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## org.springframework.web.servlet
Interface HandlerInterceptor

All Known Implementing Classes:
  ConversionServiceExposingInterceptor, HandlerInterceptorAdapter, LocaleChangeInterceptor, ThemeChangeInterceptor, UserRoleAuthorizationInterceptor, WebContentInterceptor, WebRequestHandlerInterceptorAdapter

    public interface HandlerInterceptor

Workflow interface that allows for customized handler execution chains. Applications can register any number of existing or custom interceptors for certain groups of handlers, to add common preprocessing behavior without needing to modify each handler implementation.

A HandlerInterceptor gets called before the appropriate HandlerAdapter triggers the execution of the handler itself. This mechanism can be used for a large field of preprocessing aspects, e.g. for authorization checks, or common handler behavior like locale or theme changes. Its main purpose is to allow for factoring out repetitive handler code.

Typically an interceptor chain is defined per HandlerMapping bean, sharing its granularity. To be able to apply a certain interceptor chain to a group of handlers, one needs to map the desired handlers via one HandlerMapping bean. The interceptors themselves are defined as beans in the application context, referenced by the mapping bean definition via its "interceptors" property (in XML: a <list> of <ref>).

HandlerInterceptor is basically similar to a Servlet 2.3 Filter, but in contrast to the latter it just allows custom pre-processing with the option of prohibiting the execution of the handler itself, and custom post-processing. Filters are more powerful, for example they allow for exchanging the request and response objects that are handed down the chain. Note that a filter gets configured in web.xml, a HandlerInterceptor in the application context.

As a basic guideline, fine-grained handler-related preprocessing tasks are candidates for HandlerInterceptor implementations, especially factored-out common handler code and authorization checks. On the other hand, a Filter is well-suited for request content and view content handling, like multipart forms and GZIP compression. This typically shows when one needs to map the filter to certain content types (e.g. images), or to all requests.

Since:
  20.06.2003
Author:
  Juergen Hoeller
See Also:
  `HandlerExecutionChain.getInterceptors()`, `HandlerInterceptorAdapter`, `AbstractHandlerMapping.setInterceptors(java.lang.Object[])`, `UserRoleAuthorizationInterceptor`, `LocaleChangeInterceptor`, `ThemeChangeInterceptor`, `Filter`

Method Summary
`void` | `afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)` Callback after completion of request processing, that is, after rendering the view.
`void` | `postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)` Intercept the execution of a handler.
`boolean` | `preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)` Intercept the execution of a handler.

Method Detail

### preHandle

    boolean preHandle(HttpServletRequest request,
                      HttpServletResponse response,
                      Object handler)
                      throws Exception

  Intercept the execution of a handler. Called after HandlerMapping determined an appropriate handler object, but before HandlerAdapter invokes the handler.

  DispatcherServlet processes a handler in an execution chain, consisting of any number of interceptors, with the handler itself at the end. With this method, each interceptor can decide to abort the execution chain, typically sending a HTTP error or writing a custom response.

  Parameters:
    `request` - current HTTP request
    `response` - current HTTP response
    `handler` - chosen handler to execute, for type and/or instance evaluation
  Returns:
    `true` if the execution chain should proceed with the next interceptor or the handler itself. Else, DispatcherServlet assumes that this interceptor has already dealt with the response itself.
  Throws:
    `Exception` - in case of errors

### postHandle

    void postHandle(HttpServletRequest request,
                    HttpServletResponse response,
                    Object handler,
                    ModelAndView modelAndView)
                    throws Exception

  Intercept the execution of a handler. Called after HandlerAdapter actually invoked the handler, but before the DispatcherServlet renders the view. Can expose additional model objects to the view via the given ModelAndView.

  DispatcherServlet processes a handler in an execution chain, consisting of any number of interceptors, with the handler itself at the end. With this method, each interceptor can post-process an execution, getting applied in inverse order of the execution chain.

  Parameters:
    `request` - current HTTP request
    `response` - current HTTP response
    `handler` - chosen handler to execute, for type and/or instance examination
    `modelAndView` - the `ModelAndView` that the handler returned (can also be `null`)
  Throws:
    `Exception` - in case of errors

### afterCompletion

    void afterCompletion(HttpServletRequest request,
                         HttpServletResponse response,
                         Object handler,
                         Exception ex)
                         throws Exception

  Callback after completion of request processing, that is, after rendering the view. Will be called on any outcome of handler execution, thus allows for proper resource cleanup.

  Note: Will only be called if this interceptor's `preHandle` method has successfully completed and returned `true`!

  As with the `postHandle` method, the method will be invoked on each interceptor in the chain in reverse order, so the first interceptor will be the last to be invoked.

  Parameters:
    `request` - current HTTP request
    `response` - current HTTP response
    `handler` - chosen handler to execute, for type and/or instance examination
    `ex` - exception thrown on handler execution, if any
  Throws:
    `Exception` - in case of errors

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