Doesn't permit multiple instances of the same listener by default, as it keeps listeners in a linked Set. The collection class used to hold ApplicationListener objects can be overridden through the "collectionClass" bean property.

Note that this class doesn't try to do anything clever to ensure thread safety if listeners are added or removed at runtime. A technique such as Copy-on-Write (Lea:137) could be used to ensure this, but the assumption in the basic version of the class is that listeners will be added at application configuration time and not added or removed as the application runs.

A custom collection class must be specified to allow for thread-safe runtime registration of listeners. A good candidate for this is Doug Lea's java.util.concurrent.CopyOnWriteArraySet or its non-JDK predecessor, EDU.oswego.cs.dl.util.concurrent.CopyOnWriteArraySet (or the respective CopyOnWriteArrayList version, allowing for registering the same listener multiple times). Those classes provide a thread-safe Iterator, optimized for read-mostly usage - matching this use case nicely.

Implementing ApplicationEventMulticaster's actual multicastEvent method is left to subclasses. SimpleApplicationEventMulticaster simply multicasts all events to all registered listeners, invoking them in the calling thread. Alternative implementations could me more sophisticated in those respects.