Java中的 AnnotatedElement isAnnotationPresent() 方法及示例

Java.lang.AnnotatedElement 类的isAnnotationPresent()方法用于检查实现该接口的类中是否存在指定注解类型的注解。该方法返回一个布尔值，说明相同。

句法：

public boolean isAnnotationPresent(Class annotationClass)

参数：这个方法接受一个参数annotationClass ，它是要获取的注解的类型。

返回值：此方法返回说明相同的布尔值。

异常：此方法抛出：

• NullPointerException：如果给定的注释类为空。

下面的程序演示了 isAnnotationPresent() 方法。

示例 1：

// Java program to demonstrate
// isAnnotationPresent() method
  
import java.util.*;
import java.lang.reflect.*;
import java.lang.annotation.*;
  
// create a custom Annotation
@Retention(RetentionPolicy.RUNTIME)
@interface Annotation {
  
    // This annotation has two attributes.
    public String key();
  
    public String value();
}
  
// call Annotation for method
// and pass values for annotation
@Annotation(key = "GFG", value = "GeeksForGeeks")
public class Test {
  
    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());
  
        // Check if there is any annotation
        // using isAnnotationPresent() method
        System.out.println(
            "If there is any annotation"
            + " in myAnnotatedElement: "
            + myAnnotatedElement
                  .isAnnotationPresent(
                      Annotation.class));
    }
}

输出：

AnnotatedElement represented by myAnnotatedElement: class Test
If there is any annotation in myAnnotatedElement: true

示例 2：

// Java program to demonstrate
// isAnnotationPresent() method
  
import java.util.*;
import java.lang.reflect.*;
import java.lang.annotation.*;
  
// Class with no annotations
public class Test {
  
    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 annotation
        // using isAnnotationPresent() method
        System.out.println(
            "If there is any annotation"
            + " in myAnnotatedElement: "
            + myAnnotatedElement
                  .isAnnotationPresent(
                      Annotation.class));
    }
}

输出：

AnnotatedElement represented by myAnnotatedElement: class Test
If there is any annotation in myAnnotatedElement: false

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