Error : java.util.ConcurrentModificationException

Question

i have a grid from where users select the row , when a row is clicked then its id is sending to my action class AddBookToSession.java and after wards it is returning a list into my jsp page invoice.jsp

I am getting error java.util.ConcurrentModificationException ,when users selects a row from my grid.

I read this similar question ,But still i am not able to solve my problem.

My problem is : Why i am getting java.util.ConcurrentModificationException Error and how can I solve this issue. Please help me to solve this problem.

Error in console:

   Dec 10, 2012 11:37:30 PM org.apache.catalina.core.ApplicationDispatcher invoke
   SEVERE: Servlet.service() for servlet jsp threw exception
   java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:819)
    at java.util.ArrayList$Itr.next(ArrayList.java:791)
    at org.apache.struts2.components.IteratorComponent.end(IteratorComponent.java:334)
    at org.apache.struts2.views.jsp.IteratorTag.doAfterBody(IteratorTag.java:87)
       -------------------
        ------------------

invoice.jsp

  <s:if  test="#session['BOK'].size() > 0"> 
         <table width="100%" class="userTable" valign="top" border="0"> 
          <s:iterator value="#session.BOK" status="userStatus">
       <tr class="<s:if test="%{#userStatus.odd == true}">odd</s:if> <s:else>even</s:else>">
             <td width="80%"><s:property value="bookTitile" /></td>
                 <td align="right" width="20%">
                 <s:url id="cancelURL" action="RemovebooksFromSession" namespace="/admin/setups/secure/jspHomepage/bookstransaction">
                         <s:param name="bkid" value="%{id}"></s:param>
                  </s:url>
                <sj:a href="%{cancelURL}" targets="myAdvanceDivBoxx">Rem</sj:a></td>
        </tr>
         </s:iterator>
         </table> 
         </div>
     </s:if>

AddBookToSession.java

  public String execute() 
    {  
        String bookid = request.getParameter("bid"); 
        String qnty=dao.getquantityById(Integer.parseInt(bookid));

        if(qnty.equals("0")||qnty.equals("")||qnty.equals("null")){
            return SUCCESS;
        } 
        Bookdetails book = dao.listBookDetailsById(Integer.parseInt(bookid));
        books = (ArrayList) session.get(BillTransactionBooksConstants.BOK);
        if ( books == null ) books = new ArrayList<Bookdetails>();
        boolean already_exists = false;
        for ( Bookdetails b : books ) 
        {
              if ( Integer.toString(b.getId()).equals(bookid))
            {
                already_exists = true; 
                break;
            }
        }
        if (book != null && !already_exists  ) 
        { 
            books.add(book);
            System.out.println("books size"+books.size()); 
            session.put(BillTransactionBooksConstants.BOK,books);
        }
        return SUCCESS;
    } 

Answer 1

The exception stack trace points to the s:iterator in your jsp being the place where the exception is thrown. That means that while that element goes through the book list another piece of code adds or removes from the list. That could be your Java code, or some other (e.g. RemovebooksFromSession).

Take a look at your code and try to determine if it is possible that other code runs while your jsp page is being build.

Cannot help you more with the code you posted.

Answer 2

Fundamentally, you're modifying an object while some other object is depending on the state of the first object to remain unchanged.

This is most often seen when using a non-modification-safe iterator on some sort of collection object, and then modifying the collection.

My JSP is too rusty to wade through your source, but I see a couple of cases where you appear to be using iterators, and the one in your JSP looks most suspect.

(I believe this error can also occur when you use multi-threading to manage collection objects in a non-thread-safe fashion.)

Answer 3

It is possible that the page is refreshed around the time your code executes "books.add(book);" in which case you will get the exception as you are modifying a collection at same time someone else is using it.

Is it possible to modify your code so that the "books" you modify in the AddBookToSession is copy of the internal list of books.

books = new ArrayList<Bookdetails>((ArrayList) session.get(BillTransactionBooksConstants.BOK););