Question regarding multi-threaded environment of servlet

Question

If there is a servlet, inside a servlet container such as Websphere. The servlet are executed by some threads. I would like to ask, what does the threads share? How variables are shared between them?

Do they have a local copy of the following variables?

1) private/protected/public final Semaphore permits = new Semaphore(50);

2) private/protected/public final static Semaphore permits = new Semaphore(50);

3) private/protected/public Semaphore permits = new Semaphore(50);

4) private/protected/public static Semaphore permits = new Semaphore(50);

How should I declare the semaphore so that I can use semaphore to control them? I don't want them to have each of them a copy of the semaphore. Thanks.

And, how can I check if they are accessing the same semaphore object? — lamwaiman1988, Jun 02 '11 at 07:11
That's won't work. I will only know different thread are using semaphore, but don't know whether they are using the same one. — lamwaiman1988, Jun 02 '11 at 08:17
Object reference will tell you that. See addendum to my answer below. — Adeel Ansari, Jun 02 '11 at 08:21

score 2 · Answer 1 · answered Jun 02 '11 at 06:50

Threads are run by Websphere thread pool and you should not be concerned about accessing them or sharing information via them.

Your semaphore will be shared if you declare it static in your servlet, for as long as all classes that use it live in the same application under the same classloader. However, there is a big risk in this case that if your execution path does not release permit (for example due to exception) you may end up with all threads blocked.

Adeel Ansari · Answer 2 · 2011-06-02T08:23:26.343

Each thread has its own stack, but all share the same memory space. With that in mind, a single instance can be shared, and so its state/properties, among multiple threads. Hence, we need to take care of state using synchronisation or similar techniques.

If you define a static variable or a single instance of servlet will be used -- which is highly likely but no guarantee, then it would be the same for all threads.

Nonetheless, you should create a class which provide a singleton semaphore to be used in servlet. That way servlet instances will be using the one and the same semaphore object, no matter what.

score 0 · Answer 3 · answered Jun 02 '11 at 06:48

If you want to share a semaphore, it must be created by one thread, then handed out to the others (via some form of (possibly static) getter method) when they need it.

If you create the Semaphore object in each object, they'll all have different ones, defeating the purpose.

score 0 · Answer 4 · answered Jun 02 '11 at 06:49

Servlets must be thread safe. This means your servlets should be "stateless" (unless you really know what you're doing). Essentially, use only local variables - not fields.

If you want to share state, by all means use fields of your servlet, or use classes with static fields and static getters (like the singleton pattern)

score 0 · Answer 5 · answered Jun 02 '11 at 06:56

Servlets should be threadsafe. Meaning if they have any state, they should be synchronized. Try avoiding saving some state inside servlets. They should contain only business/controller logic. Any state you want to save, put them in servletcontext/request/session which are synchronized by the container.

score 0 · Answer 6 · answered Jun 02 '11 at 07:27

Each servlet is executed in a new thread.. Technically, there is worker threads that waits for requests, when a http request comes to servlet container, this one instantiates a new servlet (depending to servlets defined in web.xml) and passes it to the worker thread. So logically each variable is not seen by other servlet instances. Now if you want to make your variable visible by all servlet instances you have to define it as static so it will be shared between all instances of the same class.

score 0 · Answer 7 · edited May 04 '13 at 17:11

You should not count on two instances of a servlet even being in the same JVM. Containers can be replicated and your Servlets need to be stateless. Besides the Java EE spec does not allow you to do any threading manipulation within. The containers may allow it, but the behaviour then becomes implementation specific. By putting semaphores in there you are interfering with the pooling and can cause issues. Use Sessions if you need to differentiate between different users.

Question regarding multi-threaded environment of servlet

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