java注解定义常量,Java注释ElementType常量是什么意思？

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)?

解决方案

This summarizes the main ones:

@CustomTypeAnnotation

public class MyAnnotatedClass {

@CustomFieldAnnotation

private String foo;

@CustomConstructorAnnotation

public MyAnnotatedClass() {

}

@CustomMethodAnnotation

public String bar(@CustomParameterAnnotation String str) {

@CustomLocalVariableAnnotation String asdf = "asdf";

return asdf + str;

}

}

ANNOTATION_TYPE is an annotation on another annotation, like this:

@CustomAnnotationTypeAnnotation

public @interface SomeAnnotation {

..

}

Package is defined in a package-info.java file in the package, like this:

@CustomPackageLevelAnnotation

package com.some.package;

import com.some.package.annotation.PackageLevelAnnotation;

For more info on PACKAGE annotations see here and here.