Detailed explanation of the working mechanism of Spring interceptor

In-depth analysis of the working principle of Spring interceptor requires specific code examples

Introduction:
Spring framework is one of the most commonly used frameworks in Java development. It has rich functions and powerful scalability. Among them, the interceptor (Interceptor), as one of the commonly used components in the Spring framework, plays a key role in actual development. This article will provide an in-depth analysis of how Spring interceptors work and provide specific code examples to help readers better understand and apply interceptors.

1. What is an interceptor?
The interceptor is an interception processing mechanism provided by the Spring framework, which is used to perform a series of processing operations before and after the target method is called. Interceptors can be used for logging, performance monitoring, permission control, etc. In Spring MVC, interceptors are mainly used to implement request pre-processing and post-processing. Common application scenarios include: login verification, cross-domain processing, data encryption and decryption, etc.

2. How the interceptor works

1. Define the interceptor interface
   In the Spring framework, the interceptor is completed by implementing the HandlerInterceptor interface. This interface contains three methods:
2. preHandle method, which intercepts before the target method is called.
3. postHandle method, intercepts processing after the target method is called.
4. afterCompletion method performs interception processing after the target method call is completed.

The following is a code example of the HandlerInterceptor interface:

public interface HandlerInterceptor {
    boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;
    void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception;
    void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception;
}

2. Registering the interceptor
   In order for the Spring framework to use the defined interceptor, the interceptor needs to be registered. Interceptors can be registered through configuration files or annotations. The following is an example of registration through a 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 example, we pass <interceptors> The </interceptors> tag registers the interceptor into the Spring framework. <mapping></mapping> tag is used to specify the intercepted request path, <exclude-mapping></exclude-mapping> tag is used to exclude the specified request path, <bean> tag is used to specify the interceptor class. </bean>

3. Execution order of interceptors
   In the Spring framework, multiple interceptors can be registered at the same time and executed according to the configured order. The execution order of interceptors follows the order of registration, that is, they are called in the order of configuration. During a request, if there are multiple interceptors, they will be executed in the order in which they were registered.
4. Interceptor application example
   In order to better understand and apply interceptors, an example is given below. Suppose we need to implement a login verification interceptor. The specific code example is as follows:

public class LoginInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        // 判断用户是否登录
        User user = (User) request.getSession().getAttribute("user");
        if (user == null) {
            // 未登录，跳转到登录页面
            response.sendRedirect(request.getContextPath() + "/login");
            return false;
        }
        return true;
    }

    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
        // 后处理
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        // 完成处理
    }
}

In the above code example, we implemented a LoginInterceptor interceptor and overridden the preHandle method. In the preHandle method, we first determine whether the user is logged in. If not, redirect to the login page.

3. Summary
This article provides an in-depth analysis of the working principle of Spring interceptor and provides an interceptor example for login verification. As one of the commonly used components in the Spring framework, interceptors play an important role. By using interceptors, functions such as login verification and permission control can be implemented to improve the security and scalability of the system.

Through the introduction of this article, readers can better understand and apply Spring interceptors, and use them flexibly in combination with specific development scenarios. I hope this article will be helpful to readers, and we welcome your valuable comments and suggestions.

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