Issue #12094 restore classloader association during comp/env creation

[janbartel]

Closes #12094

We removed explicit association/disassociation of the webapp classloader to use to create or find the appropriate java:comp/env naming context from EnvConfiguration. This is not a problem for code running in the distribution. However, if embedded code:

• fails to create an EE level classloader as the parent loader for all webapps
• pre-creates the java:comp naming Context using the common application classloader
• starts multiple WebAppContexts

then those webapps all wind up trying to share the same java:comp/env environment. The relevant example code, paraphrased from #12094, is as follows:

Main.java:

void run() throws Exception
{
        new InitialContext().lookup("java:comp");

       WebAppContext webappA = new WebAppContext();
       WebAppContext webbappB = new WebAppContext();
}

So we could require that embedded users create an empty classloader that emulates the EE classloader, but does nothing (because as all classes are on the application classloader instead):

void run() throws Exception
{
      //create useless "EE" parent classloader for webapps before creating java:comp
       Thread.currentThread().setContextClassLoader(new URLClassLoader(new URL[]{}, Thread.currentThread().getContextClassLoader());
        new InitialContext().lookup("java:comp");

       WebAppContext webappA = new WebAppContext();
       WebAppContext webbappB = new WebAppContext();
}

However, I think it's reasonable that "naive" code (ie that doesn't know about the new EE classloader level) should be able to work. So this PR re-introduces the associate/disassociate idiom that was used before. This allows embedded code to work, as well as apps using the distribution.

[erdi]

I would like to test this fix out, @janbartel. Are there any nightly builds being published for the project? I tried googling for nightlies but sadly without any luck.

[olamy]

@erdi snapshots are deployed daily here https://oss.sonatype.org/content/repositories/jetty-snapshots