{getToc} $title={Table of Contents}
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| Spring Context vs. Web Container Diagram |
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.
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init(FilterConfig config) — This is invoked only once. It is used to
initialize the filter.
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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.
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| Servlet Container Diagram |
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.
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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.
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postHandle(HttpServletRequest request, HttpServletResponse response,
Object handler,
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ModelAndView modelAndView) — This is used to perform operations before
sending the response to the client.
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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
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Filter is related to the Servlet API and HandlerIntercepter is a Spring
specific concept.
- Interceptors will only execute after Filters.
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Fine-grained pre-processing tasks are suitable for HandlerInterceptors
(authorization checks, etc.)
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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.
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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.
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| Filter vs. Interceptor in Spring Boot |
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.
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| Spring Cloud Gateway Custom Filters |
