Multithreading and Multiplexing - An Introduction to Network Programming with Java - page 74

size allocated will depend upon a number of factors related to the demands of the
particular application and the operating system for the particular platform, but, for
effi ciency's sake, should be on a kilobyte boundary. A 2 KB buffer allocation has
been chosen for the example and the pre-declaration of a ByteBuffer called buffer
has been assumed.
buffer = ByteBuffer.allocate(2048);
There is also a method called allocateDirect that may be used to set up a buffer.
This attempts to allocate the required memory as direct memory, so that data does
not need to be copied to an intermediate buffer before being written to disc. This
means that there is the potential for I/O operations to be performed considerably
more quickly. Whether the use of direct buffers is appropriate or desirable (and
there will be a cost associated with the use of them, in terms of system resources)
depends upon the needs of the particular application and the characteristics of the
underlying operating system. In practice, multiple buffers and multiple threads (in
thread pools) will be needed for heavily used servers.
Once all of the above preparatory steps have been executed, the server will enter
a traditional do…while(true) loop that accepts connecting clients and pro-
cesses their data. The fi rst step within this loop is a call to method select on the
Selector object. This returns the number of events of the type(s) that are being moni-
tored and have occurred. This method is very effi cient and appears to be based on
the Unix system call of the same name. Here's an example of its use, employing the
same Selector object called selector as was used previously in this section:
int numKeys = selector.select();
If no events have occurred since the last call of select , then execution loops back
to this call until there is at least one event detected.
For each event that is detected on a particular call to select , an object of class
SelectionKey (package java.nio.channels ) is generated and contains all the informa-
tion pertaining to the particular event. The set of SelectionKey s created by a given
call to select is called the selected set . The selected set is generated by a call to
method selectedKeys of the Selector object and is placed into a Java Set object. An
Iterator object associated with the selected set is then created by a call to the Set
object's iterator method. The lines to generate the selected set and its iterator are
shown below.
Set eventKeys = selector.selectedKeys();
Iterator keyCycler = eventKeys.iterator();
Using the above Iterator object, we can now work our way through the individ-
ual SelectionKey objects, making use of the Iterator methods hasNext and next . As
we retrieve each SelectionKey from the set, we need to typecast from type Object
(which is how each key is held within the Set object) into type SelectionKey . Here is
the code required for detection and retrieval of each key:
while (keyCycler.hasNext())