added an auto-discovery mechanism for extension points (and most notably Descriptors.)

- extensions are now stored in ExtensionList.
- discovery is extensible, but by default we use Sezpoz for annotation-based injection. This requires JDK6 for build.
- a care has been taken to preserve backward compatibility. DescriptorList and manually adding to it continues to work.
- AdministrativeMonitor is converted to use the new approach entirely, as a guinea pig of this feature.
- Descriptors for View is partially switched to this new approach, where some classes register automatically while others do manually. This tests the compatibility aspect of this feature.

A pluggable discovery is an attempt to embrace Plexus integration work, although for that to really work we need more coding.

Next stop, more tests.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@15601 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/ExtensionFinder.java

@@ -0,0 +1,115 @@
+/*
+ * The MIT License
+ *
+ * Copyright (c) 2004-2009, Sun Microsystems, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package hudson;
+
+import net.java.sezpoz.Index;
+import net.java.sezpoz.IndexItem;
+import hudson.model.Hudson;
+import hudson.model.Descriptor;
+
+import java.util.logging.Logger;
+import java.util.logging.Level;
+import java.util.List;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.lang.reflect.AnnotatedElement;
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+
+/**
+ * Discovers the implementations of an extension point.
+ *
+ * <p>
+ * This extension point allows you to write your implementations of {@link ExtensionPoint}s
+ * in arbitrary DI containers, and have Hudson discover them.
+ *
+ * <p>
+ * {@link ExtensionFinder} itself is an extension point, but to avoid infinite recursion,
+ * Hudson discovers {@link ExtensionFinder}s through {@link Sezpoz} and that alone.
+ *
+ * @author Kohsuke Kawaguchi
+ * @since 1.286
+ */
+public abstract class ExtensionFinder implements ExtensionPoint {
+    /**
+     * Discover extensions of the given type.
+     *
+     * <p>
+     * This method is called only once per the given type after all the plugins are loaded,
+     * so implementations need not worry about caching.
+     *
+     * @param <T>
+     *      The type of the extension points. This is not bound to {@link ExtensionPoint} because
+     *      of {@link Descriptor}, which by itself doesn't implement {@link ExtensionPoint} for
+     *      a historical reason.
+     * @param hudson
+     *      Hudson whose behalf this extension finder is performing lookup.
+     * @return
+     *      Can be empty but never null.
+     */
+    public abstract <T> Collection<T> findExtensions(Class<T> type, Hudson hudson);
+
+    /**
+     * The default implementation that looks for the {@link Extension} marker.
+     *
+     * <p>
+     * Uses Sezpoz as the underlying mechanism.
+     */
+    @Extension
+    public static final class Sezpoz extends ExtensionFinder {
+        public <T> Collection<T> findExtensions(Class<T> type, Hudson hudson) {
+            List<T> result = new ArrayList<T>();
+
+            ClassLoader cl = hudson.getPluginManager().uberClassLoader;
+            for (IndexItem<Extension,Object> item : Index.load(Extension.class, Object.class, cl)) {
+                try {
+                    AnnotatedElement e = item.element();
+                    Class<?> extType;
+                    if (e instanceof Class) {
+                        extType = (Class) e;
+                    } else
+                    if (e instanceof Field) {
+                        extType = ((Field)e).getType();
+                    } else
+                    if (e instanceof Method) {
+                        extType = ((Method)e).getReturnType();
+                    } else
+                        throw new AssertionError();
+
+                    if(type.isAssignableFrom(extType)) {
+                        Object instance = item.instance();
+                        if(instance!=null)
+                            result.add(type.cast(instance));
+                    }
+                } catch (InstantiationException e) {
+                    LOGGER.log(Level.WARNING, "Failed to load "+item.className(),e);
+                }
+            }
+
+            return result;
+        }
+    }
+    
+    private static final Logger LOGGER = Logger.getLogger(ExtensionFinder.class.getName());
+}