Allows to register a filtered class realm

pom.xml

@@ -207,7 +207,7 @@
               <report>summary</report>
               <report>index</report>
               <report>dependencies</report>
-              <report>issue-tracking</report>
+              <report>issue-management</report>
               <report>scm</report>
             </reports>
           </reportSet>

src/main/java/org/codehaus/plexus/classworlds/ClassWorld.java

@@ -23,9 +23,11 @@
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.function.Predicate;
 
 import org.codehaus.plexus.classworlds.realm.ClassRealm;
 import org.codehaus.plexus.classworlds.realm.DuplicateRealmException;
+import org.codehaus.plexus.classworlds.realm.FilteredClassRealm;
 import org.codehaus.plexus.classworlds.realm.NoSuchRealmException;
 
 /**
@@ -64,7 +66,40 @@ public ClassRealm newRealm( String id )
         return newRealm( id, getClass().getClassLoader() );
     }
 
-    public synchronized ClassRealm newRealm( String id, ClassLoader classLoader )
+    public ClassRealm newRealm( String id, ClassLoader classLoader )
+        throws DuplicateRealmException
+    {
+        return newRealm( id, classLoader, null );
+    }
+
+    /**
+     * Shortcut for {@link #newRealm(String, ClassLoader, Predicate)} with the class loader of the current class.
+     * @param id The identifier for this realm, must not be <code>null</code>.
+     * @param filter a predicate to apply to each resource name to determine if it should be loaded through this class loader
+     * @return the created class realm
+     * @throws DuplicateRealmException in case a realm with the given id does already exist
+     * @since 2.7.0
+     * @see FilteredClassRealm
+     */
+    public synchronized ClassRealm newRealm( String id, Predicate<String> filter )
+         throws DuplicateRealmException
+    {
+        return newRealm( id, getClass().getClassLoader(), filter );
+    }
+
+    /**
+     * Adds a class realm with filtering.
+     * Only resources/classes whose name matches a given predicate are exposed.
+     * @param id The identifier for this realm, must not be <code>null</code>.
+     * @param classLoader The base class loader for this realm, may be <code>null</code> to use the bootstrap class
+     *            loader.
+     * @param filter a predicate to apply to each resource name to determine if it should be loaded through this class loader
+     * @return the created class realm
+     * @throws DuplicateRealmException in case a realm with the given id does already exist
+     * @since 2.7.0
+     * @see FilteredClassRealm
+     */
+    public synchronized ClassRealm newRealm( String id, ClassLoader classLoader, Predicate<String> filter )
         throws DuplicateRealmException
     {
         if ( realms.containsKey( id ) )
@@ -74,8 +109,14 @@ public synchronized ClassRealm newRealm( String id, ClassLoader classLoader )
 
         ClassRealm realm;
 
-        realm = new ClassRealm( this, id, classLoader );
-
+        if ( filter == null )
+        {
+            realm = new ClassRealm( this, id, classLoader );
+        }
+        else
+        {
+            realm = new FilteredClassRealm( filter, this, id, classLoader );
+        }
         realms.put( id, realm );
 
         for ( ClassWorldListener listener : listeners )
@@ -85,7 +126,7 @@ public synchronized ClassRealm newRealm( String id, ClassLoader classLoader )
 
         return realm;
     }
-
+    
     public synchronized void disposeRealm( String id )
         throws NoSuchRealmException
     {

src/main/java/org/codehaus/plexus/classworlds/realm/ClassRealm.java

@@ -200,7 +200,7 @@ public ClassRealm getParentRealm()
     public ClassRealm createChildRealm( String id )
         throws DuplicateRealmException
     {
-        ClassRealm childRealm = getWorld().newRealm( id, null );
+        ClassRealm childRealm = getWorld().newRealm( id, (ClassLoader) null );
 
         childRealm.setParentRealm( this );
 
@@ -272,7 +272,8 @@ private Class<?> unsynchronizedLoadClass( String name, boolean resolve )
         }
     }
 
-    // java11
+    // overwrites https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/ClassLoader.html#findClass(java.lang.String,java.lang.String) 
+    // introduced in Java9
     protected Class<?> findClass( String moduleName, String name )
     {
         if ( moduleName != null )
@@ -281,7 +282,7 @@ protected Class<?> findClass( String moduleName, String name )
         }
         try
         {
-            return super.findClass( name );
+            return findClassInternal( name );
         }
         catch ( ClassNotFoundException e )
         {
@@ -306,6 +307,12 @@ protected Class<?> findClass( String name )
         throw new ClassNotFoundException( name );
     }
 
+    protected Class<?> findClassInternal( String name )
+         throws ClassNotFoundException
+    {
+        return super.findClass( name );
+    }
+
     public URL getResource( String name )
     {
         URL resource = super.getResource( name );
@@ -422,7 +429,7 @@ public Class<?> loadClassFromSelf( String name )
 
                 if ( clazz == null )
                 {
-                    clazz = super.findClass( name );
+                    clazz = findClassInternal( name );
                 }
 
                 return clazz;
@@ -495,7 +502,7 @@ public URL loadResourceFromImport( String name )
 
     public URL loadResourceFromSelf( String name )
     {
-        return super.findResource( name );
+        return findResource( name );
     }
 
     public URL loadResourceFromParent( String name )
@@ -539,7 +546,7 @@ public Enumeration<URL> loadResourcesFromSelf( String name )
     {
         try
         {
-            return super.findResources( name );
+            return findResources( name );
         }
         catch ( IOException e )
         {

src/main/java/org/codehaus/plexus/classworlds/realm/FilteredClassRealm.java

@@ -0,0 +1,84 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.codehaus.plexus.classworlds.realm;
+
+import java.io.IOException;
+import java.net.URL;
+import java.util.Collections;
+import java.util.Enumeration;
+import java.util.function.Predicate;
+
+import org.codehaus.plexus.classworlds.ClassWorld;
+
+/**
+ * Similar to {@link ClassRealm} but only exposing some resources of the underlying URL.
+ * Only supposed to be called from {@link ClassWorld}.
+ */
+public class FilteredClassRealm extends ClassRealm
+{
+    private final Predicate<String> filter;
+
+    /**
+     * Creates a new class realm.
+     *
+     * @param filter a predicate to apply to each resource name to determine if it should be loaded through this class loader
+     * @param world The class world this realm belongs to, must not be <code>null</code>.
+     * @param id The identifier for this realm, must not be <code>null</code>.
+     * @param baseClassLoader The base class loader for this realm, may be <code>null</code> to use the bootstrap class
+     *            loader.
+     */
+    public FilteredClassRealm( Predicate<String> filter, ClassWorld world, String id, ClassLoader baseClassLoader )
+    {
+        super( world, id, baseClassLoader );
+        this.filter = filter;
+    }
+
+    @Override
+    protected Class<?> findClassInternal( String name )
+        throws ClassNotFoundException
+    {
+        String resourceName = name.replace( '.', '/' ).concat( ".class" );
+        if ( !filter.test( resourceName ) )
+        {
+            throw new ClassNotFoundException(name);
+        }
+        return super.findClassInternal( name );
+    }
+
+    @Override
+    public URL findResource( String name )
+    {
+        if ( !filter.test( name ) )
+        {
+            return null;
+        }
+        return super.findResource( name );
+    }
+
+    @Override
+    public Enumeration<URL> findResources( String name )
+        throws IOException
+    {
+        if ( !filter.test( name ) )
+        {
+            return Collections.emptyEnumeration();
+        }
+        return super.findResources( name );
+    }
+}

src/site/xdoc/apiusage.xml

@@ -32,7 +32,7 @@ ClassWorld world = new ClassWorld();
     <p>
     Once a <code>ClassWorld</code> is created, realms within it
     can be created.  These realms effectively only allow loading
-    of the core JVM classes.  
+    of the core JVM classes initially.
     </p>
 
 <source><![CDATA[
@@ -42,15 +42,21 @@ ClassRealm logComponentRealm = world.newRealm( "logComponent" );
 ]]></source>
 
     <p>
-    In order to make each <code>ClassRealm</code> useful, constituent
-    must be added to that each can provide certain classes.
+    In order to make each <code>ClassRealm</code> useful, constituents
+    in form of URLs must be added to it where each can provide certain classes.
+    The URL must return either a JAR or a directory on the default file system.
     </p>
 
 <source><![CDATA[
 containerRealm.addURL( containerJarUrl );
 logComponentRealm.addURL( logComponentJarUrl );
 ]]></source>
 
+    <p>
+    <code>ClassRealm</code>s can optionally be filtered to further restrict which classes/resources
+    are exposed. The filter is provided as additional argument to <code>world.newRealm( "filteredcontainer", myPredicate );</code>
+    </p>
+
     <p>
     Now, links between the various realms need to be created to allow
     classes loaded from one to be available to classes loaded in another.

src/test/java/org/codehaus/plexus/classworlds/realm/ClassRealmImplTest.java

@@ -443,7 +443,7 @@ public void testLoadClass_ClassWorldsClassRepeatedly()
     }
 
     @Test
-    public void testLoadClass_Java11()
+    public void testLoadClassWithModuleName_Java9()
     {
         final ExtendedClassRealm mainRealm = new ExtendedClassRealm( world );
         mainRealm.addURL( getJarUrl( "a.jar" ) );
@@ -503,13 +503,17 @@ public void testGetResources_SelfBeforeParent()
         assertEquals( Arrays.asList( childUrl, parentUrl ), urls );
     }
 
-    // simulate new loadClass(Module,String) from java11
-    // it is reversed in terms of inheritance but enables to simulate the same behavior in these tests
+    /**
+     * Simulates new {@code java.lang.ClassLoader#findClass(String,String)} introduced with Java 9.
+     * It is reversed in terms of inheritance but enables to simulate the same behavior in these tests.
+     * @see <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/ClassLoader.html#findClass(java.lang.String,java.lang.String)">ClassLoader#findClass(String,String)</a>
+     */
     private static class ExtendedClassRealm extends ClassRealm
     {
+
         public ExtendedClassRealm(final ClassWorld world)
         {
-            super( world, "java11", Thread.currentThread().getContextClassLoader() );
+            super( world, "java9", Thread.currentThread().getContextClassLoader() );
         }
 
         public Class<?> simulateLoadClassFromModule(final String name)