Introduction

Introduction In the Java class library, Annotations is a special grammar data data to add additional information to the program code.Note can be used on code elements such as class, methods, fields, and parameters to provide richer program description and guidance information.By using annotations, developers can add some additional features, functions and constraints to the program without changing the original code logic. The definition and use of Java annotations are very simple. You can define an annotation in the following ways: @interface MyAnnotation { String value(); int[] numbers() default {0}; boolean enabled() default true; } The above code defines an annotation called `myannotation`, which contains three member variables:` Value`, `Numbers` and` Enabled`.The member variables of the `value` use the default name, and the variables of members of` numbers` and `enabled` define the default values.Developers can customize more member variables as needed. Using annotations can be achieved by adding the@``@`symbol and specifying the corresponding annotation name before the code element, for example: @MyAnnotation(value = "Hello", numbers = {1, 2, 3}, enabled = false) public class MyClass { // ... } In this example, the `MyClass` class is modified by`@myannotation`, and specify the corresponding value for its member variables. Using annotations can achieve many useful functions in the program.For example: 1. Checking during compilation: You can use the annotation as a class or method to add some restrictions or constraints. The compiler will perform related inspections through annotations.For example, the annotation of `@& oVerride` can be used to ensure that the method in the subclass covers the method of the same name in the parent class, and the compiler will check whether the coverage conditions are met during compilation. 2. Treatment during runtime: The annotation can be used to write some custom tools or frameworks, and the corresponding logic can be performed by scanning the information and executing the corresponding logic.For example, the annotation of `@test` can be used for testing frameworks, marking the method of testing cases. The test framework will automatically perform the corresponding test logic according to the annotation information. 3. Document generation: Note can be used to automatically generate documents or other types of files that automatically generate code.By reading the relevant injection information, the corresponding document content can be generated according to certain rules.For example, the name of the author who can be used to mark the code can be used to automatically generate the corresponding author list by reading the annotation information. In short, the annotation framework is a very useful part of the Java library.By using annotations, developers can add more metadata information to the program to provide richer functions and constraints.Through custom annotations and corresponding processors, some custom functions and tools can also be achieved, which greatly improves the development efficiency and flexibility of the Java program.