Question

我的目标是要求用户输入日期，我的应用程序将检索该日期的所有数据并显示在jsp的表格中。

现在，arraylist返回null，因此它将转到错误页面。但是在我的数据库中，有。

帮助将不胜感激。：）

以下是我的代码..

search.jsp

<form method="post" action="SearchServlet">
Date:<input name="rdate">
<input type="submit" value="Search">
</form>

SearchServlet

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws   ServletException, IOException {
String rdate = request.getParameter("rdate");
PersonBean pb = new PersonBean();
pb.setDate(rdate);
PersonDAO pd = new PersonDAO();

ArrayList personarraylist = pd.getDatePerson(pb);
System.out.println(personarraylist);
if (personarraylist.isEmpty() == true) {
response.sendRedirect("error.jsp");
} else {
request.setAttribute("person", rd.getDatePerson(pb));
RequestDispatcher view = request.getRequestDispatcher("success.jsp");
view.forward(request, response);
}
}

PersonDAO

public ArrayList getDatePerson(PersonBean pb) {

ArrayList<PersonBean> listdateperson = new ArrayList<PersonBean>();
String rdate = pb.getDate();
System.out.print(rdate);

int pendingcount = 0;

try {
currentCon = ConnectionManager.getConnection();
Statement statement = currentCon.createStatement();
String count = "SELECT COUNT(*) FROM person where date='" + rdate + "'";

PreparedStatement countstmt = currentCon.prepareStatement(count);
ResultSet countrs = countstmt.executeQuery();

while (countrs.next()) {
pendingcount = countrs.getInt(1);
System.out.print(pendingcount);
}

if (pendingcount == 0) {
PersonBean pbb = null;
pbb.setDate("0");
listdateperson.add(pbb);
}

else {

ResultSet rs = statement.executeQuery("SELECT name, date FROM person WHERE date='" + rdate + "'");
PersonBean pbbb = null;

while (rs.next()) {
PersonBean = new PersonBean();
pbbb.setName(rs.getString("name"));
pbbb.setDate(rs.getString("date"));

}

}
} catch (Exception asd) {
System.out.println(asd.getMessage());
}
return listdateperson;
}

Answer 1

1）PersonDAO if内的代码会引发NullPointerException

if (pendingcount == 0) {
 PersonBean pbb = null;  //assign null reference
 pbb.setDate("0");  //setting value to null reference throws NPE
 listdateperson.add(pbb);
}

2）缺少分配对象引用

PersonBean pbbb = null; 
while (rs.next()) {
 PersonBean = new PersonBean(); //how can you assign object reference to class PersonBean?

一定是

pbbb = new PersonBean();

您正在使用PreparedStatement，然后使用占位符（？）将参数传递给查询，这将Prevent SQL injection

另见

PreparedStatement java docs

Answer 2

因为pbbb

时没有将pendingcount != 0添加到while循环的数组列表中

将Arraylist从Servlet返回到DAO，然后返回Servlet，再返回Jsp

2 个答案:

另见