What are the types and functions of Java annotations?

Java annotations are used to provide metadata for code elements and can be used for metaprogramming, error checking, code generation, document generation and reflection. The Spring framework extensively uses annotations for configuration, simplifying application development.

Java annotations: types and functions

Annotations are added to Java code elements (classes, methods, fields, etc.) Special metadata. They provide the compiler and runtime with additional information about these elements, which can be used to perform various tasks, such as:

Annotation types

The Java language defines The following types of annotations:

• Marker Annotations: does not receive any parameters and is only used as a marker.
• Single-Valued Annotations: Receives a single parameter.
• Multi-Valued Annotations: Accepts multiple parameters.

The role of annotations

Metaprogramming:
Annotations allow metadata to be accessed at compile time and runtime, thereby enabling metaprogramming programming.

Error checking:
Annotations can be used to check whether the code follows specific rules and generate compile-time errors when errors are found.

Code generation:
Annotations can be used to generate code, such as getter and setter methods, or to generate entire classes through libraries such as Lombok.

Documentation generation:
Annotations can be used to generate Javadoc, detailing classes, methods and fields.

Reflection:
Annotations can be retrieved via reflection, allowing programs to dynamically obtain information about code elements.

Frameworks such as Spring Framework make extensive use of annotations for configuration, simplifying application development.

Practical case: Spring annotation @Controller

// 标记一个类为 Spring MVC 控制器
@Controller
public class MyController {

    // 添加处理 HTTP GET 请求的方法（方法名为请求路径）
    @GetMapping("/")
    public String index() {
        // 返回一个视图模板的名称
        return "index";
    }
}

This annotation instructs the Spring framework to recognize the MyController class as a controller and create A handler method mapped to the "/" path.

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