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SizeMonitored Demo - Instructions
Overview
The
SizeMonitored interface is widely used throughout JThreadKit. It
is used to keep tabs on the current "size" of something--for example the
number of elements in a List. The List interface
from the Collections API already has a size() method that
returns the number of elements stored in the list. But if the list is
also SizeMonitored we can have one thread call
waitUntilEmpty() (a method on the SizeMonitored
interface), while another thread removes elements from the list. There's an
interface defined in the com.jthreadkit.collection package that
is a combination of the List interface and the
SizeMonitored interface and it's called:
SizeMonitoredList
(and it's one of the options in this demo). Throughout JThreadKit, there are many other classes that implement and interfaces
that extend the SizeMonitored interface.
Running the Demo
Once the demo has been launched, start it by first choosing a SizeMonitored Type, and then click "Start". Details
about the demo are displayed in the Results text area. The demo highlights the following features of a
SizeMonitored object:
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Size Change Notification
After constructing the SizeMonitored Object, a SizeChangeListener is added to it. Each time
the size of the SizeMonitored changes, the SizeChangeListener's
sizeChanged(SizeMonitored sm, int newSize) method is called. This method is called by the thread that just
changed the value--while calling this method, this thread continues to hold the exclusive lock on the
Object returned by the getLockObject() method on SizeMonitored.
In light of this, implementations of this method should do their work
quickly (and without making method calls that potentially block!). In addition, if the size is altered again
as a result of something done during this method call, re-notification will not occur.
The left
portion of the demo display contains the Listener text area. Each time sizeChanged() is
called, the new size is appended to this text area.
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Inter-Thread Communication
In the demo, two threads are interacting with the same SizeMonitored Object. The first thread creates the
SizeMonitored Object. The second thread calls the waitForSizeToClimbTo(int atLeast) method
which causes it to block, waiting until the size is equal to the target value of 5. Once the first thread
adds five objects (Strings are used in the demo), the second thread begins removing them.
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