Java Language Annotations Getting Annotation values at run-time
Example
You can fetch the current properties of the Annotation by using Reflection to fetch the Method or Field or Class which has an Annotation applied to it, and then fetching the desired properties.
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation {
String key() default "foo";
String value() default "bar";
}
class AnnotationExample {
// Put the Annotation on the method, but leave the defaults
@MyAnnotation
public void testDefaults() throws Exception {
// Using reflection, get the public method "testDefaults", which is this method with no args
Method method = AnnotationExample.class.getMethod("testDefaults", null);
// Fetch the Annotation that is of type MyAnnotation from the Method
MyAnnotation annotation = (MyAnnotation)method.getAnnotation(MyAnnotation.class);
// Print out the settings of the Annotation
print(annotation);
}
//Put the Annotation on the method, but override the settings
@MyAnnotation(key="baz", value="buzz")
public void testValues() throws Exception {
// Using reflection, get the public method "testValues", which is this method with no args
Method method = AnnotationExample.class.getMethod("testValues", null);
// Fetch the Annotation that is of type MyAnnotation from the Method
MyAnnotation annotation = (MyAnnotation)method.getAnnotation(MyAnnotation.class);
// Print out the settings of the Annotation
print(annotation);
}
public void print(MyAnnotation annotation) {
// Fetch the MyAnnotation 'key' & 'value' properties, and print them out
System.out.println(annotation.key() + " = " + annotation.value());
}
public static void main(String[] args) {
AnnotationExample example = new AnnotationExample();
try {
example.testDefaults();
example.testValues();
} catch( Exception e ) {
// Shouldn't throw any Exceptions
System.err.println("Exception [" + e.getClass().getName() + "] - " + e.getMessage());
e.printStackTrace(System.err);
}
}
}
The output will be
foo = bar
baz = buzz
