CHAPTER 4

Writing Image I/O Plug-ins

4.1 The Plug-in Concept

The Image I/O API is designed as a pluggable framework into which any developer may add their own "plug-ins." A plug-in is defined as a set of Java language classes which may be loaded into the API at run-time and which add functionality to the AP I. In the context of the Java Image I/O API, a plug-in may provide the ability to read image data from a new file format, to write image data in a new format, to "transcode" non-image metadata between two formats, or to read or write streaming dat a from or to a new data source or sink. A plug-in may also provide support for the same format as another plug-in, perhaps providing better performance, more features, or a different view of the data stored by the format.

In order for plug-ins to be added to a running Java^TM 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 the abstract 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."

* As used on this web site, the terms "Java Virtual Machine" or "JVM" mean a virtual machine for the Java platform.

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