Alignment: Chaotic Java

Wednesday, December 21, 2005

Thread Static List in Java

We all know what a Thread is and what Static means, and how they collide. For a quick example: I have an event listener method where I want it to have a stopping condition for a suspected recursion. What I want to do is use a Collection of the items I've already received, and check for each call of the listener method if the calling item is already inside the Collection.

I can't add a Collection to the method signature because I can't change it (it came from a listener interface), so using a static Collection in my implemented handler comes to mind. However, what if the event was called from different threads at the same time?

This is, like most of my posts, not a new thing. It's just a nice tool I decided to share its simple code with; Enjoy it or diss it. Here goes:

public class ThreadStaticCollection<T> implements Collection<T>{

  private static Map<Thread, Collection<T>> threadMap = 

    new TreeMap<Thread, Collection<T>>();

  private synchronized Collection<T> get() {

    Collection<T> res = threadMap.get(Thread.currentThread());


    if (res == null) { 

      res = new LinkedList<T>();

      threadsMap.put(Thread.currentThread(), res);



    return res;



  private synchronized void purge() {

    if (get().isEmpty()) {




  public boolean add(Object item) {

    return get().add(item);


  // Goes on..

  public boolean remove(Object item) {

    boolean b = get().remove(item);


    return b;


  // Goes on..



The get() method will fetch or create the Collection currently available for the current Thread. The Collection interface is implemented by delegating all tasks to what is returned from get().

If items are removed from the Collection, a call to purge() should occur so that the Collection instance could be removed from the static Map. Also notice that both the purge and the get are synchronized for obvious reasons.

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At 12/21/2005 07:28:00 AM, Blogger Avah said...

Excuse the horrible formatting, it's the first time I use Html2Java..

At 12/21/2005 11:32:00 AM, Anonymous Anonymous said...

java.lang.ThreadLocal ??

At 12/21/2005 11:54:00 AM, Blogger Avah said...

Heehee :)

Indeed I never assumed to know everything. :)

I guess you learn something new every day, and I learned about ThreadLocal today. The idea of the ThreadStaticCollection was to provide a Collection interface without worrying about it being "ThreadLocal" underneath - Indeed, it can be reimplemented using ThreadLocal easily and I will do so.

Thank you annonymous!

At 12/22/2005 12:12:00 AM, Anonymous J. Yu said...

being a JDK class, ThreadLocal has the advantange that it can hack into Thread. it will perform better because it doesn't need synchronization.

At 12/22/2005 07:49:00 AM, Blogger Avah said...

Must say I agree.

As said, you learn something new every day. I will post a fix to this class soon, using ThreadLocal at its core. :)


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