## org.springframework.web.filter
Class ShallowEtagHeaderFilter

    java.lang.Object
      org.springframework.web.filter.GenericFilterBean
          org.springframework.web.filter.OncePerRequestFilter
              org.springframework.web.filter.ShallowEtagHeaderFilter

All Implemented Interfaces:
  Filter, BeanNameAware, DisposableBean, InitializingBean, ServletContextAware

    public class ShallowEtagHeaderFilter

extends OncePerRequestFilter

`Filter` that generates an `ETag` value based on the content on the response. This ETag is compared to the `If-None-Match` header of the request. If these headers are equal, the response content is not sent, but rather a `304 "Not Modified"` status instead.

Since the ETag is based on the response content, the response (or `View`) is still rendered. As such, this filter only saves bandwidth, not server performance.

Since:
  3.0
Author:
  Arjen Poutsma

Field Summary

Fields inherited from class org.springframework.web.filter.OncePerRequestFilter
`ALREADY_FILTERED_SUFFIX`

Fields inherited from class org.springframework.web.filter.GenericFilterBean
`logger`

Constructor Summary
`ShallowEtagHeaderFilter()`

Method Summary
`protected void` `doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)` Same contract as for `doFilter`, but guaranteed to be just invoked once per request.
`protected String` `generateETagHeaderValue(byte[] bytes)` Generate the ETag header value from the given response body byte array.
`protected boolean` `isEligibleForEtag(HttpServletRequest request, HttpServletResponse response, int responseStatusCode, byte[] responseBody)` Indicates whether the given request and response are eligible for ETag generation.

Methods inherited from class org.springframework.web.filter.OncePerRequestFilter
`doFilter, getAlreadyFilteredAttributeName, shouldNotFilter`

Methods inherited from class org.springframework.web.filter.GenericFilterBean
`addRequiredProperty, afterPropertiesSet, destroy, getFilterConfig, getFilterName, getServletContext, init, initBeanWrapper, initFilterBean, setBeanName, setServletContext`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### ShallowEtagHeaderFilter

    public ShallowEtagHeaderFilter()

Method Detail

### doFilterInternal

    protected void doFilterInternal(HttpServletRequest request,
                                    HttpServletResponse response,
                                    FilterChain filterChain)
                             throws ServletException,
                                    IOException

  Description copied from class: `OncePerRequestFilter`
  Same contract as for `doFilter`, but guaranteed to be just invoked once per request. Provides HttpServletRequest and HttpServletResponse arguments instead of the default ServletRequest and ServletResponse ones.

  Specified by:
    `doFilterInternal` in class `OncePerRequestFilter`
  Throws:
    `ServletException`
    `IOException`

### isEligibleForEtag

    protected boolean isEligibleForEtag(HttpServletRequest request,
                                        HttpServletResponse response,
                                        int responseStatusCode,
                                        byte[] responseBody)

  Indicates whether the given request and response are eligible for ETag generation.

  The default implementation returns `true` for response status codes in the `2xx` series.

  Parameters:
    `request` - the HTTP request
    `response` - the HTTP response
    `responseStatusCode` - the HTTP response status code
    `responseBody` - the response body
  Returns:
    `true` if eligible for ETag generation; `false` otherwise

### generateETagHeaderValue

    protected String generateETagHeaderValue(byte[] bytes)

  Generate the ETag header value from the given response body byte array.

  The default implementation generates an MD5 hash.

  Parameters:
    `bytes` - the response body as byte array
  Returns:
    the ETag header value
  See Also:
    `DigestUtils`