Annotations - Beginning Java 8 Language Features - page 37

You can get the fully qualified name of an annotation using the getQualifiedName() method of the TypeElement
interface.
Name qualifiedName = currentAnnotation.getQualifiedName();
// Check if it is a Version annotation
if (qualifiedName.contentEquals("com.jdojo.annotation.Version")) {
// Get Version annotation values to validate
}
Once you are sure that you have a Version annotation, you need to get all its instances from the source code. To
get information from the source code, you need to use the RoundEnvironment object. The following snippet of code
will get all elements of the source code (e.g. classes, methods, constructors, etc.) that are annotated with a Version
annotation:
Set<? extends Element> annotatedElements = roundEnv.getElementsAnnotatedWith(currentAnnotation);
At this point, you need to iterate through all elements that are annotated with a Version annotation; get the
instance of the Version annotation present on them; and validate the values of the major and minor elements. You
can perform this logic as follows:
for (Element element : annotatedElements) {
Version v = element.getAnnotation(Version.class);
int major = v.major();
int minor = v.minor();
if (major < 0 || minor < 0) {
// Print the error message here
}
}
You can print the error message using the printMessage() method of the Messager object. The processingEnv
is an instance variable defined in the AbstractProcessor class that you can use inside your processor to get the
Messager object reference, as shown below. If you pass the source code element's reference to the printMessage()
method, your message will be formatted to include the source code file name and the line number in the source code
for that element. The first argument to the printMessage() method indicates the type of the message. You can use
Kind.NOTE and Kind.WARNING as the first argument to print a note and warning, respectively.
String errorMsg = "Version cannot be negative. " +
"major=" + major + " minor=" + minor;
Messager messager = this.processingEnv.getMessager();
messager.printMessage(Kind.ERROR, errorMsg, element);
Finally, you need to return true or false from the process() method. If a processor returns true, it means it
claimed all the annotations that were passed to it. Otherwise, those annotations are considered unclaimed and they
will be passed to other processors. Listing 1-23 has the complete code for the VersionProcessor class.