Servlet 3 spec and ThreadLocal

Question

As far as I know, Servlet 3 spec introduces asynchronous processing feature. Among other things, this will mean that the same thread can and will be reused for processing another, concurrent, HTTP request(s). This isn't revolutionary, at least for people who worked with NIO before.

Anyway, this leads to another important thing: no ThreadLocal variables as a temporary storage for the request data. Because if the same thread suddenly becomes the carrier thread to a different HTTP request, request-local data will be exposed to another request.

All of that is my pure speculation based on reading articles, I haven't got time to play with any Servlet 3 implementations (Tomcat 7, GlassFish 3.0.X, etc.).

So, the questions:

• Am I correct to assume that ThreadLocal will cease to be a convenient hack to keep the request data?
• Has anybody played with any of Servlet 3 implementations and tried using ThreadLocals to prove the above?
• Apart from storing data inside HTTP Session, are there any other similar easy-to-reach hacks you could possibly advise?

EDIT: don't get me wrong. I completely understand the dangers and ThreadLocal being a hack. In fact, I always advise against using it in similar context. However, believe it or not, thread context has been used far more frequently than you probably imagine. A good example would be Spring's OpenSessionInViewFilter which, according to its Javadoc:

This filter makes Hibernate Sessions available via the current thread, which will be autodetected by transaction managers.

This isn't strictly ThreadLocal (haven't checked the source) but already sounds alarming. I can think of more similar scenarios, and the abundance of web frameworks makes this much more likely.

Briefly speaking, many people have built their sand castles on top of this hack, with or without awareness. Therefore Stephen's answer is understandable but not quite what I'm after. I would like to get a confirmation whether anyone has actually tried and was able to reproduce failing behaviour so this question could be used as a reference point to others trapped by the same problem.

Answer 1

Async processing shouldn't bother you unless you explcitly ask for it.

For example, request can't be made async if servlet or any of filters in request's filter chain is not marked with <async-supported>true</async-supported>. Therefore, you can still use regular practices for regular requests.

Of couse, if you actually need async processing, you need to use appropriate practices. Basically, when request is processed asynchronously, its processing is broken into parts. These parts don't share thread-local state, however, you can still use thread-local state inside each of that parts, though you have to manage the state manually between the parts.

Answer 2

(Caveat: I've not read the Servlet 3 spec in detail, so I cannot say for sure that the spec says what you think it does. I'm just assuming that it does ...)

Am I correct to assume that ThreadLocal will cease to be a convenient hack to keep the request data?

Using ThreadLocal was always a poor approach, because you always ran the risk that information would leak when a worker thread finished one request and started on another one. Storing stuff as attributes in the ServletRequest object was always a better idea.

Now you've simply got another reason to do it the "right" way.

Has anybody played with any of Servlet 3 implementations and tried using ThreadLocals to prove the above?

That's not the right approach. It only tells you about the particular behaviour of a particular implementation under the particular circumstances of your test. You cannot generalize.

The correct approach is to assume that it will sometimes happen if the spec says it can ... and design your webapp to take account of it.

(Fear not! Apparently, in this case, this does not happen by default. Your webapp has to explicitly enable the async processing feature. If your code is infested with thread locals, you would be advised not to do this ...)

Apart from storing data inside HTTP Session, are there any other similar easy-to-reach hacks you could possibly advise.

Nope. The only right answer is storing request-specific data in the ServletRequest or ServletResponse object. Even storing it in the HTTP Session can be wrong, since there can be multiple requests active at the same time for a given session.

Answer 3

NOTE: Hacks follow. Use with caution, or really just don't use.

So long as you continue to understand which thread your code is executing in, there's no reason you can't use a ThreadLocal safely.

try {
    tl.set(value);
    doStuffUsingThreadLocal();
} finally {
    tl.remove();
}

It's not as if your call stack is switched out randomly. Heck, if there are ThreadLocal values you want to set deep in the call stack and then use further out, you can hack that too:

public class Nasty {

    static ThreadLocal<Set<ThreadLocal<?>>> cleanMe = 
        new ThreadLocal<Set<ThreadLocal<?>>>() {
            protected Set<ThreadLocal<?>> initialValue() {
                return new HashSet<ThreadLocal<?>>();
            }
        };

    static void register(ThreadLocal<?> toClean) {
       cleanMe.get().add(toClean);
    }

    static void cleanup()  {
        for(ThreadLocal<?> tl : toClean)
            tl.remove();
        toClean.clear();
    }
}

Then you register your ThreadLocals as you set them, and cleanup in a finally clause somewhere. This is all shameful wankery that you shouldn't probably do. I'm sorry I wrote it but it's too late :/

Answer 4

You are psychic ! (+1 for that)

My aim is ... to get a proof this has stopped working in Servlet 3.0 container

Here is the proof that you were asking for.

Incidentally, it is using the exact same OEMIV filter that you mentioned in your question and, guess what, it breaks Async servlet processing !

Edit: Here is another proof.

Answer 5

I'm still wondering why people use the rotten javax.servlet API to actually implement their servlets. What I do:

• I have a base class HttpRequestHandler which has private fields for request, response and a handle() method that can throw Exception plus some utility methods to get/set parameters, attributes, etc. I rarely need more than 5-10% of the servlet API, so this isn't as much work as it sounds.

• In the servlet handler, I create an instance of this class and then forget about the servlet API.

• I can extend this handler class and add all the fields and data that I need for the job. No huge parameter lists, no thread local hacking, no worries about concurrency.

• I have a utility class for unit tests that creates a HttpRequestHandler with mock implementations of request and response. This way, I don't need a servlet environment to test my code.

This solves all my problems because I can get the DB session and other things in the init() method or I can insert a factory between the servlet and the real handler to do more complex things.

Answer 6

One solution is to not use ThreadLocal but rather use a singleton that contains a static array of the objects you want to make global. This object would contain a "threadName" field that you set. You first set the current thread's name (in doGet, doPost) to some random unique value (like a UUID), then store it as part of the object that contains the data you want stored in the singleton. Then whenever some part of your code needs to access the data, it simply goes through the array and checks for the object with the threadName that is currently running and retrieve the object. You'll need to add some cleanup code to remove the object from the array when the http request completes.