To learn annotations in depth, we must be able to define our own annotations and use them. Before defining our own annotations, we must Understand the syntax of meta-annotations and related definition annotations provided by Java.
1 package annotation; 2 3 import static java.lang.annotation.ElementType.METHOD; 4 import static java.lang.annotation.RetentionPolicy.RUNTIME; 5 6 import java.lang.annotation.Documented; 7 import java.lang.annotation.ElementType; 8 import java.lang.annotation.Retention; 9 import java.lang.annotation.RetentionPolicy;10 import java.lang.annotation.Target;11 /**12 * 13 * @author Minzhe Xu 2017年4月27日下午3:22:5214 * 15 *@Target 表示该注解用于什么地方,可能的值在枚举类 ElemenetType 中,包括: 16 *ElemenetType.CONSTRUCTOR:构造器声明 17 *ElemenetType.FIELD :域声明(包括 enum 实例) 18 *ElemenetType.LOCAL_VARIABLE:局部变量声明 19 *ElemenetType.METHOD :方法声明 20 *ElemenetType.PACKAGE :包声明 21 *ElemenetType.PARAMETER :参数声明 22 *ElemenetType.TYPE:类,接口(包括注解类型)或enum声明 23 *24 *@Retention 表示在什么级别保存该注解信息。可选的参数值在枚举类型 RetentionPolicy 中,包括: 25 *RetentionPolicy.SOURCE :注解将被编译器丢弃 26 *RetentionPolicy.CLASS :注解在class文件中可用,但会被VM丢弃 27 *RetentionPolicy.RUNTIME VM:将在运行期也保留注释,因此可以通过反射机制读取注解的信息。 28 *29 *@Documented 将此注解包含在 javadoc 中 ,它代表着此注解会被javadoc工具提取成文档。在doc文档中的内容会因为此注解的信息内容不同而不同。相当与@see,@param 等。30 *31 *@Inherited 允许子类继承父类中的注解。32 */33 34 @Target({ElementType.TYPE,ElementType.CONSTRUCTOR,ElementType.METHOD,ElementType.PARAMETER,ElementType.FIELD})35 @Retention(RetentionPolicy.RUNTIME)36 public @interface TestA {37 38 /**39 * @interface用来声明一个注解,40 * 其中的每一个方法实际上是声明了一个配置参数。41 * 方法的名称就是参数的名称,返回值类型就是参数的类型(返回值类型只能是基本类型、Class、String、enum)。42 * 可以通过default来声明参数的默认值。43 */44 String name();45 int id() default 0;46 Class gid();47 48 }
Test class
1 package annotation; 2 3 import java.util.HashMap; 4 import java.util.Map; 5 6 /** 7 * 8 * @author Minzhe Xu 2017年4月27日下午3:09:20 9 *10 */11 @TestA(gid = Long.class, name = "type")12 public class UserAnotation {13 @TestA(gid = Long.class, name = "param",id=1)14 private Integer age;15 16 @TestA(gid = Long.class, name = "constract",id=2)17 public UserAnotation(){18 19 }20 @TestA(gid = Long.class, name = "method",id=3)21 public void test1(){22 @SuppressWarnings("rawtypes")23 Map m=new HashMap(0);24 }25 26 @TestA(gid = Long.class, name = "method3",id=5)27 public void test2(@TestA(gid = Long.class, name = "inner_param",id=4) Integer a){28 29 }30 31 32 }
Use annotations through reflection
package annotation;import java.lang.annotation.Annotation;import java.lang.reflect.Constructor;import java.lang.reflect.Method;/** * * @author Minzhe Xu 2017年4月27日下午3:39:17 * */public class ParseAnotation {public static void parseTypeAnnotation() throws ClassNotFoundException{ Class clazz=Class.forName("annotation.UserAnotation"); Annotation[] annotations = clazz.getAnnotations();for(Annotation annotation:annotations){ TestA testA=(TestA) annotation; System.out.println("id="+testA.id()+";name="+testA.name()+";gid="+testA.gid()); } }public static void parseMethodAnnotation(){ Method[] methods = UserAnotation.class.getDeclaredMethods();for(Method method:methods){boolean hasAnnotation=method.isAnnotationPresent(TestA.class);if(hasAnnotation){ TestA testA=method.getAnnotation(TestA.class); System.out.println("id="+testA.id()+";name="+testA.name()+";gid="+testA.gid()); } } } @SuppressWarnings("unchecked")public static void parseConstractAnnotation(){ Constructor[] constructors = UserAnotation.class.getConstructors();for(Constructor contructor:constructors){boolean annotationPresent = contructor.isAnnotationPresent(TestA.class);//isAnnotationPresent方法来判断是否使用了某个注解if(annotationPresent){ TestA testA = (TestA) contructor.getAnnotation(TestA.class); System.out.println("id="+testA.id()+";name="+testA.name()+";gid="+testA.gid()); } } }public static void main(String[] args) throws ClassNotFoundException { parseTypeAnnotation(); parseMethodAnnotation(); parseConstractAnnotation(); } }
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