Add plugins/extension infrastructure

Plugins are stored in `${baseFolder}/plugins` and are loaded
during startup by the PluginManager.

A plugin defines it's metadata in META-INF/MANIFEST.MF:

    Plugin-Class: com.gitblit.plugins.cookbook.CookbookPlugin
    Plugin-Dependencies: foo, bar
    Plugin-Id: gitblit-plugin
    Plugin-Provider: John Doe
    Plugin-Version: 1.0

Plugins can define extension points that can be implemented
by other plugins and they can depend on other plugins:

    Plugin-Dependencies: foo, bar

During the load phase, a directed acyclic graph is built and the loading
order of the dependency chain is reversed using a topological sort;
parent followed by children. The parent plugin classloader is the
combined classloader of all parent plugins.

Change-Id: I738821fa2bff02a5dbe339a944cc7e3c4dd8e299

4 files changed, 114 insertions, 2 deletions

diff --git a/src/main/java/com/gitblit/manager/GitblitManager.java b/src/main/java/com/gitblit/manager/GitblitManager.java
index 8856715a..34c101e2 100644
--- a/src/main/java/com/gitblit/manager/GitblitManager.java
+++ b/src/main/java/com/gitblit/manager/GitblitManager.java
@@ -43,6 +43,8 @@ import org.eclipse.jgit.transport.RefSpec;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import ro.fortsoft.pf4j.PluginWrapper;
+
 import com.gitblit.Constants;
 import com.gitblit.Constants.AccessPermission;
 import com.gitblit.Constants.AccessRestrictionType;
@@ -116,6 +118,8 @@ public class GitblitManager implements IGitblit {
 
 	protected final IFederationManager federationManager;
 
+	protected final IPluginManager pluginManager;
+
 	public GitblitManager(
 			IRuntimeManager runtimeManager,
 			INotificationManager notificationManager,
@@ -124,7 +128,8 @@ public class GitblitManager implements IGitblit {
 			IPublicKeyManager publicKeyManager,
 			IRepositoryManager repositoryManager,
 			IProjectManager projectManager,
-			IFederationManager federationManager) {
+			IFederationManager federationManager,
+			IPluginManager pluginManager) {
 
 		this.settings = runtimeManager.getSettings();
 		this.runtimeManager = runtimeManager;
@@ -135,6 +140,7 @@ public class GitblitManager implements IGitblit {
 		this.repositoryManager = repositoryManager;
 		this.projectManager = projectManager;
 		this.federationManager = federationManager;
+		this.pluginManager = pluginManager;
 	}
 
 	@Override
@@ -1171,4 +1177,14 @@ public class GitblitManager implements IGitblit {
 	public boolean isIdle(Repository repository) {
 		return repositoryManager.isIdle(repository);
 	}
+
+	@Override
+	public <T> List<T> getExtensions(Class<T> clazz) {
+		return pluginManager.getExtensions(clazz);
+	}
+
+	@Override
+	public PluginWrapper whichPlugin(Class<?> clazz) {
+		return pluginManager.whichPlugin(clazz);
+	}
 }
diff --git a/src/main/java/com/gitblit/manager/IGitblit.java b/src/main/java/com/gitblit/manager/IGitblit.java
index f4221cf9..7961a06b 100644
--- a/src/main/java/com/gitblit/manager/IGitblit.java
+++ b/src/main/java/com/gitblit/manager/IGitblit.java
@@ -36,7 +36,8 @@ public interface IGitblit extends IManager,
 									IAuthenticationManager,
 									IRepositoryManager,
 									IProjectManager,
-									IFederationManager {
+									IFederationManager,
+									IPluginManager {
 
 	/**
 	 * Returns a list of repository URLs and the user access permission.
diff --git a/src/main/java/com/gitblit/manager/IPluginManager.java b/src/main/java/com/gitblit/manager/IPluginManager.java
new file mode 100644
index 00000000..670e9769
--- /dev/null
+++ b/src/main/java/com/gitblit/manager/IPluginManager.java
@@ -0,0 +1,39 @@
+/*
+ * Copyright 2014 gitblit.com.
+ *
+ * Licensed 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 com.gitblit.manager;
+
+import java.util.List;
+
+import ro.fortsoft.pf4j.PluginWrapper;
+
+public interface IPluginManager extends IManager {
+
+	/**
+	 * Retrieves the extension for given class 'clazz'.
+	 *
+	 * @param clazz extension point class to retrieve extension for
+	 * @return list of extensions
+	 */
+	public <T> List<T> getExtensions(Class<T> clazz);
+
+	/**
+     * Retrieves the {@link PluginWrapper} that loaded the given class 'clazz'.
+     *
+     * @param clazz extension point class to retrieve extension for
+     * @return PluginWrapper that loaded the given class
+     */
+    public PluginWrapper whichPlugin(Class<?> clazz);
+}
diff --git a/src/main/java/com/gitblit/manager/PluginManager.java b/src/main/java/com/gitblit/manager/PluginManager.java
new file mode 100644
index 00000000..5eb00e92
--- /dev/null
+++ b/src/main/java/com/gitblit/manager/PluginManager.java
@@ -0,0 +1,56 @@
+/*
+ * Copyright 2014 gitblit.com.
+ *
+ * Licensed 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 com.gitblit.manager;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import ro.fortsoft.pf4j.DefaultPluginManager;
+
+import com.gitblit.Keys;
+
+/**
+ * The plugin manager maintains the lifecycle of plugins. It is exposed as
+ * Dagger bean. The extension consumers supposed to retrieve plugin  manager
+ * from the Dagger DI and retrieve extensions provided by active plugins.
+ * 
+ * @author David Ostrovsky
+ * 
+ */
+public class PluginManager extends DefaultPluginManager implements
+		IPluginManager {
+
+	private final Logger logger = LoggerFactory.getLogger(getClass());
+
+	public PluginManager(IRuntimeManager runtimeManager) {
+		super(runtimeManager.getFileOrFolder(Keys.plugins.folder,
+				"${baseFolder}/plugins"));
+	}
+
+	@Override
+	public PluginManager start() {
+		logger.info("Plugin manager started");
+		loadPlugins();
+		startPlugins();
+		return this;
+	}
+
+	@Override
+	public PluginManager stop() {
+		stopPlugins();
+		return null;
+	}
+}