注解

注解的分类

　　1.标准注解

　　2.元注解

　　　　@Retention ：注解的生命周期

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    /**
     * Returns the retention policy.
     * @return the retention policy
     */
    RetentionPolicy value();
}

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
}

SOURCE:只作用在java类中，.class里没有这个注解

.CLASS:在.class里也会有这个注解

.RUNTIME:在运行时这个注解也会起到作用.

　　　　@Target ：注解的作用目标

　　　　　　

@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();
}

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
}

　TYPE:作用在类上

　Field:作用在属性上

　Method:作用在方法上

　　　　@Inherited ：是否允许之类继承该注解

　　　　@Documented

自定义注解的实现

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface PersonAnnotation {
    String name();
    int age();
    String gender() default  "男";
    String[] language();
}

获取注解的信息

public static void main(String[] args) throws Exception {
        Class clazz = Class.forName("Annotation.Person");
        PersonAnnotation annotation = (PersonAnnotation) clazz.getAnnotation(PersonAnnotation.class);
        System.out.println(annotation.age()+"
"+annotation.gender()+"
"+annotation.name());
        for(String str : annotation.language()){
            System.out.println(str+" ");
        }
    }

通过反射可以获取到注解的信息，但前提是注解的生命周期必须是Runntime.