Analyze the working principle of the "Annotations" framework in the Java library

Annotations is a special metadata in Java to provide additional information about program elements (classes, methods, variables, etc.).They are widely used in the framework in the Java library to provide functions such as configuration, compilation, processing and generating documents during runtime. working principle: 1. Definition annotation: Annotations are defined by using Java's meta -nnotations.Metropolitan annotations are special annotations used to annotate other annotations.Java provides several meta -solutions, such as@Retention,@Target, and @Documented. 2. Note processor: The annotation processor is a tool for processing the annotation in the program.They read and analyze the annotations through the reflection mechanism, and perform corresponding logic when compiling or runtime.The annotation processor can extract information from the annotations provided in the framework of the Java library and perform the corresponding operation according to the content of the annotation. 3. Visible when runtime: Annotations can specify when it is visible by using meta annotations @Retention.There are three RetentionPolicy options: Source, Class, and Runtime.If the annotation is declared using @RETENTION (RetentionPolicy.source), the annotation can be seen in the source code and will not be included in the compiled class file or in running environment.If you use @RETENTION (RetentionPolicy.class) to declare, the annotation will be included in the compiled file, but it is not visible at runtime.The annotations that use the @RETENTION (RetentionPolicy.runtime) to be declared, which can be seen in the compiled class files and runtime environments. 4. Objective element: Annotations can use meta -annotation @Target specified program element types that can be annotated.For example, annotations can be applied to class, methods, member variables, etc.When using @target annotations, you must specify an ElementType value to determine the type of target element that can be applied. 5. Note use: Once the annotations are defined, they can be used in the program.The annotation can be applied by adding the annotation name before the program element.For example, you can use the annotation @Myannotation to mark a class before the class definition.The annotation can also contain elements (member variables), and the values of these elements can be set when using the annotation. Below is a simple example, demonstrating how to define and use an annotation: import java.lang.annotation.*; // Define an annotation @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) public @interface MyAnnotation { String value() default ""; int count() default 0; } // Use annotations public class MyClass { @MyAnnotation(value = "Hello", count = 5) public void myMethod() { // ... } } // Analysis annotation import java.lang.reflect.*; public class AnnotationParser { public static void main(String[] args) throws Exception { Method method = MyClass.class.getMethod("myMethod"); MyAnnotation annotation = method.getAnnotation(MyAnnotation.class); System.out.println("Value: " + annotation.value()); System.out.println("Count: " + annotation.count()); } } In the above example, we define an annotation called Myannotation, which has two elements Value and Count.Then, we used this annotation on the MyMethod method of the MyClass class.Finally, we used the reflex mechanism to analyze the annotations in the Mymethod method and print the value of the annotation element. The working principle of the Annotations framework is based on the JAVA's reflection mechanism and the mutual cooperation of the annotation processor.Many of the frameworks in the Java class library use Annotations to provide configuration and processing functions.By using annotations, the code can be more easy to read, maintain, and provide more flexibility and scalability.