Add MLClassLoaderContextSelector to avoid multiple log4j2 contexts in one application (#98)

Author: ichttt

src/main/java/cpw/mods/modlauncher/log/MLClassLoaderContextSelector.java

@@ -0,0 +1,19 @@
+package cpw.mods.modlauncher.log;
+
+import cpw.mods.cl.ModuleClassLoader;
+import org.apache.logging.log4j.core.selector.ClassLoaderContextSelector;
+
+/**
+ * A custom context selector to avoid initializing multiple log4j contexts due to {@link ModuleClassLoader#getParent()} always returning null (as a {@link ModuleClassLoader} can have multiple parents).
+ * As all {@link ModuleClassLoader}s should get the same log4j context, we just return a static string with "MCL", otherwise we use the default logic
+ */
+public class MLClassLoaderContextSelector extends ClassLoaderContextSelector {
+
+    @Override
+    protected String toContextMapKey(ClassLoader loader) {
+        if (loader instanceof ModuleClassLoader) {
+            return "MCL";
+        }
+        return super.toContextMapKey(loader);
+    }
+}

src/main/resources/log4j2.component.properties

@@ -0,0 +1 @@
+Log4jContextSelector=cpw.mods.modlauncher.log.MLClassLoaderContextSelector