Java中的类 getAnnotation() 方法和示例

Java.lang.Class 类的getAnnotation()方法用于获取指定注解类型的注解，如果此类注解存在。该方法以对象的形式返回该类。

句法：

public T getAnnotation(Class annotationClass)

参数：此方法接受一个参数annotationClass ，它是要获取的注释的类型。

返回值：该方法返回注解类的指定对象。

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 getAnnotation() 方法。

示例 1：

// Java program to demonstrate
// getAnnotation() method
  
import java.util.*;
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 class
        Class myClass = Test.class;
  
        System.out.println("Class represented by myClass: "
                           + myClass.toString());
  
        // Get the annotation
        // using getAnnotation() method
        System.out.println(
            "Annotation of myClass: "
            + myClass.getAnnotation(
                  Annotation.class));
    }
}

输出：

Class represented by myClass: class Test
Annotation of myClass: @Annotation(key=GFG, value=GeeksForGeeks)

示例 2：

// Java program to demonstrate
// getAnnotation() method
  
import java.util.*;
import java.lang.annotation.*;
  
// Class with no annotations
// getAnnotation() will return null
public class Test {
  
    public static void main(String[] args)
        throws ClassNotFoundException
    {
  
        // returns the Class object for this class
        Class myClass = Test.class;
  
        System.out.println("Class represented by myClass: "
                           + myClass.toString());
  
        // Get the annotation
        // using getAnnotation() method
        System.out.println(
            "Annotation of myClass: "
            + myClass.getAnnotation(
                  Annotation.class));
    }
}

输出：

Class represented by myClass: class Test
Annotation of myClass: null

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