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Java Annotations


  • What are Annotations?
  • Use of Annotations
  • Annotations Placements
  • Built in Java Annotations
  • Custom Annotations

What Are Annotations?

  • Metadata about the java code.
  • Introduced from Java 5
  • Mostly do not effect the code execution, sometimes they might.
  • E.g. @Override
  • Use of @ points to the compiler that it is an annotation.

Use of Annotations

  • Compilers can use them for suppressing warnings and detecting errors.
  • Software tools can gather information from them and generate code, XML information etc.
  • These annotations could also be modified so that they are available at runtime as well.

Annotations Placements

  • Annotations could be placed before java packages, types, constructors, methods, fields, parameters, and local variables.
  • The annotation can contain multiple elements like @Entity(tableName = “vehicles”, primaryKey = “id”)
  • In case there is a single element, convention is to call that element “value” e.g. @Entity(value=“entity”)
  • It can also be left as blank when there is just a single element like @Entity(“entity”)
Annotations are defined using the @ symbol followed by an annotation type, and they can also include elements with values.

Built In Java Annotations

  • @Deprecated:- This annotation is used to mark a class, method or field as deprecated, meaning it should no longer be used.
  • @Override:- The @Override Java annotation is used above methods that override methods in a superclass. If the method does not match a method in the superclass, the compiler will give you an error.
  • @SuppressWarnings:- The @SuppressWarnings annotation makes the compiler suppress warnings for a given method.
  • Demo

Custom Annotations

  • An annotation type is a Java interface, except that you must add an ‘@’ sign before the interface keyword when declaring it.
  • It can be created like.
					public @interface CustomAnnotation 
String value();
Integer id();

  • Element default values can be specified.

Custom Annotations (Continued…)

  • For custom annotations we can use the following options:-
    • @Retention:- Used to specify if the custom annotation is available at runtime.
    • @Target:- Used to specify with which java element the annotation could be used..
    • @Inherited:- Used to specify that custom annotation used with parent class gets inherited by subclasses as well.
    • @Documented:- Used to specify that custom annotation should be read and used by JavaDoc tool.