[英]Getting list of strings from servlet to jsp
我有一個JSP頁面,其中有兩個標簽。 首先,我嘗試輸入諸如塔塔,現代,豐田,奧迪等汽車制造商的名稱。當用戶首先選擇任何選項時,它應該顯示該制造商的汽車型號,例如Innova,Land Cruiser等。所以當用戶選擇第一個標記中的任何選項,我正在調用一個servlet,該servlet從數據庫中的列表中獲取所有模型,並將該列表設置為session的屬性,並將請求轉發回JSP。 但是在jsp中,當我嘗試獲取列表時,它給出了NULL POINTER EXCEPTION。 怎么解決呢? 代碼如下:
DbReviewCar.java
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
Connection conn= null;
PreparedStatement pstmt= null;
ResultSet rs;
String sql= null;
String maker= request.getParameter("make");
List modellist= new ArrayList();
/*if(maker==null)
{
modellist.add("ferrari");
modellist.add("hummer");
request.getSession().setAttribute("value", modellist);
request.getRequestDispatcher("CarReview.jsp").forward(request,response);
}
else
{*/
try {
Class.forName("com.mysql.jdbc.Driver");
conn= DriverManager.getConnection("jdbc:mysql://localhost/cardetails", "root", "Welcome123");
sql= "select model from cars where make=?;";
pstmt= conn.prepareStatement(sql);
pstmt.setString(1, maker);
rs= pstmt.executeQuery();
while(rs.next())
{
String mod= rs.getString(1);
modellist.add(mod);
System.out.println(mod+">>>>>>>>>>>>>>>>>>.");
}
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
request.getSession().setAttribute("value", modellist);
request.getRequestDispatcher("CarReview.jsp").forward(request,response);
}
CarReview.jsp
這是我的JSP文件
<form action="DbReviewCar" method="get" name="myform">
<table>
<tr>
<td>
<tr>
<td>Make:</td>
<td><select name="make" onchange="this.form.submit()"><option>select</option>
<option>Maruti</option>
<option>Ford</option>
<option>Honda</option>
<option>Skoda</option>
<option>Tata</option>
<option>Audi</option>
<option>Toyota</option></select><br></br></td>
</tr>
<%
List list = new ArrayList();
list.addAll((List) (request.getSession().getAttribute("value")));
%>
<tr>
<td>Model:</td>
<td><select name="model">
<%
for (int i = 0; i < list.size(); i++) {
%>
<option value=<%=list.get(i)%>><%=list.get(i)%></option>
<%
}
%>
</select><br></br></td>
</tr>
<tr>
<td>Rating For Style:</td>
<td><input type="text" name="style"><br></br></td>
</tr>
<tr>
<td>Rating for comfort:</td>
<td><input type="text" name="comfort"><br></br></td>
</tr>
<tr>
<td>Rating for Performance:</td>
<td><input type="text" name="performance"><br></br></td>
</tr>
<tr>
<td>Rating for FuelEconomy:</td>
<td><input type="text" name="economy"><br></br></td>
</tr>
<tr>
<td>Review:</td>
<td><textarea cols="18" rows="3"></textarea><br></br></td>
</tr>
<tr>
<td><Button>Save</Button></td>
<td><input type="reset" name="cancel" value="Cancel" /></td>
</tr>
</table>
</form>
首次加載jsp時,未設置“值”屬性。 嘗試檢查null是否為值:
request.getSession().getAttribute("value")
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