No concurrent calls on stateful bean!

[toni]

Hi,

if I send many simultaneous HTTP requests to my seam application, which carry out a particular action, then I get an exception.

To reproduce this I have create a short test application, which you find below.

The test application exists of a JSF Table, which displays a set of entities. Each entity bean can be deleted by clicking on a "delete" link.

If I delete the entity beans sequentially, then everything works correctly. However, if I very quickly click on many links, then sometimes I get the following exception:

Caused by: javax.ejb.ConcurrentAccessException: no concurrent calls on stateful bean 'jboss.j2ee:service=EJB3,name=TemplateAction' (EJB3 4.3.13) at org.jboss.ejb3.stateful.StatefulInstanceInterceptor.invoke(StatefulInstanceInterceptor.java:73)

I read of course, that this happens for reentrant calls on beans (A->B->A) or when one starts a thread inside a EJB, but that's not what I'm doing.

I already upgraded to SEAM 1.1, because I thought might/could be a bug. But I guess it's my lack of understanding.

To reproduce the error, I have create the following example, so that people here can reproduce the error. Just copy and paste the source code it into an existing project and call "/test.seam".

It takes only 3-4 minutes. I appreciate any comments and help.

Toni


Test.java
-------------
@Entity
@Name("test")

public class Test
{
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
long id;

String testText;

public long getId()
{
return id;
}

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

public String getTestText()
{
return testText;
}

public void setTestText(String testText)
{
this.testText = testText;
}
}


BasicTest.java
---------------------
@Local
public interface BasicTest
{
public String list();

public String create();

public String select();

public String save();

public String delete();

public void destroy();

public DataModel getDataModel();
}


TestAction.java
------------------------
@Name("testAction")
@Stateful

public class TestAction implements BasicTest
{
@In(create = true)
EntityManager entityManager;

@In(create = true)
@Out
@Valid
Test test;

@Out(required = false)
DataModel dataModel;

@Begin(join = true)
public DataModel getDataModel()
{
if (dataModel == null)
dataModel = new ListDataModel();
dataModel.setWrappedData(entityManager.createQuery("from Test").getResultList());
return dataModel;
}

@Begin(join = true)
public String list()
{
return "tests";
}

public String create()
{
test = new Test();
return "test";
}

public String select()
{
test = (Test) dataModel.getRowData();
return "test";
}

@End
public String save()
{
entityManager.merge(test);
return "tests";
}

@End
public String delete()
{
entityManager.remove(dataModel.getRowData());
return "tests";
}

@Destroy
@Remove
public void destroy()
{
}
}


----------- test.jsp --------------

<%@ taglib uri="http://java.sun.com/jsf/html" prefix="h" %>
<%@ taglib uri="http://java.sun.com/jsf/core" prefix="f" %>
<%@ taglib uri="http://jboss.com/products/seam/taglib" prefix="s" %>

<f:view>

<h:form>
<s:validateAll>

<h:panelGrid columns="1">

<h:outputText value="Type"/>
<h:inputText size="20" value="#{test.testText}"/>

</h:panelGrid>

<h:messages/>
<h:commandButton type="submit" value="Save" action="#{testAction.save}"/>

</s:validateAll>
</h:form>

</f:view>




----------- tests.jsp ----------------------

<%@ taglib uri="http://java.sun.com/jsf/html" prefix="h" %>
<%@ taglib uri="http://java.sun.com/jsf/core" prefix="f" %>
<%@ taglib uri="http://jboss.com/products/seam/taglib" prefix="s" %>

<f:view>


<h:form>
<h:outputText value="No Test Entities available" rendered="#{testAction.dataModel.rowCount == 0}"/>

<h:dataTable value="#{testAction.dataModel}" var="t" rendered="#{testAction.dataModel.rowCount > 0}">

<h:column>
<f:facet name="header">
<h:outputText value="Type"/>
</f:facet>
<h:outputText value="#{t.testText}"/>
</h:column>

<h:column>
<f:facet name="header">
<h:outputText value="Action"/>
</f:facet>
<h:commandLink value="delete" action="#{testAction.delete}"/>
</h:column>

</h:dataTable>

</h:form>

<h:outputLink value="test.seam">
<f:param name="conversationId" value="#{conversation.id}"/>
<h:outputText value="Create Another Test Entity"/>
</h:outputLink>

</f:view>





-------------- add to faces config ---------------------

<navigation-rule>
<navigation-case>
<from-outcome>tests</from-outcome>
<to-view-id>/tests.jsp</to-view-id>
</navigation-case>
<navigation-case>
<from-outcome>test</from-outcome>
<to-view-id>/test.jsp</to-view-id>
</navigation-case>
</navigation-rule>

[norman.richards]

I think Seam normally serializes requests within a conversation, but if you are accessing a stateful component in the session then you can run into problems like this. You'll need to add @Synchronized to your session bean to get Seam to enforce serialized access to a component across different conversations.

[toni]

Thanks for the reply!

I looked into the documentation and found the following: "Specifies that a stateful component has multiple concurrent clients, and so access to the component must be synchronized. This annotation is not required session scoped components, which are synchronized by default."

However, I have the feeling that also requests to conversational scoped statefull components get serialized, because I can sent several requests simultaneously, but not too many, otherwise I run into an an exception after some time passes.

Is there a default timeout, which we can set in the components.xml for serialized requests to a component? And shouldn't the default be to serialize all requests to a component?

How should be handle simultaneous requests to statefull components? I mean users often click many times onto a link or a button, because they get impatient. How does seam deal with those kind of user requests?

[pdpantages]

Hello, I ran into something like this a while ago (when using Seam 1.0.1) and ended up using

import org.jboss.annotation.ejb.SerializedConcurrentAccess;
....
@SerializedConcurrentAccess

This fixed my exceptions and I have had no probs so far after adding it.

Is this equivalent to @Syncrhonized? , or should I switch to the latter? I have had no trouble (that I know of) in Seam 1.0.1; I just upgraded to 1.1.0. & all still seems OK.....

Thanks, PdP

[norman.richards]

I have never used @SerializedConcurrentAccess, but I'm sure it is not the exactly the same. (practically speaking, it may be roughly equivalent though - but I haven't looked into it)


You are actually right, seam does serialize access to stateful session beans. I didn't realize we did that. I went ahead and tried the test and the only way I could get anything to happen was by adding a Thread.sleep() to the delete action. And then, I got the expected result:

The conversation ended, timed out or was processing another request

I wonder what is happening in your case to make it not work...