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In order for plug-ins to be added
to a running JavaTM Virtual
Machine*, they must be compiled into Java bytecode files (class
files). These class files will contain the code for subclasses of
various classes defined by the API. For example, a plug-in that pr
ovides the capability to read images will include a new subclass of
javax.imageio.ImageReader class defined
by the API. The usual reversed Internet domain name convention may
be used to guarantee uniqueness of class names.
Because loading and instantiating an entire plug-in may be expensive, an additional class is used as a "stand-in" to provide information about the plug-in. This class may be used, for example, to determine what formats are handled by the plug-in p rior to actually instantiating the plug-in.
The "stand-in" object is lightweight enough that it can be loaded and a single instance instantiated every time the API is used within a given invocation of the Java virtual machine. This pattern, in which a small class is used to provide informat ion about an available service, is referred to as a "service provider interface."