元注解学习笔记
元注解
package annotation.meta;
import java.lang.annotation.*;
/**
* Target 目标，描述注解的使用范围
*
*/
@Target(value = {
ElementType.METHOD, ElementType.TYPE})
/**
* Retention 保留，表示注解的保留范围，用于描述注解的生命周期
* RetentionPolicy 保留政策，共三个取值
* SOURCE 源码级
* CLASS 类级
* RUNTIME 运行级
*/
@Retention(value = RetentionPolicy.RUNTIME)
/**
* 表示该注解将生成在Javadoc中
*/
@Documented
/**
* Inherited 子类可以继承父类中的该注解
*/
@Inherited
@interface MyAnnotation {
}
Target
描述注解的使用范围
/*
* Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
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*
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*/
package java.lang.annotation;
/**
* Indicates the contexts in which an annotation type is applicable. The
* declaration contexts and type contexts in which an annotation type may be
* applicable are specified in JLS 9.6.4.1, and denoted in source code by enum
* constants of {@link ElementType java.lang.annotation.ElementType}.
*
* <p>If an {@code @Target} meta-annotation is not present on an annotation type
* {@code T} , then an annotation of type {@code T} may be written as a
* modifier for any declaration except a type parameter declaration.
*
* <p>If an {@code @Target} meta-annotation is present, the compiler will enforce
* the usage restrictions indicated by {@code ElementType}
* enum constants, in line with JLS 9.7.4.
*
* <p>For example, this {@code @Target} meta-annotation indicates that the
* declared type is itself a meta-annotation type. It can only be used on
* annotation type declarations:
* <pre>
* @Target(ElementType.ANNOTATION_TYPE)
* public @interface MetaAnnotationType {
* ...
* }
* </pre>
*
* <p>This {@code @Target} meta-annotation indicates that the declared type is
* intended solely for use as a member type in complex annotation type
* declarations. It cannot be used to annotate anything directly:
* <pre>
* @Target({})
* public @interface MemberType {
* ...
* }
* </pre>
*
* <p>It is a compile-time error for a single {@code ElementType} constant to
* appear more than once in an {@code @Target} annotation. For example, the
* following {@code @Target} meta-annotation is illegal:
* <pre>
* @Target({ElementType.FIELD, ElementType.METHOD, ElementType.FIELD})
* public @interface Bogus {
* ...
* }
* </pre>
*
* @since 1.5
* @jls 9.6.4.1 @Target
* @jls 9.7.4 Where Annotations May Appear
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
/**
* Returns an array of the kinds of elements an annotation type
* can be applied to.
* @return an array of the kinds of elements an annotation type
* can be applied to
*/
ElementType[] value();
}
取值范围：
/*
* Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
package java.lang.annotation;
/**
* The constants of this enumerated type provide a simple classification of the
* syntactic locations where annotations may appear in a Java program. These
* constants are used in {@link Target java.lang.annotation.Target}
* meta-annotations to specify where it is legal to write annotations of a
* given type.
*
* <p>The syntactic locations where annotations may appear are split into
* <em>declaration contexts</em> , where annotations apply to declarations, and
* <em>type contexts</em> , where annotations apply to types used in
* declarations and expressions.
*
* <p>The constants {@link #ANNOTATION_TYPE} , {@link #CONSTRUCTOR} , {@link
* #FIELD} , {@link #LOCAL_VARIABLE} , {@link #METHOD} , {@link #PACKAGE} ,
* {@link #PARAMETER} , {@link #TYPE} , and {@link #TYPE_PARAMETER} correspond
* to the declaration contexts in JLS 9.6.4.1.
*
* <p>For example, an annotation whose type is meta-annotated with
* {@code @Target(ElementType.FIELD)} may only be written as a modifier for a
* field declaration.
*
* <p>The constant {@link #TYPE_USE} corresponds to the 15 type contexts in JLS
* 4.11, as well as to two declaration contexts: type declarations (including
* annotation type declarations) and type parameter declarations.
*
* <p>For example, an annotation whose type is meta-annotated with
* {@code @Target(ElementType.TYPE_USE)} may be written on the type of a field
* (or within the type of the field, if it is a nested, parameterized, or array
* type), and may also appear as a modifier for, say, a class declaration.
*
* <p>The {@code TYPE_USE} constant includes type declarations and type
* parameter declarations as a convenience for designers of type checkers which
* give semantics to annotation types. For example, if the annotation type
* {@code NonNull} is meta-annotated with
* {@code @Target(ElementType.TYPE_USE)}, then {@code @NonNull}
* {@code class C {...}} could be treated by a type checker as indicating that
* all variables of class {@code C} are non-null, while still allowing
* variables of other classes to be non-null or not non-null based on whether
* {@code @NonNull} appears at the variable's declaration.
*
* @author Joshua Bloch
* @since 1.5
* @jls 9.6.4.1 @Target
* @jls 4.1 The Kinds of Types and Values
*/
public enum ElementType {
/** Class, interface (including annotation type), or enum declaration */
TYPE,
/** Field declaration (includes enum constants) */
FIELD,
/** Method declaration */
METHOD,
/** Formal parameter declaration */
PARAMETER,
/** Constructor declaration */
CONSTRUCTOR,
/** Local variable declaration */
LOCAL_VARIABLE,
/** Annotation type declaration */
ANNOTATION_TYPE,
/** Package declaration */
PACKAGE,
/**
* Type parameter declaration
*
* @since 1.8
*/
TYPE_PARAMETER,
/**
* Use of a type
*
* @since 1.8
*/
TYPE_USE
}
Retention
保留，描述注解的生命周期
/*
* Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
package java.lang.annotation;
/**
* Indicates how long annotations with the annotated type are to
* be retained. If no Retention annotation is present on
* an annotation type declaration, the retention policy defaults to
* {@code RetentionPolicy.CLASS}.
*
* <p>A Retention meta-annotation has effect only if the
* meta-annotated type is used directly for annotation. It has no
* effect if the meta-annotated type is used as a member type in
* another annotation type.
*
* @author Joshua Bloch
* @since 1.5
* @jls 9.6.3.2 @Retention
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
/**
* Returns the retention policy.
* @return the retention policy
*/
RetentionPolicy value();
}
取值范围:
- SOURCE 源码级
- CLASS 类级
- RUNTIME 运行级
/*
* Copyright (c) 2003, 2004, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
package java.lang.annotation;
/**
* Annotation retention policy. The constants of this enumerated type
* describe the various policies for retaining annotations. They are used
* in conjunction with the {@link Retention} meta-annotation type to specify
* how long annotations are to be retained.
*
* @author Joshua Bloch
* @since 1.5
*/
public enum RetentionPolicy {
/**
* Annotations are to be discarded by the compiler.
*/
SOURCE,
/**
* Annotations are to be recorded in the class file by the compiler
* but need not be retained by the VM at run time. This is the default
* behavior.
*/
CLASS,
/**
* Annotations are to be recorded in the class file by the compiler and
* retained by the VM at run time, so they may be read reflectively.
*
* @see java.lang.reflect.AnnotatedElement
*/
RUNTIME
}
Documented
表示该注解将生成在Javadoc中
/*
* Copyright (c) 2003, 2004, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
package java.lang.annotation;
/**
* Indicates that annotations with a type are to be documented by javadoc
* and similar tools by default. This type should be used to annotate the
* declarations of types whose annotations affect the use of annotated
* elements by their clients. If a type declaration is annotated with
* Documented, its annotations become part of the public API
* of the annotated elements.
*
* @author Joshua Bloch
* @since 1.5
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Documented {
}
Inherited
子类可以继承父类中的该注解
/*
* Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
package java.lang.annotation;
/**
* Indicates that an annotation type is automatically inherited. If
* an Inherited meta-annotation is present on an annotation type
* declaration, and the user queries the annotation type on a class
* declaration, and the class declaration has no annotation for this type,
* then the class's superclass will automatically be queried for the
* annotation type. This process will be repeated until an annotation for this
* type is found, or the top of the class hierarchy (Object)
* is reached. If no superclass has an annotation for this type, then
* the query will indicate that the class in question has no such annotation.
*
* <p>Note that this meta-annotation type has no effect if the annotated
* type is used to annotate anything other than a class. Note also
* that this meta-annotation only causes annotations to be inherited
* from superclasses; annotations on implemented interfaces have no
* effect.
*
* @author Joshua Bloch
* @since 1.5
* @jls 9.6.3.3 @Inherited
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Inherited {
}