fix for #53

pf4j/src/main/java/ro/fortsoft/pf4j/PluginClassLoader.java

@@ -55,6 +55,7 @@ public class PluginClassLoader extends URLClassLoader {
      */
 	@Override
     public Class<?> loadClass(String className) throws ClassNotFoundException {
+        synchronized (getClassLoadingLock(className)) {
             log.debug("Received request to load class '{}'", className);
             // if the class it's a part of the plugin engine use parent class loader
             if (className.startsWith(PLUGIN_PACKAGE_PREFIX)) {
@@ -64,8 +65,8 @@ public class PluginClassLoader extends URLClassLoader {
                 } catch (ClassNotFoundException e) {
                     // try next step
                     // TODO if I uncomment below lines (the correct approach) I received ClassNotFoundException for demo (ro.fortsoft.pf4j.demo)
-//                log.error(e.getMessage(), e);
-//                throw e;
+                    //                log.error(e.getMessage(), e);
+                    //                throw e;
                 }
             }
 
@@ -102,6 +103,7 @@ public class PluginClassLoader extends URLClassLoader {
             // use the standard URLClassLoader (which follows normal parent delegation)
             return super.loadClass(className);
         }
+    }
 
     /**
      * Load the named resource from this plugin. This implementation checks the plugin's classpath first