常連

### 前提・実現したいこと
現在、条件分岐を使って、<th>に表示されている連続した日付と、formからDBに送られたn月m日の出発点･到着点、時刻などを
一致する日付の行に車両のその日の情報を表示という機能を実装したいのですが、
自分でやってみたところ、
<c:if>を使って一致するものを表示という形を取ろうと思ったのですが、エラーになってしまったのでヒントを頂きたいです。
### 発生している問題・エラーメッセージ

```
HTTPステータス 500 - An exception occurred processing JSP page /WEB-INF/views/dispatch/index.jsp at line 38
type 例外レポート

メッセージ An exception occurred processing JSP page /WEB-INF/views/dispatch/index.jsp at line 38

説明 The server encountered an internal error that prevented it from fulfilling this request.

例外

org.apache.jasper.JasperException: An exception occurred processing JSP page /WEB-INF/views/dispatch/index.jsp at line 38

35:                   session.setAttribute("1月","日");
36:                   %>
37: 
38:                    <c:if test="${truck.task_date == '1月'+stts.count+'日'}" var="flg"/>
39:                   <c:forEach var="truck" items="${trucks}" varStatus="status">
40:                   <tbody>
41:                   <c:if test="${flg}">


Stacktrace:
	org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:579)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:476)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	controllers.dispatch.DispatchIndexServlet.doGet(DispatchIndexServlet.java:49)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	filters.LoginFilter.doFilter(LoginFilter.java:72)
	filters.EncodingFilter.doFilter(EncodingFilter.java:38)
原因

java.lang.NumberFormatException: For input string: "1月"
	java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	java.lang.Long.parseLong(Long.java:589)
	java.lang.Long.valueOf(Long.java:803)
	org.apache.el.lang.ELArithmetic$LongDelegate.coerce(ELArithmetic.java:212)
	org.apache.el.lang.ELArithmetic.coerce(ELArithmetic.java:384)
	org.apache.el.lang.ELArithmetic.add(ELArithmetic.java:257)
	org.apache.el.parser.AstPlus.getValue(AstPlus.java:40)
	org.apache.el.parser.AstPlus.getValue(AstPlus.java:38)
	org.apache.el.parser.AstEqual.getValue(AstEqual.java:38)
	org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:184)
	org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(PageContextImpl.java:943)
	org.apache.jsp.WEB_002dINF.views.dispatch.index_jsp._jspx_meth_c_005fif_005f1(index_jsp.java:537)
	org.apache.jsp.WEB_002dINF.views.dispatch.index_jsp._jspService(index_jsp.java:221)
	org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:438)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	controllers.dispatch.DispatchIndexServlet.doGet(DispatchIndexServlet.java:49)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	filters.LoginFilter.doFilter(LoginFilter.java:72)
	filters.EncodingFilter.doFilter(EncodingFilter.java:38)
```

### 該当のソースコード

```JSP
 <table class="Truck_table">
         <thead>
            <tr>
               <th class="space1" rowspan="2" colspan="4"></th>
<c:forEach begin="1" end="31" varStatus="stts">
               <th class="date" colspan="4">1月<c:out value="${stts.count}"></c:out>日</th></c:forEach>
               </tr>
               <tr>
               <c:forEach begin="1" end="31">
                  <th class="task_type">宵積卸地</th>
                  <th class="task_type">積地</th>
                  <th class="task_type">卸地</th>
                  <th class="task_type">宵積</th>
                  </c:forEach>
                  </tr>
                  </thead>
                  <%
                  session.setAttribute("1月","日");
                  %>

                   <c:if test="${truck.task_date == '1月'+stts.count+'日'}" var="flg"/>
                  <c:forEach var="truck" items="${trucks}" varStatus="status">
                  <tbody>
                  <c:if test="${flg}">
                  <tr>
                  <td class="office_name" rowspan="4"><c:out value="${truck.office_name}"></c:out></td>
                  <td class ="truck_code" colspan="2" rowspan="4"><c:out value="${truck.truck_code}"></c:out></td>
                  <td class ="truck_type" rowspan="4"><c:out value="${truck.truck_type}"></c:out></td>

                  <td class ="time"><c:out value="${truck.mw_time}"></c:out></td>
                  <td class ="time"><c:out value="${truck.sa_time}"></c:out></td>
                  <td class ="time"><c:out value="${truck.wa_time}"></c:out></td>
                  <td class ="time"><c:out value="${truck.es_time}"></c:out></td>
                  </tr>

                  <tr>
                      <td class ="area" id="1"><c:out value="${truck.m_wholesale_area}"></c:out></td>
                      <td class ="area" id="1"><c:out value="${truck.stack_area}"></c:out></td>
                      <td class ="area" id="1"><c:out value="${truck.wholesale_area}"></c:out></td>
                      <td class ="area" id="1"><c:out value="${truck.e_stack_area}"></c:out></td>
                   </tr>

                  <tr>
                      <td class ="area" id="2"><c:out value="${truck.m_wholesale_area}"></c:out></td>
                      <td class ="area" id="2"><c:out value="${truck.stack_area}"></c:out></td>
                      <td class ="area" id="2"><c:out value="${truck.wholesale_area}"></c:out></td>
                      <td class ="area" id="2"><c:out value="${truck.e_stack_area}"></c:out></td>
                   </tr>

                   <tr>
                   <td class ="time"></td>
                   <td class ="time"><c:out value="${truck.sa_time2}"></c:out></td>
                   <td class ="time"><c:out value="${truck.wa_time2}"></c:out></td>
                   <td class ="time"></td>
                   </tr>
                   <tr>
                   <td class ="area" id="3">ボタン</td>
                   <td class ="area" id="3"><c:out value="${truck.stack_area2}"></c:out></td>
                   <td class ="area" id="3"><c:out value="${truck.wholesale_area2}"></c:out></td>
                   <td class ="area" id="3">ボタン</td>
                   </tr>
                   </c:if>
                  </tbody>
                 </c:forEach>
         </table>
```

```Java
import java.io.IOException;
import java.util.List;

import javax.persistence.EntityManager;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import models.Trucks;
import utils.DBUtil;


@WebServlet("/dispatch/index")
public class DispatchIndexServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;


    public DispatchIndexServlet() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    	EntityManager em = DBUtil.createEntityManager();

        List<Trucks> trucks = em.createNamedQuery("getAllTrucks",Trucks.class)
        		                       .getResultList();

        long trucks_count = (long)em.createNamedQuery("getTrucksCount", Long.class)
                                       .getSingleResult();

        em.close();




        request.setAttribute("trucks", trucks);
        request.setAttribute("trucks_count", trucks_count);
        if(request.getSession().getAttribute("flush") != null) {
            request.setAttribute("flush", request.getSession().getAttribute("flush"));
            request.getSession().removeAttribute("flush");
        }
        RequestDispatcher rd = request.getRequestDispatcher("/WEB-INF/views/dispatch/index.jsp");
        rd.forward(request, response);
	}
}

```
### 補足情報（FW/ツールのバージョンなど）
Eclipse4.6Neon使用、MySQL

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