export_filter.apt

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                                            Export filter
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                                          Fabrizio Giustina
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                                              26-11-2005
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Export filter? What's that?

 When displaytag exports data in any non-html format, it needs to change the content type returned to the
 browser and reset any other content generated by the surrounding page.

 Sometimes this can't be done: if content has already been sent back to the user, the response can't be
 reset and you get an error. This could happen because:


 * Too many chars have been already written to the response, so that the response buffer was full and response
 has been automatically flushed.

 * Something (tags? java snippets?) before the <<<display:table>>> tag has explicitely flushed the response
 (<<<response.flushBuffer()>>>).

 * Your page is dynamically included into another page. This happens for example using Struts tiles.

 []


 Another problem is related to exporting binary files. The output of binary data is not supported in
jsps: it may work on some application server, but it may end up with errors in others. Because of this
restriction an "external help" may be required. CSV, XML and text-based Excel formats don't require a binary
output, but if you want to try PDF, POI-based Excel, or some other custom binary format you may encounter
problems.


* The solution

 The first attempt can be using a larger page buffer in your jsp pages, for example:
 <<<\<%@ page buffer = "16kb" %/>>>>
 However, this can work only if you are in the first situation listed above.

 In j2ee 1.3/jsp 1.2 containers, you can take advantage of filters to solve the problem. Displaytag ships
 with a filter that works together with the table tag during export, disallowing the flushing the response
 when export has been requested.



* Installing the export filter

 Configure the Filter in your web.xml:

+--------------------------------------------------------+
  <filter>
    <filter-name>ResponseOverrideFilter</filter-name>
    <filter-class>org.displaytag.filter.ResponseOverrideFilter</filter-class>
  </filter>
+--------------------------------------------------------+

 And add mappings for urls the filter will intercept, for example:

+--------------------------------------------------------+
  <filter-mapping>
    <filter-name>ResponseOverrideFilter</filter-name>
    <url-pattern>*.do</url-pattern>
  </filter-mapping>
  <filter-mapping>
    <filter-name>ResponseOverrideFilter</filter-name>
    <url-pattern>*.jsp</url-pattern>
  </filter-mapping>
+--------------------------------------------------------+