Threads - The Java Programming Language - page 568

// ...
}
The static field users holds a ThreadLocal variable whose initial value in
each thread will be User.UNKNOWN_USER . This initial value is defined by
overriding the method initialValue , whose default behavior is to return
null . The programmer can set the user for a given thread by invoking
setUser . Once set, the method currentUser will return that value when in-
voked in that thread. As shown in the method setValue , the current user
might be used to determine privileges.
You can clear the value in a ThreadLocal by invoking its remove method. If
get is invoked again, then initialValue will again be used to determine
what to return.
When a thread dies, the values set in THReadLocal variables for that
thread are not reachable, and so can be collected as garbage if not oth-
erwise referenced.
When you spawn a new thread, the values of THReadLocal variables for
that thread will be the value returned by its initialValue method. If
you want the value in a new thread to be inherited from the spawning
thread, you can use the class InheritableThreadLocal , a subclass of
THReadLocal . It has a protected method childValue that is invoked to get
the child's (spawned thread's) initial value. The method is passed the
parent's value for the variable and returns the value for the child. The
default implementation of childValue returns the parent's value, but you
can subclass to do something different, such as cloning the parent's
value for the child.
Use of ThreadLocal objects is an inherently risky tool. You should use
them only where you thoroughly understand the thread model that will
be used. The problems arise with thread pooling, a technique that is
sometimes used to reuse threads instead of creating new ones for each
task. In a thread pooling system a thread might be used several times.
Any ThreadLocal object used in such an environment would contain the