Explore the implementation principle of Spring interceptor

Revealing the implementation mechanism of Spring interceptor

Introduction

When developing web applications, we often need to wait before or after the request reaches the controller Do something. For example, authenticate users, record logs, handle exceptions, etc. The Spring framework provides us with interceptors (Interceptors) to implement these operations. Interceptors can pre-process and post-process requests and responses.

This article will delve into the implementation mechanism of Spring interceptor. We will understand what interceptors are, how they work, and demonstrate how to implement custom interceptors through specific code examples.

The concept of interceptor

Interceptor is a mechanism in the Spring framework used to pre-process and post-process requests. It is similar to filters in Servlets, but unlike filters, interceptors are method-level based. It can intercept methods in the specified Controller and execute custom logic before and after the method is executed. Interceptors can help us implement some general processing that has nothing to do with business logic and improve code reusability and maintainability.

How the interceptor works

The interceptor is implemented through AOP (aspect-oriented programming). Spring interceptor is based on the HandlerInterceptor interface, which defines three methods: preHandle, postHandle and afterCompletion. The specific workflow is as follows:

1. When the client sends a request to DispatcherServlet, DispatcherServlet will find the corresponding HandlerMapping based on the requested URL and obtain the corresponding HandlerExecutionChain.
2. HandlerExecutionChain contains a series of HandlerInterceptor.

3. DispatcherServlet will call the preHandle method of each interceptor in turn and check the return value of the method.

   • If true is returned, the request continues processing and enters the preHandle method of the next interceptor.
   • If false is returned, the request will be interrupted here and returned to the client.
4. When all interceptor preHandle methods return true, DispatcherServlet will call the handleRequest method of the Handler object in HandlerExecutionChain.
5. The Handler object in HandlerExecutionChain executes business logic and returns ModelAndView.
6. DispatcherServlet will call the postHandle method of each interceptor in turn and pass ModelAndView to them.
7. The postHandle method of the interceptor can modify and enhance ModelAndView.
8. DispatcherServlet passes ModelAndView to ViewResolver for view parsing and rendering.
9. When the view is rendered, DispatcherServlet will call the afterCompletion method of each interceptor in turn to further clean up some resources.

Implementation of custom interceptor

The following uses a specific example to demonstrate how to implement a custom interceptor.

public class MyInterceptor implements HandlerInterceptor {

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        //  在方法执行之前进行逻辑处理
        System.out.println("拦截器preHandle方法执行");
        return true;
    }

    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
        // 在方法执行之后进行逻辑处理
        System.out.println("拦截器postHandle方法执行");
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        // 在视图渲染完成后进行逻辑处理
        System.out.println("拦截器afterCompletion方法执行");
    }
}

In the above code, we implemented the HandlerInterceptor interface and rewritten three of its methods. In the preHandle method, we can perform some pre-processing; in the postHandle method, we can modify and enhance the ModelAndView; in the afterCompletion method, we can perform some resource cleanup operations.

Next, we need to configure the custom interceptor into the Spring container. This can be achieved through XML configuration or annotations.

XML configuration method

Add the following configuration in the Spring configuration file:

<mvc:interceptors>
    <mvc:interceptor>
        <mvc:mapping path="/**" />
        <mvc:exclude-mapping path="/login" />
        <bean class="com.example.MyInterceptor" />
    </mvc:interceptor>
</mvc:interceptors>

In the above configuration, we use <interceptor></interceptor> tag to define the interceptor, and specify the URL path to be intercepted through the <mapping></mapping> tag. Use the <exclude-mapping></exclude-mapping> tag to exclude some URL paths that do not need to be intercepted.

Configuring the interceptor using annotation

Add the @Component annotation to the interceptor class, and use the @Order annotation to specify the execution order of the interceptor .

@Component
@Order(1)
public class MyInterceptor implements HandlerInterceptor {
    // 省略代码
}

Add the following configuration in Spring's configuration class:

@Configuration
public class WebConfig implements WebMvcConfigurer {

    @Autowired
    private MyInterceptor myInterceptor;
    
    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(myInterceptor).addPathPatterns("/**").excludePathPatterns("/login");
    }
}

In the above configuration, we add the interceptor to the interceptor by implementing the WebMvcConfigurer interface and rewriting the addInterceptors method. in the registry.

Conclusion

Through this article, we have understood the concept and working principle of Spring interceptor, and demonstrated how to implement a custom interceptor through specific code examples. Interceptors are a very important feature in the Spring framework, which can help us implement some common processing logic and improve code reusability and maintainability. I hope this article has been helpful to your understanding of Spring interceptors.

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