>>>>> On Tue, 09 Dec 2014 08:04:50 +1100, Stuart Douglas <sdouglas_at_redhat.com> said:
MT> Section 6.2.3 includes the following text:
MT> <quote>
MT> A Filter and the target servlet or resource at the end of the filter
MT> chain must execute in the same invocation thread.
MT> </quote>
MT>
MT> As a result of a bug raised against Apache Tomcat [1] I am seeking
MT> clarification of the meaning of that sentence.
MT>
MT> As written, the meaning seems unambiguous. However, does it apply when
MT> the request is in asynchronous mode? To put it another way is the
MT> following legal?
MT> - filter calls ServletRequest.startAsync()
MT> - filter calls AsyncContext.start(Runnable)
MT> - That Runnable called FilterChain.doFilter()
SD> Another potential problem with doing this is that AsyncContext.start()
SD> does not wait for the current call stack to return to the container (at
SD> least it is not required to). This means that you will have two threads
SD> potentially modifying the response, which is not thread safe.
SD> We could modify the behaviour of AsyncContext.start() to wait for the
SD> request to return to the container before actually running the task.
SD> I think that this is actually a real thread safety problem in the
SD> Servlet API, any code that uses AsyncContext.start() and then wants to
SD> modify the response cannot be sure the original call stack has returned,
SD> and could potentially have thread safety issues.
Being new to this spec, I went looking for some catch-all text that
calls out these obvious thread safety issues. Indeed, there is section
2.3.3.4 Thread Safety:
Spec> Other than the startAsync and complete methods, implementations of
Spec> the request and response objects are not guaranteed to be thread
Spec> safe. This means that they should either only be used within the
Spec> scope of the request handling thread or the application must
Spec> ensure that access to the request and response objects are thread
Spec> safe.
Spec> If a thread created by the application uses the container-managed
Spec> objects, such as the request or response object, those objects
Spec> must be accessed only within the object's life cycle as
Spec> defined in sections Section 3.12, "Lifetime of the Request
Spec> Object" on page 3-31and Section 5.7, "Lifetime of the
Spec> Response Object" on page 5-50 respectively. Be aware that
Spec> other than the startAsync, and complete methods, the request and
Spec> response objects are not thread safe. If those objects were
Spec> accessed in the multiple threads, the access should be
Spec> synchronized or be done through a wrapper to add the thread
Spec> safety, for instance, synchronizing the call of the methods to
Spec> access the request attribute, or using a local output stream for
Spec> the response object within a thread.
Need we say more?
Ed
--
| edward.burns_at_oracle.com | office: +1 407 458 0017
| 56 days til DevNexus 2015
| 66 days til JavaLand 2015
| 76 days til CONFESS 2015