Backward compat API WebAppContext.getServerClassMatcher() is not updating the WebAppClassloader

[joakime]

Jetty version(s)
12.0.9

Jetty Environment
ee10 (but could be more)

Java version/vendor (use: java -version)
Any

OS type/version
Any

Description
If a project is using the old API WebAppContext.getServerClassMatcher() and then modifies the server classes, those changes are not applied to the WebAppClassloader of the WebAppContext.

This applies to Embedded usage and XML deployables.

If the deployable has, for example ...

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "http://www.eclipse.org/jetty/configure_9_3.dtd">

<Configure class="org.eclipse.jetty.ee10.webapp.WebAppContext">
  <Set name="contextPath">/demo</Set>
  <Set name="war"><Property name="jetty.webapps" />/demo.war
  </Set>
  <Ref refid="Server">
    <Call id="lowResourceMonitor" name="getBean">
      <Arg>
        <Call class="java.lang.Class" name="forName">
          <Arg>org.eclipse.jetty.server.LowResourceMonitor</Arg>
        </Call>
      </Arg>
    </Call>
  </Ref>
  <Call name="setAttribute">
    <Arg>org.eclipse.jetty.server.LowResourceMonitor</Arg>
    <Arg>
      <Ref refid="lowResourceMonitor" />
    </Arg>
  </Call>
  <!-- tweak what classes from the Server this WebAppContext can see -->
  <Get name="serverClassMatcher">
    <Call name="add">
      <!-- needed for all example servlets to actually see LowResourceMonitor -->
      <Arg>-org.eclipse.jetty.server.LowResourceMonitor</Arg>
    </Call>
    <Call name="add">
      <!-- needed for example servlets that use Server.getBean().
           FromBaseRequestServlet
           and FromRequestAttributeChannelServlet
           and FromRequestAttributeConnectionServlet -->
      <Arg>-org.eclipse.jetty.server.Server</Arg>
    </Call>
    <Call name="add">
      <!-- needed for FromBaseRequestServlet -->
      <Arg>-org.eclipse.jetty.server.Request</Arg>
    </Call>
  </Get>
</Configure>

These changes are not applied to the WebAppClassloader.
To work, the project needs the following change.

diff --git a/standalone/lowresource-access/webapps/demo.xml b/standalone/lowresource-access/webapps/demo.xml
index c5cf4dc..4752183 100644
--- a/standalone/lowresource-access/webapps/demo.xml
+++ b/standalone/lowresource-access/webapps/demo.xml
@@ -20,7 +20,7 @@
     </Arg>
   </Call>
   <!-- tweak what classes from the Server this WebAppContext can see -->
-  <Get name="serverClassMatcher">
+  <Get name="hiddenClassMatcher">
     <Call name="add">
       <!-- needed for all example servlets to actually see LowResourceMonitor -->
       <Arg>-org.eclipse.jetty.server.LowResourceMonitor</Arg>

The project at https://github.com/jetty/jetty-examples/tree/12.0.x/standalone/lowresource-access can be used as an example.
Note: you will need to revert the commit jetty/jetty-examples@14d5c89 to replicate this bug.

[gregw]

Likely due to changes introduced in #11566 or #11722