ObjectOS :: Auto :: Annotations in the framework and its role in the Java class library (meta-Nanotations in Objectos :: Auto :: Annotations Framework and their Roles in Java Class Librarr IES)
Yuan Note is a special annotation that is used to annotate other annotations.In the Java class library, there are many behaviors that are used for definition, configuration and control annotations.In ObjectOS :: Auto :: Annotations framework, some commonly used meta -solutions are also defined to make further customization on the basis of annotations.
The following is the commonly used in ObjectOS :: auto :: Annotations framework and their role in the Java library:
1. @Target: Specify the type of target element that can be applied to the annotation.Can be used for class, interface, enumeration, field, method, etc.For example, using @Target (ElementType.Method) can apply the annotation to the method.
2. @Retention: Specify the life cycle of the annotation.Java provides three life cycles: Source, Class, and Runtime.Source -level annotations were discarded during compilation, Class -level annotations were retained during compilation, and it was not visible at runtime, and Runtime -level annotations were visible at runtime.
3. @Documented: Specify whether the annotation is included in the generated Java document.If an annotation is annotated by @Documented, it will be included in the generated document, otherwise it will not.
4. @Inherited: Specify whether the annotation can be inherited.If a class is marked as a @Inherited annotation, the subclass will inherit the annotation.
5. @repeatable: Specify whether the annotation can be repeated.Before the Java 8, the annotation can only be applied once in the same position.However, by labeling the annotation as @Repetable and using a container annotation to pack it, it can make it use many times in the same position.
These meta -notes play an important role in the Java library.They provide a mechanism to define and control annotations, enabling developers to more flexibly use annotations to describe and customize code behaviors.
The following is a simple Java code example using meta -annotations:
import java.lang.annotation.*;
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface MyAnnotation {
String value() default "";
}
public class MyClass {
@MyAnnotation("Hello")
public void myMethod() {
System.out.println("My Method");
}
}
public class Main {
public static void main(String[] args) {
MyClass myClass = new MyClass();
Class<?> cls = myClass.getClass();
MyAnnotation myAnnotation = cls.getMethod("myMethod").getAnnotation(MyAnnotation.class);
System.out.println(myAnnotation.value()); // Output: Hello
}
}
In the above example, we define a custom annotation @Myannotation, and the annotation is applied to the method of the MyClass class.By using reflexes, we can get annotations and get its value.
The use of these meta -annotations makes the annotations become a powerful and flexible tool in Java development, which can easily increase the readability, additional metadata, and implementation of specific behaviors.
