Threads - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 267

Figure 8.4 Display of TaskSplitApplet program. Pressing the “Go „ button can
result in a different sequence in the completion times of the thread each time.
8.5.3 Exclusive thread operations
Threading becomes trickier when threads perform operations that can conflict
with each other. For example, Figure 8.5 depicts a situation where two thread
processes both want to access an object but for different purposes (this is derived
from an example in the Sun Java Tutorial). The Filler thread wants to put
a number into the bin variable in the Box .Itcan only do so when the cav-
ity is empty. The Getter ,onthe other hand, wants to retrieve the number
from Cavity and leave the Cavity empty. Ideally, Filler and Getter
would alternate their calls to the methods put() and get() .However,ifno
special steps are taken, it is quite easy for Getter to invoke get() when
the Cavity is empty and for Filler to invoke put() when the Cavity is
still full.
This type of situation is called a data race because each thread is racing to
do its task without waiting for the other thread to finish its activity. A synchro-
nization scheme prevents this problem. Synchronization forces threads to wait
in single file at the method or code block of an object where the conflict can
occur.
In this case, this means that the Box object only allows one thread at a time
to invoke either its put() or get() .Itisasifonly one thread object owns the
lock on the door to a Box object. That thread must give up the lock before any
other thread can access any synchronized method on the object . (Note that the
lock terminology is by convention. Giving up the key might be more illuminating.
The term monitor is also used.)