定義

注解（Annotation）也叫元數(shù)據(jù)至壤，它是一種代碼級別的說明，它是JDK 1.5以及以后版本引入的一個特征枢纠，與類像街、接口、枚舉是在同一個層次晋渺，它可以聲明在包镰绎、類、字段木西、方法畴栖、局部變量、方法參數(shù)等的前面八千，用來對這些數(shù)據(jù)進(jìn)行說明吗讶，注釋

功能

• 編寫文檔：通過代碼里標(biāo)識元數(shù)據(jù)生成文檔（javadoc生成java文檔）
• 代碼分析：配合反射獲取注解和相關(guān)參數(shù)（spring 基于注解編程）
• 編譯檢查：override etc ...

Annotation內(nèi)部結(jié)構(gòu)

注解本質(zhì)上是繼承Annotation的一個接口,

元注解

元注解用于定義注解的注解，java中有如下元注解：

• @Target
• @Retention
• @Document
• @Inherited

@Target 元注解用于聲明注解可以使用的作用域恋捆，如果沒有使用@Target照皆，則除了TYPE_PARAMETER，其他地方都可以使用該注解

@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
}

@Retention指定注解的聲明周期沸停，默認(rèn)為CLASS

@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
}

@Documented用于聲明是否生成javadoc

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Documented {
}

@Inherited 用于聲明查詢父類的注解

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Inherited {
}

注解屬性

在注解接口的中定義的方法即為注解屬性其膜毁，返回值有如下要求：

• 基本數(shù)據(jù)類型
• String
• Enum
• Annotation
• 以上類型的數(shù)組

自定義和獲取注解

@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface TestAnno {
    int age();

    String name();

    //defaut
    String desc() default "test";

    TestAnno1 anno();

    String[] arrays();


}

public @interface TestAnno1 {
}


@TestAnno(age = 1, name = "a", arrays = {"a", "b"}, anno = @TestAnno1)
public class Test {
    public static void main(String[] args) {
        TestAnno anno = Test.class.getAnnotation(TestAnno.class);
        System.out.println("age: " + anno.age());
        System.out.println("name: " + anno.name());
        System.out.println("arrays: " + Arrays.toString(anno.arrays()));

    }
}

運(yùn)行結(jié)果如下：

age: 1
name: a
arrays: [a, b]