Java @Retention 注解

在Java，注释用于将元数据附加到程序元素，例如类、方法、实例等。一些注释用于注释其他注释。这些类型的注释称为元注释。 @Retention 也是一个带有一些保留策略的元注释。这些保留策略决定了注释在什么时候被丢弃。存在三种类型的保留策略：SOURCE、CLASS 和 RUNTIME。

• RetentionPolicy.SOURCE：使用 SOURCE 保留策略注释的注释在运行时被丢弃。
• RetentionPolicy.CLASS：使用 CLASS 保留策略注释的注释记录在 .class 文件中，但在运行时被丢弃。 CLASS 是Java的默认保留策略。
• RetentionPolicy.RUNTIME：使用 RUNTIME 保留策略注释的注释在运行时保留，并且可以在运行时在我们的程序中访问。

执行：

在这里，我们将使用保留策略（例如 SOURCE、CLASS 和 RUNTIME）创建三个自定义注释。这些自定义注解稍后用于注解三个类，即 A、B 和 C。在 main 方法中，我们检查注解是否在运行时附加到类。

例子

Java

// Java Program to Illustrate Retention Annotations
  
// Importing required classes from java.lang package
import java.lang.annotation.Annotation;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
  
// Here we will be creating 3 annotations with
// RetentionPolicy as SOURCE, CLASS, & RUNTIME
  
// Retention Annotation 1
@Retention(RetentionPolicy.SOURCE)
  
// Interaface
@interface SourceRetention
{
    String value() default "Source Retention";
}
  
// Retention Annotation 2
@Retention(RetentionPolicy.CLASS)
  
// Interface
@interface ClassRetention
{
    String value() default "Class Retention";
}
  
// Retention Annotation 3
@Retention(RetentionPolicy.RUNTIME)
  
// Interface
@interface RuntimeRetention
{
    String value() default "Runtime Retention";
}
  
// Annotating classes A, B, and C
// with our custom annotations
@SourceRetention
class A {
}
  
@ClassRetention
class B {
}
  
@RuntimeRetention
class C {
};
  
// Main class
public class RetentionPolicyDemo {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Obtaining the array of annotations used to
        // annotate class A, B, and C. Array a and b will be
        // empty as their annotation are attached before
        // runtime while array c will contain the
        // RuntimeRetention annotation as it was marked with
        // RUNTIME retention policy
        Annotation a[]
            = new A().getClass().getAnnotations();
        Annotation b[]
            = new B().getClass().getAnnotations();
        Annotation c[]
            = new C().getClass().getAnnotations();
  
        // Printing the number of retained annotations of
        // each class at runtime
        System.out.println(
            "Number of annotations attached to "
            + "class A at Runtime: " + a.length);
  
        System.out.println(
            "Number of annotations attached to "
            + "class B at Runtime: " + b.length);
  
        System.out.println(
            "Number of annotations attached to "
            + "class C at Runtime: " + c.length);
  
        // Since the class C is annotated with an annotation
        // which which has retention policy as runtime so it
        // can be accessed during runtime while annotations
        // of other two classes are discarded before runtime
        // so they can't be accessed
        System.out.println(
            "Annotation attached to class C: " + c[0]);
    }
}

输出

Number of annotations attached to class A at Runtime: 0
Number of annotations attached to class B at Runtime: 0
Number of annotations attached to class C at Runtime: 1
Annotation attached to class C: @RuntimeRetention(value="Runtime Retention")