注解(Annotation)是java的重要组成部分，用于标记，从jdk1.5开始引入的新特性。
Jdk早起引入的注解有：@Deprecated，@Override，@SuppressWarnings
简单注解格式（以spring Autowired为例）
1 @Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE}) 2 @Retention(RetentionPolicy.RUNTIME) 3 @Documented 4 public @interface Autowired { 5 6 /** 7 * Declares whether the annotated dependency is required. 8 * <p>Defaults to {@code true}. 9 */ 10 boolean required() default true; 11 12 }
注解是一冲标记，@Target代表可以标记的类型，可以标记在ElementType枚举出的类型
1 public enum ElementType { 2 /** Class, interface (including annotation type), or enum declaration */ 3 TYPE, 4 5 /** Field declaration (includes enum constants) */ 6 FIELD, 7 8 /** Method declaration */ 9 METHOD, 10 11 /** Formal parameter declaration */ 12 PARAMETER, 13 14 /** Constructor declaration */ 15 CONSTRUCTOR, 16 17 /** Local variable declaration */ 18 LOCAL_VARIABLE, 19 20 /** Annotation type declaration */ 21 ANNOTATION_TYPE, 22 23 /** Package declaration */ 24 PACKAGE, 25 26 /** 27 * Type parameter declaration 28 * 29 * @since 1.8 30 */ 31 TYPE_PARAMETER, 32 33 /** 34 * Use of a type 35 * 36 * @since 1.8 37 */ 38 TYPE_USE 39 }
@Retention代表作用域RetentionPolicy
1 public enum RetentionPolicy { 2 /** 3 * Annotations are to be discarded by the compiler. 4 */ 5 SOURCE, 6 7 /** 8 * Annotations are to be recorded in the class file by the compiler 9 * but need not be retained by the VM at run time. This is the default 10 * behavior. 11 */ 12 CLASS, 13 14 /** 15 * Annotations are to be recorded in the class file by the compiler and 16 * retained by the VM at run time, so they may be read reflectively. 17 * 18 * @see java.lang.reflect.AnnotatedElement 19 */ 20 RUNTIME 21 }
包换三种类型，分别对应的有效作用域是.java文件、.class文件、内存中的字节码。
javac将java源文件编译成class文件会去掉SOURCE的注解，类加载器将class文件加载到内存中会去掉CLASS的注解，运行期RUNTIME有效的注解会一直存在。
注解中包换一些value，例如required，boolean类型，默认值为true。
运行期获取注解
1 Autowired autowired= (Autowired)Object.class.getAnnotation(Autowired.class);