Understand and apply how Spring interceptors work

Decrypting the operating principles and practices of Spring interceptors

Introduction:
In web development, interceptors are a very important concept. It can perform some additional processing logic before or after the request is processed. In the Spring framework, we can use interceptors to implement various functions, such as authentication, logging, parameter verification, etc. This article will delve into how Spring interceptors work and provide some practical example code.

1. How the Spring interceptor works
In Spring, the interceptor is implemented through AOP (aspect-oriented programming). Interceptors mainly involve three core concepts: Interceptor Chain, HandlerInterceptor interface and its implementation class, and interceptor configuration.

1. Interceptor Chain: The interceptor chain consists of a series of interceptors, which are executed one by one in the configured order. The execution order of the interceptor chain can be controlled by encoding order, annotation order, or order in the XML configuration file.
2. HandlerInterceptor interface and its implementation class: HandlerInterceptor is an interface defined in the Spring framework and is used to define the behavior of the interceptor. Classes that implement this interface can implement custom interception logic based on requirements.
3. Interceptor configuration: In Spring, interceptors can be configured through annotations or XML configuration files. Through the configuration file, we can specify the path of the interceptor, the order in which the interceptor is applied, etc.

2. Practical Example
Next, we will use a simple example to demonstrate how to implement and use Spring interceptor. The sample code is based on Spring Boot and Spring MVC. The specific steps are as follows:

1. Create a Spring Boot project:
   First, we need to create a Spring Boot project. You can create a basic Spring Boot project by selecting Spring Initializr in the IDE, or add related dependencies manually.
2. Create a custom interceptor class:
   Create a new package in the src/main/java directory and name it com.example.interceptor. Then create a class named AuthInterceptor under the package and implement the HandlerInterceptor interface. In this class, we can define the interception logic that needs to be executed. The following is a sample code:

package com.example.interceptor;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;

public class AuthInterceptor implements HandlerInterceptor {

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
        // 在请求被处理之前执行的逻辑
        // 这里可以放置需要进行身份验证的逻辑
        return true; // 返回true表示继续执行后续的拦截器和处理器方法，返回false表示中断执行
    }

    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler,
            ModelAndView modelAndView) throws Exception {
        // 在请求被处理之后执行的逻辑
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
            throws Exception {
        // 在整个请求结束之后执行的逻辑
    }
}

3. Configuring the interceptor:
   Next, we need to configure the interceptor in the configuration file of the Spring Boot project. In the src/main/resources directory, find the application.properties or application.yml file (according to your own project configuration file type), and add the following configuration:

# 配置拦截器
spring.mvc.interceptor.include=/api/** # 拦截所有以/api/开头的请求
spring.mvc.interceptor.exclude=/api/login # 排除对/api/login请求的拦截
spring.mvc.interceptor.order=1 # 配置拦截器的顺序

4. Start the application:
   Start the application in the IDE or use the Maven command. After startup, you can visit http://localhost:8080/api/test for testing. The interceptor will execute the corresponding logic before the request is processed.

Conclusion:
This article deeply explores the operating principle of Spring interceptor and provides a practical example to demonstrate how to use Spring interceptor. By understanding the working principles and practical applications of interceptors, we can better apply interceptors to meet actual needs and improve the security and scalability of web applications.

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