Java Reference
In-Depth Information
@Target({ElementType.TYPE, ElementType.CONSTRUCTOR, ElementType.METHOD,
ElementType.PACKAGE, ElementType.LOCAL_VARIABLE, ElementType.TYPE_USE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Version {
int major();
int minor();
}
Suppose you annotate a
Test
class with your
Version
annotation type as follows:
package com.jdojo.annotation;
@Version(major=1, minor=0)
public class Test {
// Code for Test class goes here
}
When you generate documentation for the
Test
class using the Javadoc tool, the
Version
annotation on the
Test
class declaration is also generated as part of the documentation. If you remove the
Documented
annotation from
the
Version
annotation type declaration, the
Test
class documentation would not contain information about its
Version
annotation.
The Repeatable Annotation
Prior to Java 8, it was not allowed to repeat an annotation in the same context. For example, the following repeated use
of the
Version
annotation would generate a compile-time error:
@Version(major=1, minor=1)
@Version(major=1, minor=2)
public class Test {
// Code goes here
}
Java 8 added a
Repeatable
meta-annotation type. An annotation type declaration must be annotated with a
@Repeatable
annotation if its repeated use is to be allowed. The
Repeatable
annotation type has only one element
named
value
whose type is a class type of another annotation type.
Creating a repeatable annotation type is a two-step process:
•
Declare an annotation type (say
T
) and annotate it with the
Repeatable
meta-annotation.
Specify the value for the annotation as another annotation that is known as containing
annotation for the repeatable annotation type being declared.
•
Declare the containing annotation type with one element that is an array of the repeatable
annotation.
Listing 1-14 and Listing 1-15 contain declarations for
ChangeLog
and
ChangeLogs
annotation types.
ChangeLog
is
annotated with the
@Repeatable(ChangeLogs.class)
annotation, which means that it is a repeatable annotation type
and its containing annotation type is
ChangeLogs
.
