Java中的 AnnotatedElement getDeclaredAnnotation() 方法及示例

Java.lang.reflect.AnnotatedElement 接口的getDeclaredAnnotation()方法用于获取指定声明的注解类型的声明注解，如果这样的声明注解存在于实现该接口的类中。该方法以对象的形式返回该类。

句法：

public T getDeclaredAnnotation(Class declared annotationClass)

参数：此方法接受一个参数声明的annotationClass ，它是要获取的声明注释的类型。

返回值：该方法返回声明的实现该接口的注解类的指定对象。

异常：此方法抛出：

• NullPointerException：如果给定的声明注释类为空。

下面的程序演示了 getDeclaredAnnotation() 方法。

示例 1：

// Java program to demonstrate
// getDeclaredAnnotation() method
  
import java.util.*;
import java.lang.annotation.*;
import java.lang.reflect.*;
  
// create a custom DeclaredAnnotation
@Retention(RetentionPolicy.RUNTIME)
@interface DeclaredAnnotation {
  
    // This declared annotation has two attributes.
    public String key();
  
    public String value();
}
  
// call DeclaredAnnotation for method
// and pass values for declared annotation
@DeclaredAnnotation(key = "GFG", value = "GeeksForGeeks")
public class Test {
  
    public Object obj;
  
    public static void main(String[] args)
        throws ClassNotFoundException
    {
  
        // returns the Class object
        // for this myAnnotatedElement
        AnnotatedElement myAnnotatedElement
            = Test.class;
  
        System.out.println(
            "AnnotatedElement represented"
            + " by myAnnotatedElement: "
            + myAnnotatedElement.toString());
  
        // Get the declared annotation
        // using getDeclaredAnnotation() method
        System.out.println(
            "DeclaredAnnotation of myAnnotatedElement: "
            + myAnnotatedElement
                  .getDeclaredAnnotation(
                      DeclaredAnnotation.class));
    }
}

输出：

AnnotatedElement represented by myAnnotatedElement: class Test
DeclaredAnnotation of myAnnotatedElement: @DeclaredAnnotation(key=GFG, value=GeeksForGeeks)

示例 2：

// Java program to demonstrate
// getDeclaredAnnotation() method
  
import java.util.*;
import java.lang.annotation.*;
import java.lang.reflect.*;
  
@Deprecated
public class Test {
  
    public Object obj;
  
    public static void main(String[] args)
        throws ClassNotFoundException
    {
        try {
  
            // returns the Class object
            // for this myAnnotatedElement
            AnnotatedElement myAnnotatedElement
                = Test.class;
  
            System.out.println(
                "AnnotatedElement represented"
                + " by myAnnotatedElement: "
                + myAnnotatedElement.toString());
  
            // Get the declared annotation
            // using getDeclaredAnnotation() method
            System.out.println(
                "DeclaredAnnotation of myAnnotatedElement: "
                + myAnnotatedElement
                      .getDeclaredAnnotation(
                          Deprecated.class));
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

输出：

AnnotatedElement represented by myAnnotatedElement: class Test
DeclaredAnnotation of myAnnotatedElement: @java.lang.Deprecated()

参考： https://docs.oracle.com/javase/9/docs/api/ Java/lang/reflect/AnnotatedElement.html#getDeclaredAnnotation-java.lang.Class-