SpringBoot系列（三）元注解

元注解，注解的注解，SpringBoot有四个元注解，分别是@Target、@Retention、@Documented、@Inherited。下面就是对元注解的详细讲解和源码展示。

@Taget

该注解的作用：告诉java将自定义的注解放到什么地方，例如类、方法、构造器、变量等，它是一个枚举类型，有如下属性：

• ElementType.TYPE ：类、接口（包括注解类型）、或enum声明
• ElementType.FIELD ：字段声明（包括枚举常量）
• ElementType.METHOD ：方法
• ElementType.PARAMETER ：参数
• ElementType.CONSTRUCTOR ：构造器
• ElementType.LOCAL_VARIABLE ：局部变量
• ElementType.ANNOTATION_TYPE ：注解类型
• ElementType.PACKAGE ：包
• ElementType.TYPE_PARAMETER ：类型参数
• ElementType.TYPE_USE ：类型的注解，如泛型

@Target的源码

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>
 *    &#064;Target(ElementType.ANNOTATION_TYPE)
 *    public &#064;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>
 *    &#064;Target({})
 *    public &#064;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>
 *    &#064;Target({ElementType.FIELD, ElementType.METHOD, ElementType.FIELD})
 *    public &#064;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();
}

ElementType枚举值

package java.lang.annotation;
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

该注解的作用：该注解用于说明自定义注解的生命周期，在注解中有三个生命周期。它是一个枚举类型，有如下属性：

• RetentionPolicy.SOURCE：编译阶段丢弃，自定义注解在编译结束之后就不再有意义，所以它们不会写入class字节码文件。@Overide、@SuppressWarnings 都属于这类注解。
• RetentionPolicy.CLASS：类加载时丢弃，编译时保留（在class文件中存在,但JVM将会忽略,运行时无法获得）。默认使用这种方式。
• RetentionPolicy.RUNTIME：始终不会丢弃， 运行期也保留读注解，可以被JVM或其他使用反射机制读取该注解的信息。 自定义的注解通常使用这种方式。

@Retention的源码

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();
}

RetentionPolicy的枚举值

package java.lang.annotation;

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
}

@Inherited

该注解的作用：该注解是一个标记注解，表明被标注的类型是可以被继承的。如果一个使用了 @Inherited 修饰的 Annotation 类型被用于一个Class，则这个 Annotation 将被用于该 Class 的子类。

@Inherited的源码

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 {
}

@Documented

该注解的作用：是否将注解信息添加在 Javadoc中。

@Documented的源码

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 {
}