Java Reflection 教程(8):Annotations类标注

本文详细介绍如何使用Java Reflection API获取类、方法及参数上的注解,并展示如何读取这些注解的值。

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前面Java  标注(Annotations)介绍 介绍了Java代码中Annotation的用法,使用Java Reflection API 也可以用来方法类,类成员或类方法的Annotation定义。

1.类Annotations

可以通过Class 的getAnnotations 来方法类的标注定义:

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());
 }
}


或者通过指定Annotation类型来访问类的标注:

 

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. 类方法Annotations

类似的方法可以访问类方法的标注,如下例:

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


或者指定Annotation类型:

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());
}

3. 参数Annotations

访问参数的标注,如下例:

 

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

4.  类成员Annotation

访问类成员标注的例子如下:

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


或者

 

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());
}


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