLException {
List<Dvd> dvds = new ArrayList<Dvd>();
Connection connection = null;
PreparedStatement preparedStatement = null;
ResultSet resultSet = null;
try {
connection = getConnection();
preparedStatement = connection.prepareStatement("SELECT * FROM sedivyj_dvd where user_id = ?;");
preparedStatement.setLong(1, user_id);
resultSet = preparedStatement.executeQuery();
while (resultSet.next()) {
Dvd dvd = new Dvd();
dvd.setId(resultSet.getInt("id"));
dvd.setUser_id(resultSet.getString("user_id"));
dvd.setName(resultSet.getString("name"));
dvd.setBorrower(resultSet.getString("borrower"));
dvd.setMail(resultSet.getString("mail"));
dvd.setBorrow_date(resultSet.getString("borrow_date"));
dvd.setBorrow_until(resultSet.getString("borrow_until"));
dvds.add(dvd);
}
} catch (SQLException e) {
throw e;
} finally {
cleanUp(connection, preparedStatement);
}
return dvds;
}
假设nickname
是问题所在(还考虑到不存在的用户):
List<Dvd> dvds = new ArrayList<Dvd>;
try {
final String nickname = request.getParameter("nickname");
if (nickname != null) {
final User user = this.userDao.getByLogin(nickname);
if (user != null) {
dvds = this.dvdDao.getDvdsByUserId(user.getId());
} else {
// handle non-existent user
}
} else {
// handle no "nickname" parameter was present
}
} catch (SQLException e) {
throw new ServletException("Unable to get dvds", e);
}
request.setAttribute("dvds", dvds);
final RequestDispatcher dispatcher = request.getRequestDispatcher("WEB-INF/loans.jsp");
dispatcher.forward(request, response);
看来您的代码在请求中需要 “昵称”参数才能正常工作。 (如果您没有“昵称”,则无法找出用户标识,并无法检索该用户的DVD ...)
由于请求缺少参数(或者拼写错误或其他原因),因此您似乎正在获得NPE。 在发送请求的过程中,这绝对是一个错误。
正确的解决方案是:
发送某种如果错误响应getParameter("nickname")
返回null
, 和
修复发送缺少参数的请求的Web表单。
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