Java Field getAnnotation()方法及示例

java.lang.reflect.Field 的 getAnnotation() 方法用于返回指定类型的Field对象的注释，如果存在这样的注释，则返回null。

语法

public <T extends Annotation> T
  getAnnotation(Class<T> annotationClass)

参数： 该方法 的annotationClass 是对应于注释类型的Class对象。

返回：如果该元素上存在指定的注释类型，该方法返回该元素的注释，否则为空。

异常：如果给定的注释类是空的，这个方法会抛出 NullPointerException 。

以下程序说明了getAnnotation()方法：
程序1 :

// Java program to illustrate
// getAnnotation() method
  
import java.lang.annotation.*;
import java.lang.reflect.AnnotatedType;
import java.lang.reflect.Field;
import java.util.Arrays;
  
public class GFG {
  
    // initialize field with
    // default value in annotation
    @annotations(3125462345.32155365326)
  
    private double realNumbers;
  
    public static void main(String[] args)
        throws NoSuchFieldException
    {
  
        // create Field object
        Field field
            = GFG.class
                  .getDeclaredField("realNumbers");
  
        // apply getAnnotation()
        annotations annotations
            = field.getAnnotation(
                annotations.class);
  
        // print results
        System.out.println(annotations);
    }
  
    @Target({ ElementType.FIELD })
    @Retention(RetentionPolicy.RUNTIME)
    private @interface annotations {
        double value() default 99.9;
    }
}

输出:

@GFG$annotations(value=3.1254623453215537E9)

程序2

// Java program to illustrate
// getAnnotation() method
  
import java.lang.annotation.*;
import java.lang.reflect.AnnotatedType;
import java.lang.reflect.Field;
import java.util.Arrays;
  
public class GFG {
    private int @SpecialNumber[] number;
  
    public static void main(String[] args)
        throws NoSuchFieldException
    {
        // get Field object
        Field field
            = GFG.class
                  .getDeclaredField("number");
  
        // apply getAnnotation() method
        AnnotatedType annotatedType
            = field.getAnnotation();
  
        // print the results
        System.out.println(
            "Type: "
            + annotatedType.getType()
                  .getTypeName());
        System.out.println(
            "Annotations: "
            + Arrays.toString(
                  annotatedType
                      .getAnnotations()));
        System.out.println(
            "Declared Annotations: "
            + Arrays.toString(
                  annotatedType
                      .getDeclaredAnnotations()));
    }
  
    @Target({ ElementType.TYPE_USE })
    @Retention(RetentionPolicy.RUNTIME)
    private @interface SpecialNumber {
    }
}

输出: