我可以从 web-fragment.xml 中引用 web.xml 中的 servlet 或过滤器吗？

Question

So I have a web.xml that defines a servlet or a filter that has some servlet-mapping or filter-mapping associated with it, then I want to add some more servlet-mapping/filter-mapping entries in web-fragment.xml, will that work? Say web.xml:

<servlet>
<servlet-name>A</servlet-name>
<servlet-class>com.A</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>A</servlet-name>
<url-pattern>/Path1</url-pattern>
</servlet-mapping>

then in web-fragment.xml I have

<servlet-mapping>
<servlet-name>A</servlet-name>
<url-pattern>/Path2</url-pattern>
</servlet-mapping>

From my testing it doesn't work but I don't understand why would it be so.

Answer 1

I experienced this same issue. I believe the answer is that he 3.1 servlet spec says

<servlet-mapping> elements with the same <servlet-name> are additive across web-fragments. <servlet-mapping> specified in the web.xml overrides values specified in the web-fragments with the same <servlet-name>

(And something analogous for filter-mappings.)

They are additive only across web-fragments , not across both the web.xml and the web-fragments . If they occur in the web.xml, they override all the ones on the fragments. This seemed to be born out by my testing with Tomcat using the logEffectiveWebXml feature.

So you could make this work by moving these mapping out of the web.xml into a web-fragment.xml. (In my a case, I decided to abandon the web-fragments approach as it became too hard to manage the ordering across multiple files.)

Answer 2

As of Servlet 3.0 Specification web-fragment is introduced for 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.

I guess you are not linking your fragments to web.xml, so probably you have to do something like this:

 <absolute-ordering>
    <name>fragment3</name>
    <name>fragment2</name>
    <name>fragment1</name>
    <others/>
    ...
</absolute-ordering>