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One of the latest trends in programming, particularly in Java programming, is
the use of metadata. Metadata, simply put, is data about data. Metadata can be
used to create documentation, to track down dependencies in code, and even to
perform rudimentary compile-time checking. A rash of tools for metadata, such as
XDoclet (see Resources), add these features to the core Java language and have
been part of the Java programming landscape for a while.
Until the availability of J2SE 5.0 (aka Tiger, now in its second beta release),
the core Java language came closest to a metadata facility with the Javadoc
methodology. You use a special set of tags to mark up your code and then execute
the javadoc command to turn the tags into a formatted HTML page that documents
the classes the tags are attached to. Javadoc is an inadequate metadata tool,
though, because you have no solid, practical, standardized way to get at the
data for any purpose other than generating documentation. The fact that HTML
code is often mixed into the Javadoc output decreases its value for any other
purpose even further.
Tiger incorporates a far more versatile metadata facility into the core Java
language through a new feature called annotations. Annotations are modifiers you
can add to your code and apply to package declarations, type declarations,
constructors, methods, fields, parameters, and variables. Tiger includes
built-in annotations and also supports custom annotations you can write
yourself. This article will give you an overview of metadata's benefits and
introduce you to Tiger's built-in annotations. Part 2 in this article series
will explore custom annotations. My thanks to O'Reilly Media, Inc., which has
graciously allowed me to use the code sample from the annotations chapter of my
book on Tiger for this article (see Resources).
The value of metadata
In general, metadata's benefits fall into three categories: documentation,
compiler checking, and code analysis. Code-level documentation is the
most-often-cited use. Metadata provides a helpful way to indicate if methods are
dependent on other methods, if they are incomplete, if a certain class must
reference another class, and so on. This is indeed useful, but documentation is
probably the least relevant reason for adding metadata to the Java language.
Javadoc already provides a fairly easy-to-understand and robust way to document
code. Besides, who wants to write a documentation tool when one already exists
and works fine for the most part?
Annotations, a new feature in J2SE 5.0 (Tiger), brings a much-needed metadata facility to the core Java language. In this first of a two-part series, author Brett McLaughlin explains why metadata is so useful, introduces you to annotations in the Java language, and delves into Tiger's built-in annotations. Part 2 covers custom annotations.
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