implemented optional dependencies

Plugin authors could now define optional dependencies to other plugins in their pom.
Extensions which are using classes from optional dependencies must specify this
with the "requires" attribute of the extension annotation.
Extensions with "requires" attribute are not installed if one of the specified plugins,
is not installed.

scm-webapp/src/main/java/sonia/scm/plugin/ExtensionCollector.java

@@ -39,6 +39,8 @@ import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 import com.google.common.collect.Multimap;
 import com.google.common.collect.Sets;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 //~--- JDK imports ------------------------------------------------------------
 
@@ -47,6 +49,7 @@ import java.util.Collections;
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Set;
+import java.util.stream.Collectors;
 
 /**
  *
@@ -56,23 +59,33 @@ import java.util.Set;
 public final class ExtensionCollector
 {
 
-  /**
-   * Constructs ...
-   *
-   *
-   * @param modules
-   */
-  ExtensionCollector(Iterable<ScmModule> modules)
-  {
-    for (ScmModule module : modules)
-    {
+  private static final Logger LOG = LoggerFactory.getLogger(ExtensionCollector.class);
+
+  private final Set<String> pluginIndex;
+
+  public ExtensionCollector(ClassLoader moduleClassLoader, Set<ScmModule> modules, Set<InstalledPlugin> installedPlugins) {
+    this.pluginIndex = createPluginIndex(installedPlugins);
+
+    for (ScmModule module : modules) {
       collectRootElements(module);
     }
-
-    for (ScmModule module : modules)
-    {
-      collectExtensions(module);
+    for (ScmModule plugin : PluginsInternal.unwrap(installedPlugins)) {
+      collectRootElements(plugin);
     }
+
+    for (ScmModule module : modules) {
+      collectExtensions(moduleClassLoader, module);
+    }
+
+    for (InstalledPlugin plugin : installedPlugins) {
+      collectExtensions(plugin.getClassLoader(), plugin.getDescriptor());
+    }
+  }
+
+  private Set<String> createPluginIndex(Set<InstalledPlugin> installedPlugins) {
+    return installedPlugins.stream()
+      .map(p -> p.getDescriptor().getInformation().getName())
+      .collect(Collectors.toSet());
   }
 
   //~--- methods --------------------------------------------------------------
@@ -245,20 +258,35 @@ public final class ExtensionCollector
     }
   }
 
-  /**
-   * Method description
-   *
-   *
-   * @param module
-   */
-  private void collectExtensions(ScmModule module)
-  {
-    for (Class extension : module.getExtensions())
-    {
-      appendExtension(extension);
+  private void collectExtensions(ClassLoader defaultClassLoader, ScmModule module) {
+    for (ExtensionElement extension : module.getExtensions()) {
+      if (isRequirementFulfilled(extension)) {
+        Class<?> extensionClass = loadExtension(defaultClassLoader, extension);
+        appendExtension(extensionClass);
+      }
     }
   }
 
+  private Class<?> loadExtension(ClassLoader classLoader, ExtensionElement extension) {
+    try {
+      return classLoader.loadClass(extension.getClazz());
+    } catch (ClassNotFoundException ex) {
+      throw new RuntimeException("failed to load clazz", ex);
+    }
+  }
+
+  private boolean isRequirementFulfilled(ExtensionElement extension) {
+    if (extension.getRequires() != null) {
+      for (String requiredPlugin : extension.getRequires()) {
+        if (!pluginIndex.contains(requiredPlugin)) {
+          LOG.debug("skip loading of extension {}, because the required plugin {} is not installed", extension.getClazz(), requiredPlugin);
+          return false;
+        }
+      }
+    }
+    return true;
+  }
+
   /**
    * Method description
    *