Question

I think I found more bugs in my web application. Normally, I do not worry about concurrency issues, but when you get a ConcurrentModificationException, you begin to rethink your design.

I am using JBoss Seam in conjuction with Hibernate and EHCache on Jetty. Right now, it is a single application server with multiple cores.

I briefly looked over my code and found a few places that haven’t thrown an exception yet, but I am fairly sure they can.

The first servlet filter I have basically checks if there are messages to notify the user of an event that occurred in the background (from a job, or another user). The filter simply adds messages to the page in a modal popup. The messages are stored on the session context, so it is possible another request could pull the same messages off the session context.

Right now, it works fine, but I am not hitting a page with many concurrent requests. I am thinking that I might need to write some JMeter tests to ensure this doesn’t happen.

The second servlet filter logs all incoming requests along with the session. This permits me to know where the client is coming from, what browser they’re running, etc. The problem I am seeing more recently is on image gallery pages (where there are many requests at about the same time), I end up getting a concurrent modification exception because I’m adding a request to the session.

The session contains a list of requests, this list appears to be being hit by multiple threads.

@Entity
public class HttpSession 
{
  protected List<HttpRequest> httpRequests;

  @Fetch(FetchMode.SUBSELECT)
  @OneToMany(mappedBy = "httpSession")
  public List<HttpRequest> getHttpRequests()
  {return(httpRequests);}

  ...
}

@Entity
public class HttpRequest
{
  protected HttpSession httpSession;

  @ManyToOne(optional = false)
  @JoinColumn(nullable = false)
  public HttpSession getHttpSession()
  {return(httpSession);}

  ...
}

In that second servlet filter, I am doing something of the sort:

httpSession.getHttpRequests().add(httpRequest);
session.saveOrUpdate(httpSession);

The part that errors out is when I do some comparison to see what changed from request to request:

for(HttpRequest httpRequest:httpSession.getHttpRequests())

That line there blows up with a concurrent modification exception.

Things to walk away with:
1. Will JMeter tests be useful here?
2. What books do you recommend for writing web applications that scale under concurrent load?
3. I tried placing synchronized around where I think I need it, ie on the method that loops through the requests, but it still fails. What else might I need to do?

I added some comments:

I had thought about making the logging of the http requests a background task. I can easily spawn a background task to save that information. I am trying to remember why I didn’t evaluate that too much. I think there is some information that I would like to have access to on the spot.

If I made it asynchronous, that would speed up the throughput quite a bit – well I’d have to use JMeter to measure those differences.

I would still have to deal with the concurrency issue there.

Thanks,

Walter

Answer 1

A ConcurrentModificationException occurrs when any collection is modified while iterating over it. You can do it in a single thread, e.g.:

for( Object o : someList ) {
  someList.add( new Object() );
}

Wrap your list with Collections.synchronizedList or return an unmodifiable copy of the list.