java.lang.reflect.Constructor.getAnnotation()方法範例

2019-10-16 22:48:59

如果存在註釋,java.lang.reflect.Constructor.getAnnotation(Class <T> annotationClass)方法返回指定型別的元素的注釋,否則返回為null

宣告

以下是java.lang.reflect.Constructor.getAnnotation(Class <T> annotationClass)方法的宣告。

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

引數

  • annotationClass - 與注釋型別對應的Class物件。

返回值

如果存在於此元素,返回該元素的注釋指定的注釋型別,否則返回:null

例外

  • NullPointerException - 如果給定的注釋類為空。

例子

以下範例顯示了java.lang.reflect.Constructor.getAnnotation(Class <T> annotationClass)方法的用法。

import java.lang.annotation.Annotation;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Constructor;

public class ConstructorDemo {
   public static void main(String[] args) {
      Constructor[] constructors = SampleClass.class.getConstructors();

      Annotation annotation = constructors[0].getAnnotation(CustomAnnotation.class);
      if(annotation instanceof CustomAnnotation){
         CustomAnnotation customAnnotation = (CustomAnnotation) annotation;
         System.out.println("name: " + customAnnotation.name());
         System.out.println("value: " + customAnnotation.value());
      }
   }
}

@CustomAnnotation(name="SampleClass",  value = "Sample Class Annotation")
class SampleClass {
   private String sampleField;

   @CustomAnnotation(name="sampleClassConstructor",  value = "Sample Constructor Annotation")
   public SampleClass(){
   }

   public SampleClass(String sampleField){
      this.sampleField = sampleField;
   }

   public String getSampleField() {
      return sampleField;
   }

   public void setSampleField(String sampleField) {
      this.sampleField = sampleField;
   } 
}

@Retention(RetentionPolicy.RUNTIME)
@interface CustomAnnotation {
   public String name();
   public String value();
}

讓我們編譯並執行上面的程式,這將產生以下結果 -

name: sampleClassConstructor
value: Sample Constructor Annotation