Oleg Estekhin created MNG-5326:
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Summary: Class realm created for plugins that use extensions leaks
into other plugins
Key: MNG-5326
URL: https://jira.codehaus.org/browse/MNG-5326
Project: Maven 2 & 3
Issue Type: Bug
Components: Class Loading
Affects Versions: 3.0.4
Reporter: Oleg Estekhin
Attachments: extensions-vs-realms.zip
The attached zip contains a test project that displays this issue.
In the terms of the test project, the "clean" plugin will be our demo plugin
that suffers from extra class realms, and "jar" and "source" plugins take a
role of some other plugins that use extensions.
The build is run with "mvn -X clean" to invoke the "clean" plugin, so "jar" and
"source" plugins are not even used during the build.
The "grep Created new class realm"-like part of the build output looks like
this:
{quote}
Line 43: [DEBUG] Created new class realm maven.api
Line 100: [DEBUG] Created new class realm
extension>org.apache.maven.plugins:maven-jar-plugin:2.4
Line 117: [DEBUG] Created new class realm
project>extensions-vs-realms:extensions-vs-realms-1:1.0
Line 138: [DEBUG] Created new class realm
extension>org.apache.maven.plugins:maven-source-plugin:2.1.2
Line 150: [DEBUG] Created new class realm
project>extensions-vs-realms:extensions-vs-realms-2:1.0
Line 209: [DEBUG] Created new class realm
plugin>org.apache.maven.plugins:maven-clean-plugin:2.4.1
Line 268: [DEBUG] Created new class realm
plugin>org.apache.maven.plugins:maven-clean-plugin:2.4.1-1301626664
Line 327: [DEBUG] Created new class realm
plugin>org.apache.maven.plugins:maven-clean-plugin:2.4.1-689777577
{quote}
Maven creates additional class realms to handle extensions in the sub-modules
(even if the test project does not actually contain any extensions), and then
proceeds to create a normal class realm for the "clean" plugin for the
top-level project and two additional class realms for the "clean" plugin for
sub-modules.
It means that the "clean" plugin and all of its dependencies will be loaded
multiple times using multiple class loaders, which in some special cases could
case issues with plugins or their dependencies that use singletons or some
other static "single instance in the whole JVM" features. The example of such
an issue is
https://code.google.com/p/maven-svn-revision-number-plugin/issues/detail?id=16,
where the s-r-n plugin uses SvnKit+SqlJet library, and SqlJet library contains
a singleton that must be consistent with a singleton inside the JVM code. When
s-r-n plugin and its dependencies are loaded multiple times, each class loader
hierarchy will have its own SqlJet singleton, and when used concurrently during
the parallel build the state of each individual SqlJet singleton will not match
the state of the sun.nio.ch.FileChannelImpl$SharedFileLockTable singleton
inside the JVM.
The questions and issues to consider:
# If some plugin declares that it wants extensions, but there are actually no
extensions declared, could the Maven skip the additional class realm at all?
# In the example the "jar" and "source" plugins are not used in the build, but
the Maven still creates all the additional class realms/class loaders. Could it
skip this part if it sees that such plugins will not be used?
# In the example the "clean" plugin does not use extensions, but it is reloaded
inside class realms that contain extensions. Could all plugins that do not use
extensions be used from the normal project's realm, and only plugins that do
use extensions be loaded from their special class realms?
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