目录
什么是注解
为什么学习注解
注解的基本作用
常用的注解类和元注解
@Override
@Documented
@Retention
@Target
@Inherited
注解和注释的区别
巨人的肩膀
注解（Annotation）就是一个元数据，即描述数据的数据【是不是感觉和注释差不多，两者的区别看这里：注解和注释的区别】
官方术语：如果你想把某个方法声明为服务，那么使用注解（Annotation）会更好一些，因为这种情况下需要注解和方法紧密耦合起来，开发人员也必须意识到这一点。
自我理解：更好的优化理解代码
自我表现：对菜鸟装一手
自我现实：
参考的代码出错了，都不知道哪里错，那么你就可以凉凉了
面试一问三不知道，恭喜你凉凉了
编译检查
在反射中使用注解（Annotation）
根据注解（Annotation）生成帮助文档
注解处理器
在框架中的作用
注解类：用于注解的一个类，如@Override
元注解（meta-annotation）：自定义注解上还有注解，共四个：@Target、@Retention、@Documented、@Inherited
@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); }
代码片段01——调用@Override代码地方
@Target(ElementType.METHOD) @Retention(RetentionPolicy.SOURCE) public @interface Override { }
代码片段02——@Override代码内容
从代码片段01，我们可以知道注解@Override绝对是我们学习Android的人员最常用的注解了。那么它具体是什么作用呢？在代码片段02中，我们可以理解发现里面可以说是什么都没写，它的具体作用是只能是注解的基本作用之一，检测编译；具体体现是验证@Override下面的方法名是否是你父类中所有的，如果没有则报错。比如上面的举例onCreate，如果没有@Override，且你的oncreate改为全部小写依旧可以编译通过，这时候编译器默认你创建了新的方法。
@Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.ANNOTATION_TYPE) public @interface Documented { }
代码片段03——@Documented代码内容
在代码片段03中，我们可以理解发现里面可以说是什么都没写，它的具体作用是只能是注解的基本作用之一，根据注解（Annotation）生成帮助文档；具体体现是简单的标记注解，标识是否将注解信息包含在java文档中.
/**
* 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(); }
代码片段04——@Retention代码内容
在代码片段04中，我们可以看到@Retention是根据RetentionPolicy类中的相应的结果集设置注解的保留时间【即设置注解的生命周期何时结束】，默认设置是RetentionPolicy.CLASS。查看RetentionPolicy类中的相应的结果集。
/**
* 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 }
代码片段05——RetentionPolicy类代码内容
在代码片段05中可知，这个类是设置注解什么时候结束的常量，即控制注解的生命周期的常量。分别为SOURCE，CLASS，RUNTIME。
RetentionPolicy.SOURCE：有效期在源码阶段，在编译阶段丢弃，这些注解在编译结束后不会有任何意义，也不会写入字节码中。
RetentionPolicy.CLASS：有效期至字节码文件。在类加载的时候丢弃，注解默认使用这种方式。
RetentionPolicy.RUNTIME：始终有效，在运行时也会保留。因此可以使用反射读取该注解的信息，自定义注解通常使用这种方式。
/**
* 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(); }
代码片段06——@Target代码内容
在代码片段06中，我们看到@Target是表示在什么地方使用该注解。默认情况下，则该注解可以放在任意地方。查看代码片段07，可以设置的参数。
/** * 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 }
代码片段07——ElementType类t代码内容
ElementType.TYPE：用于描述类，接口（包括注解类型），Enum
ElementType.FIELD：用于描述实例变量（包括枚举的常量）
ElementType.METHOD：用于描述方法
ElementType.PARAMETER：用于描述参数
ElementType.CONSTRUCTOR：用于构造方法
ElementType.LOCAL_VARIABLE：用于描述局部变量
ElementType.ANNOTATION_TYPE ：用于描述注解
ElementType.PACKAGE：用于描述包
ElementType.TYPE_PARAMETER：since 1.8 表示该注解能写在类型变量的声明语句中
ElementType.TYPE_USE：since 1.8 表示该注解能写在使用类型的任何语句中
@Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.ANNOTATION_TYPE) public @interface Inherited { }
代码片段08——@Inherited代码内容
代码08片段表示该注解类型被自动继承，如果用户在当前类中查询这个元注解类型，但是当前类的声明中不包含这个元注解类型，那么将自动查询其父类，直至查到该注解或到达顶层类
定义不同
注解：元数据，它是一种描述数据的数据。所以，可以说注解就是源代码的元数据。
注释：是对源代码说明的文字
作用对象不同
注解：是给编译器看的。
注释：是给人看的。
书写范围不同
注解：遵守一定的书写规范，以@开头，与工具一起使用
注释：可以在代码的任何地方书写
运行范围不同
注解：可以参与编译器的任何阶段，对数据有一定的操作作用
注释：被编译器忽略，不参与编译
巨人的肩膀
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