Thursday May 07, 2009

Servlet 3.0 web-fragment.xml

In JSR 315: Java Servlet 3.0 Specification, web-fragment.xml is introduced for pluggability of library jars which are packaged under WEB-INF/lib. The content of web.xml and web-fragment.xml are almost the same. One can define servlets, filters and listeners there. One can also specify metadata-complete=true in a given web-fragment.xml. In the latter case, the annotation processing of classes in that jar would be skipped. With web-fragment.xml, library jars can be self-contained and provide web related metadata information.

The basic differences of web.xml and web-fragment.xml are summarized in the following table:

 web.xmlweb-fragment.xml
LocationWEB-INF of the war fileMETA-INF directory of JAR file inside WAR file's WEB-INF/lib
Ordering related element<absolute-ordering><ordering>

Ordering of web fragments

If there are more than one web-fragment jars, then one may like to specify the order of processing web-fragment.xml and annotations. This is important. For instance, filters will be executed in the order specified in web.xml. Similary for listeners. In Servlet 3.0, <absolute-ordering> is introduced in web.xml and <ordering> is introduced in web-fragment.xml. The ordering of web-fragments is specified in the following priority:
  • from <absolute-ordering> in web.xml if it exists
  • from <ordering> for each web-fragment.xml if it exists
  • otherwise unspecified

absolute-ordering in web.xml

The <absolute-ordering> in web.xml provides a way to specify the ordering of loading web-fragment.xml and annotation processing of web fragment. For instance,
<web-app>
    ...
    <absolute-ordering>
        <name>A</name>
         <others/>
        <name>B</name>
    <absolute-ordering>
</web-app>
In the above example, the web fragment A would be processed first and web fragment B would be processed last. Note the name A and B are specified in name element of web-fragment.xml (see examples below).

ordering in web-fragment.xml

If there is no <absolute-ordering> in web.xml, then one would look at <ordering> in web-fragment.xml. The details are described in section 8.2.3 of Servlet 3.0 spec. Let us look at some examples.
  • There is only one jar having <ordering> in web-fragment.xml.
    <web-fragment>
        <name>A</name>
        ...
        <ordering>
            <before>
                <others/>
            </before>
        </ordering>
    </web-fragment>
    In this case, web-fragment A would be processed first.

  • There are two jars having <ordering> in web-fragment.xml, namely
    web-fragment A:
    <web-fragment>
        <name>A</name>
        ...
        <ordering>
            <before>
                <others/>
            </before>
        </ordering>
    </web-fragment>

    web-fragment B:

    <web-fragment>
        <name>B</name>
        ...
        <ordering>
            <before>
                <others/>
            </before>
        </ordering>
    </web-fragment>
    Both web-fragment A and B would like to be processed first. In this case, one only guarantee that both A and B are processed before other web-fragments. But the ordering of A and B are not determined, that is arbitrary in this case.

  • There are two jars having <ordering> in web-fragment.xml, namely
    web-fragment A:
    <web-fragment>
        <name>A</name>
        ...
        <ordering>
            <before>
                <others/>
            </before>
        </ordering>
    </web-fragment>

    web-fragment B:

    <web-fragment>
        <name>B</name>
        ...
        <ordering>
            <after>
                <name>A</name>
            </after>
            <before>
                <others/>
            </before>
        </ordering>
    </web-fragment>
    In this case, A would be processed first, then followed by B, and then other web-fragments.
If one would like to have a deterministic ordering, then I would recommend to use absolute-ordering in web.xml.
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Shing Wai Chan

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