📜  Java中的构造函数 getAnnotation() 方法及示例

📅  最后修改于: 2022-05-13 01:55:16.672000             🧑  作者: Mango

Java中的构造函数 getAnnotation() 方法及示例

如果存在这样的注解,则使用Constructor类的getAnnotation()方法获取指定类型的此构造函数对象注解,否则为 null。指定的类型作为参数传递。

句法:

public  T
  getAnnotation(Class annotationClass)

参数:该方法接受一个参数annotationClass ,它表示对应于注解类型的Class对象。

返回值:如果此元素上存在指定的注释类型,则此方法返回此元素的注释,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 demonstrate
// Constructor.getAnnotation() method
  
import java.lang.annotation.Annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.AnnotatedType;
import java.lang.reflect.Constructor;
  
public class GFG {
  
    public static void main(String... args)
        throws NoSuchMethodException
    {
  
        // Create Constructor Object
        Constructor[] constructors
            = Demo.class.getConstructors();
  
        // Create annotation object
        Annotation annotation
            = constructors[0]
                  .getAnnotation(PathVar.class);
  
        if (annotation instanceof PathVar) {
  
            PathVar customAnnotation
                = (PathVar)annotation;
            System.out.println(
                "Path: "
                + customAnnotation.Path());
        }
    }
}
  
// Demo class
class Demo {
    public Demo(@PathVar(Path = "Demo/usr")
                String str) {}
}
  
// PathVar interface
@Target({ ElementType.TYPE_USE })
@Retention(RetentionPolicy.RUNTIME)
@interface PathVar {
    public String Path();
}

输出:

Path: Demo/usr

方案二:

// Java program to demonstrate
// Constructor.getAnnotation() method
  
import java.lang.annotation.Annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.Constructor;
  
public class GFG {
  
    public static void main(String... args)
        throws NoSuchMethodException
    {
  
        // Create Constructor Object
        Constructor[] constructors
            = Maths.class.getConstructors();
  
        // Create annotation object
        Annotation annotation
            = constructors[0]
                  .getAnnotation(Calculator.class);
  
        System.out.println(
            "Annotation:"
            + annotation.toString());
    }
}
  
// Demo class
@Calculator(add = "Adding value",
            subtract = "Subtracting Value")
class Maths {
  
    @Calculator(add = "Adding value",
                subtract = "Subtracting Value")
    public Maths() {}
}
  
// Calculator interface
@Retention(RetentionPolicy.RUNTIME)
@interface Calculator {
    public String add();
    public String subtract();
}

输出:

Annotation : @Calculator(add=Adding value, subtract=Subtracting Value)

参考资料:https: Java Java.lang.Class)