GUACAMOLE-541: Prioritize classes defined within extensions over classes inherited from the webapp classpath or lib directory.

guacamole/src/main/java/org/apache/guacamole/extension/Extension.java

@@ -21,12 +21,6 @@ package org.apache.guacamole.extension;
 
 import java.io.File;
 import java.io.IOException;
-import java.net.MalformedURLException;
-import java.net.URL;
-import java.net.URLClassLoader;
-import java.security.AccessController;
-import java.security.PrivilegedActionException;
-import java.security.PrivilegedExceptionAction;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -387,34 +381,8 @@ public class Extension {
                 extension.close();
             }
 
-            try {
-
             // Create isolated classloader for this extension
-                classLoader = AccessController.doPrivileged(new PrivilegedExceptionAction<ClassLoader>() {
+            classLoader = ExtensionClassLoader.getInstance(file, parent);
-
-                    @Override
-                    public ClassLoader run() throws GuacamoleException {
-
-                        try {
-
-                            // Classloader must contain only the extension itself
-                            return new URLClassLoader(new URL[]{file.toURI().toURL()}, parent);
-
-                        }
-                        catch (MalformedURLException e) {
-                            throw new GuacamoleException(e);
-                        }
-
-                    }
-
-                });
-
-            }
-
-            // Rethrow any GuacamoleException
-            catch (PrivilegedActionException e) {
-                throw (GuacamoleException) e.getException();
-            }
 
         }
 

guacamole/src/main/java/org/apache/guacamole/extension/ExtensionClassLoader.java

@@ -0,0 +1,159 @@
+/*
+ * 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.apache.guacamole.extension;
+
+import java.io.File;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.security.AccessController;
+import java.security.PrivilegedActionException;
+import java.security.PrivilegedExceptionAction;
+import org.apache.guacamole.GuacamoleException;
+import org.apache.guacamole.GuacamoleServerException;
+
+/**
+ * ClassLoader implementation which prioritizes the classes defined within a
+ * given extension .jar file. Unlike the standard URLClassLoader, classes
+ * within the parent ClassLoader are only used if they are not defined within
+ * the given .jar. If classes are defined in both the parent and the extension
+ * .jar, the versions defined within the extension .jar are used.
+ */
+public class ExtensionClassLoader extends URLClassLoader {
+
+    /**
+     * The ClassLoader to use if class resolution through the extension .jar
+     * fails.
+     */
+    private final ClassLoader parent;
+
+    /**
+     * Returns an instance of ExtensionClassLoader configured to load classes
+     * from the given extension .jar. If a necessary class cannot be found
+     * within the .jar, the given parent ClassLoader is used. Calling this
+     * function multiple times will not affect previously-returned instances of
+     * ExtensionClassLoader.
+     *
+     * @param extension
+     *     The extension .jar file from which classes should be loaded.
+     *
+     * @param parent
+     *     The ClassLoader to use if class resolution through the extension
+     *     .jar fails.
+     *
+     * @return
+     *     A ExtensionClassLoader instance which loads classes from the
+     *     given extension .jar file.
+     *
+     * @throws GuacamoleException
+     *     If the given file is not actually a file, or the contents of the
+     *     file cannot be read.
+     */
+    public static ExtensionClassLoader getInstance(final File extension,
+            final ClassLoader parent) throws GuacamoleException {
+
+        try {
+            // Attempt to create classloader which loads classes from the given
+            // .jar file
+            return AccessController.doPrivileged(new PrivilegedExceptionAction<ExtensionClassLoader>() {
+
+                @Override
+                public ExtensionClassLoader run() throws GuacamoleException {
+                    return new ExtensionClassLoader(extension, parent);
+                }
+
+            });
+        }
+
+        catch (PrivilegedActionException e) {
+            throw (GuacamoleException) e.getException();
+        }
+
+    }
+
+    /**
+     * Returns a URL which points to the given extension .jar file.
+     *
+     * @param extension
+     *     The extension .jar file to generate a URL for.
+     *
+     * @return
+     *     A URL which points to the given extension .jar.
+     *
+     * @throws GuacamoleException
+     *     If the given file is not actually a file, or the contents of the
+     *     file cannot be read.
+     */
+    private static URL getExtensionURL(File extension)
+            throws GuacamoleException {
+
+        // Validate extension file is indeed a file
+        if (!extension.isFile())
+            throw new GuacamoleException(extension + " is not a file.");
+
+        try {
+            return extension.toURI().toURL();
+        }
+        catch (MalformedURLException e) {
+            throw new GuacamoleServerException(e);
+        }
+
+    }
+
+    /**
+     * Creates a new ExtensionClassLoader configured to load classes from the
+     * given extension .jar. If a necessary class cannot be found within the
+     * .jar, the given parent ClassLoader is used. Calling this function
+     * multiple times will not affect previously-returned instances of
+     * ExtensionClassLoader.
+     *
+     * @param extension
+     *     The extension .jar file from which classes should be loaded.
+     *
+     * @param parent
+     *     The ClassLoader to use if class resolution through the extension
+     *     .jar fails.
+     *
+     * @throws GuacamoleException
+     *     If the given file is not actually a file, or the contents of the
+     *     file cannot be read.
+     */
+    private ExtensionClassLoader(File extension, ClassLoader parent)
+            throws GuacamoleException {
+        super(new URL[]{ getExtensionURL(extension) }, null);
+        this.parent = parent;
+    }
+
+    @Override
+    protected Class<?> findClass(String string) throws ClassNotFoundException {
+
+        // Search only within the given URLs
+        try {
+            return super.findClass(string);
+        }
+
+        // Search parent classloader ONLY if not found within given URLs
+        catch (ClassNotFoundException e) {
+            return parent.loadClass(string);
+        }
+
+    }
+
+}