04. Defining Custom Annotations

Defining Custom Annotations

In this section, you will learn how to define your own custom Java annotations.

Defining Custom Annotations

In addition to the built-in annotations from the previous section, you can also define your own custom Java annotations.

Example

@Retention(RetentionPolicy.SOURCE)
@Target(ElementType.TYPE)  // Applies to class, interface, or enum
public @interface ConvertsTo {
  Class<?> targetClass();
  String setterPrefix() default "set";
}

Elements

Annotations are allowed to have parameterized values, which are called elements. Because annotations are always available at compile-time, the type of an element has to be a compile-time constant, an enum, a class literal, or an array initializer.

Elements can also have default values. Element values are set like so:

Object input = ... ;  // Let's pretend we know this is a List<String>
@SuppressWarnings(value = "unchecked")
List<String> result = (List<String>) input;

If an annotation only has one element, and the element is named "value", you can skip naming the "value":

Object input = ... ;  // Let's pretend we know this is a List<String>
@SuppressWarnings("unchecked")
List<String> result = (List<String>) input;

Retention Policies

Here are the possible retention policies for an annotation:

QUIZ QUESTION::

Match each RetentionPolicy with its description.

ANSWER CHOICES:

Annotation Targets

The target types determine which parts of the program can be given a particular annotation. Here are the possible target types:

Further Reading

• Annotations Tutorial