Java中的字段 getAnnotation() 方法及示例
Java.lang.reflect.Field的getAnnotation()方法用于返回返回指定类型的Field 对象,如果存在这样的注解,否则为null。这是获取Field 对象注解的重要方法。
句法:
public T
getAnnotation(Class annotationClass)
参数:该方法annotationClass是注解类型对应的Class对象。
返回:此方法返回此元素的指定注释类型的注释(如果存在于此元素上),否则返回 null。
异常:如果给定的注释类为空,此方法将抛出NullPointerException 。
下面的程序说明了 getAnnotation() 方法:
方案一:
// 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$annotations(value=3.1254623453215537E9)
方案二:
// Java program to illustrate
// getAnnotation() method
import java.lang.annotation.*;
import java.lang.reflect.AnnotatedType;
import java.lang.reflect.Field;
import java.util.Arrays;
public class GFG {
private int @SpecialNumber[] number;
public static void main(String[] args)
throws NoSuchFieldException
{
// get Field object
Field field
= GFG.class
.getDeclaredField("number");
// apply getAnnotation() method
AnnotatedType annotatedType
= field.getAnnotation();
// print the results
System.out.println(
"Type: "
+ annotatedType.getType()
.getTypeName());
System.out.println(
"Annotations: "
+ Arrays.toString(
annotatedType
.getAnnotations()));
System.out.println(
"Declared Annotations: "
+ Arrays.toString(
annotatedType
.getDeclaredAnnotations()));
}
@Target({ ElementType.TYPE_USE })
@Retention(RetentionPolicy.RUNTIME)
private @interface SpecialNumber {
}
}
输出:
@GFG$annotations(value=WelcomeTOGFG)
参考文献: https: Java/lang/reflect/Field.html#getAnnotation–