Filter vs. Interceptor in Spring Boot

What is a Servlet?

It is a Java class that is used to extend the capabilities of servers that host applications accessed by means of a request-response programming model.

Java Servlet technology defines HTTP-specific servlet classes. The?javax.servlet?and?javax.servlet.http?packages provide interfaces and classes for writing servlets.?Servlet?in “javax.servlet” package declares three essential methods for the life cycle of a servlet — init(), service(), and destroy().

What is a Servlet Container?

Servlets run in a?servlet container?which handles the networking side (e.g. parsing an HTTP request, connection handling etc).?Tomcat?is the most popular one.

Filter

It?is a Java class which is executed by the?servlet container?for each incoming HTTP?request?and for each HTTP?response.

Requests always first pass through Filter instances, before reaching a Servlet.

If you have multiple custom filters in your application, you can define the order with “@Order” annotation.

• init(FilterConfig config)?— This is invoked only once. It is used to initialize the filter.
• doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)?— This method is invoked every time a user send a request to any resource, to which the filter is mapped. It is used to perform filtering tasks.
• destroy()?— This is invoked only once when filter is taken out of the service.

Interceptor

Spring Interceptors are similar to Servlet Filters. An interceptor just allows custom pre-processing with the option of prohibiting the execution of the handler itself, and custom post-processing, having access to Spring Context.

• preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)?— This is used to perform operations before sending the request to the controller. This method should return true to return the response to the client.
• postHandle(HttpServletRequest request, HttpServletResponse response, Object handler,
• ModelAndView modelAndView)?— This is used to perform operations before sending the response to the client.
• afterCompletion(HttpServletRequest request, HttpServletResponse response,
• Object handler, Exception exception)?— This is used to perform operations after completing the request and response.

HandlerInterceptor :?HandlerInterceptor instances are executed as part of the request handling inside the DispatcherServlet (which implements?javax.servlet.Servlet).

HandlerInterceptorAdapter:?If you would like to provide a custom implementation and only care for a few of their methods (if you do not want to create empty methods that requires overriding), it is better to implement an adapter.

Filters vs HandlerInterceptors

• Filter?is related to the Servlet API and?HandlerIntercepter?is a Spring specific concept.
• Interceptors will only execute after?Filters.
• Fine-grained pre-processing tasks are suitable for?HandlerInterceptors (authorization checks, etc.)
• Content handling related or generic flows are well-suited for?Filters (such as multipart forms, zip compression, image handling, logging requests, authentication etc.)
• Interceptor’s?postHandle?method will allow you to add more model objects to the view but you can not change the HttpServletResponse since it's already committed.
• Filter’s?doFilter?method is much more versatile than?Interceptor’s?postHandle. You can change the request or response and pass it to the chain or even block the request processing.
• A?HandlerInterceptor?gives more fine-grained control than a filter because you have access to the actual target “handler”. You can even check if the handler method has a specific annotation.

Spring Cloud Gateway Custom Filters

GlobalFilter

Global Filters affect every single request.

GatewayFilter

Gateway Filters applies to only some routes.

Order:?If we want to configure the position of the filter in the filter chain, we can retrieve an?OrderedGatewayFilter?instance.