目錄
注解（Annotation）就是一個元數據，即描述數據的數據【是不是感覺和注釋差不多，兩者的區別看這里：注解和注釋的區別】
- 官方術語：如果你想把某個方法聲明為服務，那么使用注解（Annotation）會更好一些，因為這種情況下需要注解和方法緊密耦合起來，開發人員也必須意識到這一點。
- 自我理解：更好的優化理解代碼
- 自我表現：對菜鳥裝一手
- 自我現實：
- 參考的代碼出錯了，都不知道哪里錯，那么你就可以涼涼了
- 面試一問三不知道，恭喜你涼涼了
- 編譯檢查
- 在反射中使用注解（Annotation）
- 根據注解（Annotation）生成幫助文檔
- 注解處理器
- 在框架中的作用
注解類：用於注解的一個類，如@Override
元注解（meta-annotation）：自定義注解上還有注解，共四個：@Target、@Retention、@Documented、@Inherited
@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); }
代碼片段01——調用@Override代碼地方
@Target(ElementType.METHOD) @Retention(RetentionPolicy.SOURCE) public @interface Override { }
代碼片段02——@Override代碼內容
從代碼片段01，我們可以知道注解@Override絕對是我們學習Android的人員最常用的注解了。那么它具體是什么作用呢？在代碼片段02中，我們可以理解發現里面可以說是什么都沒寫，它的具體作用是只能是注解的基本作用之一，檢測編譯；具體體現是驗證@Override下面的方法名是否是你父類中所有的，如果沒有則報錯。比如上面的舉例onCreate，如果沒有@Override，且你的oncreate改為全部小寫依舊可以編譯通過，這時候編譯器默認你創建了新的方法。
@Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.ANNOTATION_TYPE) public @interface Documented { }
代碼片段03——@Documented代碼內容
在代碼片段03中，我們可以理解發現里面可以說是什么都沒寫，它的具體作用是只能是注解的基本作用之一，根據注解（Annotation）生成幫助文檔；具體體現是簡單的標記注解，標識是否將注解信息包含在java文檔中.
/**
* 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(); }
代碼片段04——@Retention代碼內容
在代碼片段04中，我們可以看到@Retention是根據RetentionPolicy類中的相應的結果集設置注解的保留時間【即設置注解的生命周期何時結束】，默認設置是RetentionPolicy.CLASS。查看RetentionPolicy類中的相應的結果集。
/**
* 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 }
代碼片段05——RetentionPolicy類代碼內容
在代碼片段05中可知，這個類是設置注解什么時候結束的常量，即控制注解的生命周期的常量。分別為SOURCE，CLASS，RUNTIME。
RetentionPolicy.SOURCE：有效期在源碼階段，在編譯階段丟棄，這些注解在編譯結束后不會有任何意義，也不會寫入字節碼中。RetentionPolicy.CLASS：有效期至字節碼文件。在類加載的時候丟棄，注解默認使用這種方式。RetentionPolicy.RUNTIME：始終有效，在運行時也會保留。因此可以使用反射讀取該注解的信息，自定義注解通常使用這種方式。
/**
* 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(); }
代碼片段06——@Target代碼內容
在代碼片段06中，我們看到@Target是表示在什么地方使用該注解。默認情況下，則該注解可以放在任意地方。查看代碼片段07，可以設置的參數。
/** * 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 }
代碼片段07——ElementType類t代碼內容
ElementType.TYPE：用於描述類，接口（包括注解類型），EnumElementType.FIELD：用於描述實例變量（包括枚舉的常量）ElementType.METHOD：用於描述方法ElementType.PARAMETER：用於描述參數ElementType.CONSTRUCTOR：用於構造方法ElementType.LOCAL_VARIABLE：用於描述局部變量ElementType.ANNOTATION_TYPE：用於描述注解ElementType.PACKAGE：用於描述包ElementType.TYPE_PARAMETER：since 1.8 表示該注解能寫在類型變量的聲明語句中ElementType.TYPE_USE：since 1.8 表示該注解能寫在使用類型的任何語句中
@Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.ANNOTATION_TYPE) public @interface Inherited { }
代碼片段08——@Inherited代碼內容
代碼08片段表示該注解類型被自動繼承，如果用戶在當前類中查詢這個元注解類型，但是當前類的聲明中不包含這個元注解類型，那么將自動查詢其父類，直至查到該注解或到達頂層類
- 定義不同
- 注解：元數據，它是一種描述數據的數據。所以，可以說注解就是源代碼的元數據。
- 注釋：是對源代碼說明的文字
- 作用對象不同
- 注解：是給編譯器看的。
- 注釋：是給人看的。
- 書寫范圍不同
- 注解：遵守一定的書寫規范，以@開頭，與工具一起使用
- 注釋：可以在代碼的任何地方書寫
- 運行范圍不同
- 注解：可以參與編譯器的任何階段，對數據有一定的操作作用
- 注釋：被編譯器忽略，不參與編譯
巨人的肩膀