Java Annotations: ElementType?

Question

java.lang.annotation.ElementType:

A program element type. The constants of this enumerated type provide a simple classification of the declared elements in a Java program. These constants are used with the Target meta-annotation type to specify where it is legal to use an annotation type.

There are the following constants:

• ANNOTATION_TYPE - Annotation type declaration
• CONSTRUCTOR - Constructor declaration
• FIELD - Field declaration (includes enum constants)
• LOCAL_VARIABLE - Local variable declaration
• METHOD - Method declaration
• PACKAGE - Package declaration
• PARAMETER - Parameter declaration
• TYPE - Class, interface (including annotation type), or enum declaration

Can someone explain what each of them are (where they'd be annotated in actual code)?

Answer 1

Let's say the annotation to which you specify the ElementType is called YourAnnotation:

• ANNOTATION_TYPE - Annotation type declaration. Note: This goes on other annotations

  @YourAnnotation
  public @interface AnotherAnnotation {..}
  

• CONSTRUCTOR - Constructor declaration

  public class SomeClass {
      @YourAnnotation
      public SomeClass() {..}
  }
  

• FIELD - Field declaration (includes enum constants)

  @YourAnnotation
  private String someField;
  

• LOCAL_VARIABLE - Local variable declaration. Note: This can't be read at runtime, so it is used only for compile-time things, like the @SuppressWarnings annotation.

  public void someMethod() {
      @YourAnnotation int a = 0;
  }
  

• METHOD - Method declaration

  @YourAnnotation
  public void someMethod() {..}
  

• PACKAGE - Package declaration. Note: This can be used only in package-info.java.

  @YourAnnotation
  package org.yourcompany.somepackage;
  

• PARAMETER - Parameter declaration

  public void someMethod(@YourAnnotation param) {..}
  

• TYPE - Class, interface (including annotation type), or enum declaration

  @YourAnnotation
  public class SomeClass {..}
  

You can specify multiple ElementTypes for a given annotation. E.g.:

@Target({ElementType.CONSTRUCTOR, ElementType.METHOD})

Answer 2

TYPE:

Annotation:

@Target({ElementType.TYPE})    // This annotation can only be applied to
public @interface Tweezable {  // class, interface, or enum declarations.
}

and an example usage:

@Tweezable
public class Hair {
    ...
}

Answer 3

It seems pretty obvious, but...

@BlahType
public class MyAnnotatedClass {
  @BlahField
  private String foo;

  @BlahConstructor
  public MyAnnotatedClass() {
  }

  @BlahMethod
  public String bar(@BlahParameter String str) {
    @BlahLocalVariable String asdf = "asdf";
    return asdf + str;
  }
}

Then ANNOTATION_TYPE is just on an annotation definition, and package is defined in a package-info.java file in the package, see here.