Sharing Objects - Java Concurrency in Practice - page 44

3.2.1. Safe Construction Practices
ThisEscape illustrates an important special case of escape—when the this references
escapes during construction. When the inner EventListener instance is published, so is
the enclosing ThisEscape instance. But an object is in a predictable, consistent state only
after its constructor returns, so publishing an object from within its constructor can publish
an incompletely constructed object. This is true evenifthepublicationisthelaststatementin
the constructor. If the this reference escapes during construction, the object is considered
not properly constructed . [8]
Do not allow the this reference to escape during construction.
A common mistake that can let the this reference escape during construction is to start a
thread from a constructor. When an object creates a thread from its constructor, it almost al-
ways shares its this reference with the new thread, either explicitly (by passing it to the
constructor) or implicitly (because the Thread or Runnable is an inner class of the own-
ing object). The new thread might then be able to see the owning object before it is fully
constructed. There's nothing wrong with creating a thread in a constructor, but it is best not to
start the thread immediately. Instead, expose a start or initialize method that starts
the owned thread. (See Chapter 7 for more on service lifecycle issues.) Calling an override-
able instance method (one that is neither private nor final ) from the constructor can
also allow the this reference to escape.