java--基础--21.2--注解--案例

java–基础–21.2–注解–案例

1、类注解

可以在运行时获取类、方法或字段的注解。下面是获取类注解的示例：

Class aClass = TheClass.class;
Annotation[] annotations = aClass.getAnnotations();
for(Annotation annotation : annotations){
        if(annotation instanceof MyAnnotation){
            MyAnnotation myAnnotation = (MyAnnotation) annotation;
            System.out.println("name: " + myAnnotation.name());
            System.out.println("value: " + myAnnotation.value());
         }
}

也可以获取指定的类注解，如下：

Class aClass = TheClass.class;
Annotation annotation = aClass.getAnnotation(MyAnnotation.class);

if(annotation instanceof MyAnnotation){
         MyAnnotation myAnnotation = (MyAnnotation) annotation;
         System.out.println("name: " + myAnnotation.name());
         System.out.println("value: " + myAnnotation.value());
}

2、方法注解

下面是方法注解的示例:

public class TheClass {
         @MyAnnotation(name="someName",  value = "Hello World")
          public void doSomething(){}
}

获取方法的所有注解，如下：

Method method = ... //obtain method object
Annotation[] annotations = method.getDeclaredAnnotations();
for(Annotation annotation : annotations){
         if(annotation instanceof MyAnnotation){
             MyAnnotation myAnnotation = (MyAnnotation) annotation;
             System.out.println("name: " + myAnnotation.name());
             System.out.println("value: " + myAnnotation.value());
         }
}

获取方法的特定注解:

Method method = ... // obtain method object
Annotation annotation = method.getAnnotation(MyAnnotation.class);
if(annotation instanceof MyAnnotation){
       MyAnnotation myAnnotation = (MyAnnotation) annotation;
       System.out.println("name: " + myAnnotation.name());
        System.out.println("value: " + myAnnotation.value());
}

4、参数注解

也可以给方法声明的参数添加注解，如下：

public class TheClass {
         public static void doSomethingElse(
            @MyAnnotation(name="aName", value="aValue") String parameter){
         }
}

从Method对象获取参数注解：

Method method = ... //obtain method object
Annotation[][] parameterAnnotations = method.getParameterAnnotations();
Class[] parameterTypes = method.getParameterTypes();
int i=0;
for(Annotation[] annotations : parameterAnnotations){
         Class parameterType = parameterTypes[i++];
         for(Annotation annotation : annotations){
            if(annotation instanceof MyAnnotation){
                MyAnnotation myAnnotation = (MyAnnotation) annotation;
                System.out.println("param: " + parameterType.getName());
                System.out.println("name : " + myAnnotation.name());
                System.out.println("value: " + myAnnotation.value());
            }
         }
}

注意 Method.getParameterAnnotations() 方法返回的是二维的Annotation数组,每个方法参数都有一个一维的Annotation数组。

5、字段注解

下面是字段注解示例:

public class TheClass {
         @MyAnnotation(name="someName",  value = "Hello World")
         public String myField = null;
}

获取字段的所有注解，如下：

Field field = ... //obtain field object
Annotation[] annotations = field.getDeclaredAnnotations();
for(Annotation annotation : annotations){
         if(annotation instanceof MyAnnotation){
            MyAnnotation myAnnotation = (MyAnnotation) annotation;
            System.out.println("name: " + myAnnotation.name());
            System.out.println("value: " + myAnnotation.value());
         }
}

获取指定字段的注解，如下：

Field field = ... // obtain method object
Annotation annotation = field.getAnnotation(MyAnnotation.class);
if(annotation instanceof MyAnnotation){
         MyAnnotation myAnnotation = (MyAnnotation) annotation;
         System.out.println("name: " + myAnnotation.name());
         System.out.println("value: " + myAnnotation.value());
}