Java Annotation

[wangbinyq]

How Do Annotations Work in Java?

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}

• Annotations are only metadata and they do not contain any business logic.
• It's the consumer's job to something using the metadata
• @Document - Whether to put the annotation in Javadocs
• @Retention - When the annotation is needed. Defines for how long the annotation should be kept.
  • RetentionPolicy.SOURCE - Discard during the compile.
  • RetentionPolicy.CLASS - Discard during class load. Bytecode-level post-processing. Default
  • RetentionPolicy.RUNTIME - Do not discard. Used for reflection at runtime.
• @Target? - Places the annotation can go.
  • Default to everywhere.
  • ElementType.TYPE (class, interface, enum)
  • ElementType.FIELD (instance field)
  • ElementType.(METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE, ANNOTATION_TYPE, PACKAGE)
• @Inherited - Whether subclasses get the annotation

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@interface Todo {
  public enum Priority {LOW, MEDIUM, HIGH}
  public enum Status {STARTED, NOT_STARTED}
  String author() default "Yash";
  Priority priority() default Priority.LOW;
  Status status() default Status.NOT_STARTED;
}

@Todo(priority = Todo.Priority.MEDIUM, author = "Yashwant", status = Todo.Status.STARTED)
public void incompleteMethod1() {
  //Some business logic is written
  //But it’s not complete yet
}

Class businessLogicClass = BusinessLogic.class;
for(Method method : businessLogicClass.getMethods()) {
  Todo todoAnnotation = (Todo)method.getAnnotation(Todo.class);
  if(todoAnnotation != null) {
    System.out.println(" Method Name : " + method.getName());
    System.out.println(" Author : " + todoAnnotation.author());
    System.out.println(" Priority : " + todoAnnotation.priority());
    System.out.println(" Status : " + todoAnnotation.status());
  }
}

[wangbinyq]

Annotations in Java JSR 330 Dependency Injection Annotations