fixed a bug where XStream activity in plugins wasn't able to resolve classes.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@1131 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/PluginManager.java

@@ -119,6 +119,16 @@ public final class PluginManager {
 
         @Override
         protected Class<?> findClass(String name) throws ClassNotFoundException {
+            // first, use the context classloader so that plugins that are loading
+            // can use its own classloader first.
+            ClassLoader cl = Thread.currentThread().getContextClassLoader();
+            if(cl!=null)
+                try {
+                    return cl.loadClass(name);
+                } catch(ClassNotFoundException e) {
+                    // not found. try next
+                }
+
             for (PluginWrapper p : activePlugins) {
                 try {
                     return p.classLoader.loadClass(name);