Java Reference
In-Depth Information
Solution
Use the Java Annotations or “Metadata” facility.
Discussion
The continuing success of the open source tool
XDoclet
—originally used to generate the te-
dious auxiliary classes and deployment descriptor files for the widely criticized EJB2 frame-
work—led to a demand for a similar mechanism in standard Java. Java
Annotations
were the
result. The
annotation
mechanism uses an interface-like syntax, in which both declaration
and use of Annotations use the name preceded by an at character (
@
). This was chosen, ac-
cording to the designers, to be reminiscent of “Javadoc tags, a preexisting ad hoc annotation
facility in the Java programming language.” Javadoc is ad hoc only in the sense that its
@
tags
were never fully integrated into the language; most were ignored by the compiler, but
@de-
precated
was always understood by the compiler (see
Dealing with Deprecation Warnings
)
.
Annotations can be read at runtime by use of the Reflection API; this is discussed in
Using
and Defining Annotations
,
where I also show you how to define your own annotations. An-
notations can also be read post-compile time by tools such as the RMI and EJB stub generat-
ors (and others to be invented, perhaps by you, gentle reader!).
Annotations are also read by
javac
at compile time to provide extra information to the com-
piler.
For example, a common coding error is overloading a method when you mean to override it,
by mistakenly using the wrong argument type. Consider overriding the equals method in
Ob-
ject
. If you mistakenly write:
public
public
boolean
boolean
equals
(
MyClass obj
) {
...
}
then you have created a new overload that will likely never be called, and the default version
in
Object
will be called. To prevent this, an annotation included in
java.lang
is the
Over-
ride
annotation. This has no parameters but simply is placed before the method call. For ex-
ample:
/**
* AnnotationOverrideDemo - Simple demonstation of Metadata being used to
