SpringBoot注解之：@Target和@Retention

@Target

先来看一下源码，发现只有一个参数ElementType[] value()

package java.lang.annotation;
​
@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();
}
​

我们进入ElementType的源码

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
}

@Target(ElementType.TYPE)——接口、类、枚举、注解 @Target(ElementType.FIELD)——字段、枚举的常量 @Target(ElementType.METHOD)——方法 @Target(ElementType.PARAMETER)——方法参数 @Target(ElementType.CONSTRUCTOR) ——构造函数 @Target(ElementType.LOCAL_VARIABLE)——局部变量 @Target(ElementType.ANNOTATION_TYPE)——注解 @Target(ElementType.PACKAGE)——包

@Retention

老规矩，还是先来看一下源码,发现也是只有一个参数RetentionPolicy

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();
}

我们进入到这个RetentionPolicy里看一下，发现有三个值

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
}

RetentionPolicy.SOURCE:这种类型的Annotations只在源代码级别保留,编译时就会被忽略,在class字节码文件中不包含。

RetentionPolicy.CLASS:这种类型的Annotations编译时被保留,默认的保留策略,在class文件中存在,但JVM将会忽略,运行时无法获得。

RetentionPolicy.RUNTIME:这种类型的Annotations将被JVM保留,所以他们能在运行时被JVM或其他使用反射机制的代码所读取和使用。