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/Extension.java

@@ -0,0 +1,73 @@
+/*
+ * 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.Indexable;
+
+import java.lang.annotation.Documented;
+import static java.lang.annotation.ElementType.*;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import hudson.ExtensionFinder.Sezpoz;
+
+/**
+ * Marks a field, a method, or a class for automatic discovery, so that Hudson can locate
+ * implementations of {@link ExtensionPoint}s automatically.
+ *
+ * <p>
+ * (In contrast, in earlier Hudson, the registration was manual.)
+ *
+ * <p>
+ * In a simplest case, put this on your class, and Hudson will create an instance of it
+ * and register it to the appropriate {@link ExtensionList}.
+ *
+ * <p>
+ * If you'd like Hudson to call
+ * a factory method instead of a constructor, put this annotation on your static factory
+ * method. Hudson will invoke it and if the method returns a non-null instance, it'll be
+ * registered. The return type of the method is used to determine which {@link ExtensionList}
+ * will get the instance.
+ *
+ * Finally, you can put this annotation on a static field if the field contains a reference
+ * to an instance that you'd like to register.
+ *
+ * <p>
+ * This is the default way of having your implementations auto-registered to Hudson,
+ * but Hudson also supports arbitrary DI containers for hosting your implementations.
+ * See {@link ExtensionFinder} for more details.
+ *
+ * @author Kohsuke Kawaguchi
+ * @since 1.286
+ * @see Sezpoz
+ * @see ExtensionFinder
+ * @see ExtensionList
+ */
+@Indexable
+@Retention(RetentionPolicy.RUNTIME)
+@Target({TYPE, FIELD, METHOD})
+@Documented
+public @interface Extension {
+}