从JSP类检索数据（带有Servlet的MVC）

Question

Im currently working on a school project where me and my group are supposed to build a MVC-project where we have the classes Building and Room. The relation is OnetoMany, Building-(OneToMany)-Room. The problem occurs when we want to search for a building name and later show all rooms in that building. We can extract the relevant data when we run our servlet without our jsp, but when we start the project by running our searchBuilding.jsp we can only type the building name, and when expected to see the connected rooms, we instead get the message "java.lang.ClassCastException: java.util.ArrayList cannot be cast to org.ics.ejb.Room". It might be a bit messy with all the code but i would really appreciate if someone could help me spot the problem since im supposed to show this to my professor tomorrow.

This is our code: BUILDING:

@Entity
    @Table(name = "Building")
    public class Building {
    private String bname;
private List<Room> rooms; // Building can have many Rooms

@Id
@Column(name = "Bname")
public String getBname() {
    return bname;
}

public void setBname(String bname) {
    this.bname = bname;
}

@OneToMany(mappedBy = "building", fetch = FetchType.EAGER)
public List<Room> getRooms() {
    return rooms;
}

public void setRooms(List<Room> rooms) {
    this.rooms = rooms;
}   

}

ROOM:

@NamedQueries({
@NamedQuery(name="Room.findByBname",
 query="SELECT r FROM Room r WHERE r.bname LIKE :bname"),
})

@Entity
@Table(name = "Room")
public class Room implements Serializable {
/**
 * 
 */
private static final long serialVersionUID = 1L;
private RoomId id;
private String bname;
private Building building;

public String getBname() {
    return bname;
}

public void setBname(String bname) {
    this.bname = bname;
}

@Id
public RoomId getId() {
    return id;
}

public void setId(RoomId id) {
    this.id = id;
}

@ManyToOne
@JoinColumn(name = "Bname", insertable = false, updatable = false)
public Building getBuilding() {
    return this.building;
}

public void setBuilding(Building building) {
    this.building = building;
}
}

ROOMID:

@Embeddable
public class RoomId implements Serializable {

private String bname;
private String rcode;

public RoomId() {
}

public RoomId(String bname, String rcode) {
    this.bname = bname;
    this.rcode = rcode;
}

@Column(name = "Bname", nullable = false)
public String getbname() {
    return bname;
}

public void setbname(String bname) {
    this.bname = bname;
}

@Column(name = "Rcode", nullable = false)
public String getrcode() {
    return rcode;
}

public void setrcode(String rcode) {
    this.rcode = rcode;
}

public boolean equals(Object other) {
    if ((this == other)) {
        return true;
    }

    if ((other == null)) {
        return false;
    }

    if (!(other instanceof RoomId)) {
        return false;
    }

    RoomId castOther = (RoomId) other;

    return ((this.getbname() == castOther.getbname()) || (this.getbname() != null
            && castOther.getbname() != null &&

    this.getbname().equals(castOther.getbname())))

            &&

((this.getrcode() == castOther.getrcode()) ||                                        (this.getrcode() != null               && castOther.getrcode() != null &&

            this.getrcode().equals(castOther.getrcode())));
}

public int hashCode() {
    return super.hashCode();
}

}

BUILDINGEAO:

@Stateless
public class BuildingEAOImpl implements BuildingEAOImplLocal {
@PersistenceContext(unitName = "LabEJBSql")
private EntityManager em;

public BuildingEAOImpl() {
    // TODO Auto-generated constructor stub
}

public Building findByBname(String bname) {
    return em.find(Building.class, bname);
}
}

FACADE:

@Stateless
public class Facade implements FacadeRemote, FacadeLocal {
@EJB
BuildingEAOImplLocal BuildingEAO;
@EJB
RoomEAOImplLocal RoomEAO;

public Facade() {
    // TODO Auto-generated constructor stub
}

public List<Room> findRoomsByBname(String bname) { 
     return RoomEAO.findByBname(bname); 
     }
 }

SERVLET:

@WebServlet("/TestClientServlet")
public class TestClientServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
@EJB
private FacadeLocal facade;

/**
 * @see HttpServlet#HttpServlet()
 */
public TestClientServlet() {
    super();
    // TODO Auto-generated constructor stub
}

protected void service(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {
    PrintWriter out = response.getWriter();
    out.println("<!DOCTYPE html><html><head>");
    out.println("<title>Lab1</title>");
    out.println("<meta charset=\"ISO-8859-1\">");
    out.println("</head><body>");



    /*//THIS WORKS PERFECTLY FINE
List<Room> rooms = facade.findRoomsByBname("EC2");
    for (Room r : rooms) {
        out.println("<h4>Hittade: " + r.getId().getbname() + " "
                + r.getId().getrcode() + "</h4>");
        out.println("</body></html>");
    }
}*/

protected void doGet(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {

    PrintWriter out = response.getWriter();
    out.println("TestClientServlet-doGet");
    out.close();

}

protected void doPost(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {

    String url = null;
    // Get hidden field
    String operation = request.getParameter("operation");

    if (operation.equals("showbuilding")) {
        String bname = request.getParameter("txtBname");

        List<Room> r = facade.findRoomsByBname(bname);
        request.setAttribute("rooms", r);
        url = "/ShowBuilding.jsp";
    } else if (operation.equals("searchbuilding")) {
        System.out.println("TestClientServlet-searchbuilding");
        url = "/SearchBuilding.jsp";
    } else {
        url = "/SearchBuilding.jsp";
    }
    System.out.println(url);

    RequestDispatcher dispatcher = getServletContext()
            .getRequestDispatcher(url);
    dispatcher.forward(request, response);
}
*/
}

SEARCHBUILDING.JSP:

<html> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=ISO-
8859-1"> 
<title>Search Building</title> 
</head> 
<body> 
<form action="/BuildRoomClientProject/TestClientServlet" method="post"> 

<table cellspacing="0" cellpadding="0" border="0" align="left"> 
<tr> 
<td><h2>Search Building:</h2></td> 
</tr> 
<tr> 
<td> 

<input type= "text" name= "txtBname" size ="25" maxlength="25">
<input type="submit" name="submit" value="Search" /> 
</td> 
<td></td> 
</tr> 
</table> 

<input name="operation" value="showbuilding" type="hidden"> 

</form> 
</body> 
</html> 

SHOWBUILDING.JSP:

<%@ page contentType="text/html;charset=windows-1252"%> 
<%@ page import = "org.ics.ejb.Building" %> 
<%@ page import = "org.ics.ejb.Room" %> 
<%@ page import = "org.ics.ejb.RoomId" %> 
<html> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> 
<title> 
Show Building 
</title> 
</head> 
<body> 
<%  Room r = (Room)request.getAttribute("rooms"); %>
<% Building b = (Building)request.getAttribute("building"); %>
<% for (int i=0; i<100; i++){
System.out.println(r.getBname() + " " + r.getId().getrcode());
}%> 
<h2> 
Building: 
</h2> 
<p>
<%= r.getId().getrcode()%>
<%= b.getBname()%> 
</p>
<form action="/BuildRoomClientProject/TestClientServlet" method="post"> 
<input type="submit" name="submit" value="Tillbaka"> 
<input name="operation" value="searchbuilding" type="hidden"> 
</form> 
</body> 
</html>

Answer 1

The "rooms" attribute is being set as a ArrayList of rooms in the servlet. But when you are trying to get it in the jsp, you are expecting a single room. Change

<%  Room r = (Room)request.getAttribute("rooms"); %>

to

<%  List<Room> rooms = (List<Room>)request.getAttribute("rooms"); %>

and then iterate through each room.