fix guthub issue 715

Signed-off-by: Ceki Gulcu <ceki@qos.ch>

logback-classic/src/main/java/ch/qos/logback/classic/util/ContextInitializer.java

@@ -21,6 +21,7 @@
 import ch.qos.logback.core.spi.ContextAwareImpl;
 import ch.qos.logback.core.status.InfoStatus;
 import ch.qos.logback.core.util.EnvUtil;
+import ch.qos.logback.core.util.Loader;
 import ch.qos.logback.core.util.StatusListenerConfigHelper;
 
 import java.util.Comparator;
@@ -65,7 +66,12 @@ public void autoConfig() throws JoranException {
         autoConfig(Configurator.class.getClassLoader());
     }
 
+
     public void autoConfig(ClassLoader classLoader) throws JoranException {
+
+        // see https://github.com/qos-ch/logback/issues/715
+        classLoader = Loader.systemClassloaderIfNull(classLoader);
+
         String versionStr = EnvUtil.logbackVersion();
         if (versionStr == null) {
             versionStr = CoreConstants.NA;

logback-core/src/main/java/ch/qos/logback/core/util/Loader.java

@@ -56,6 +56,21 @@ public Boolean run() {
         });
     }
 
+    /**
+     * This method is used to sanitize the <code>cl</code> argument in case it is null.
+     *
+     * @param cl a class loader, may be null
+     * @return the system class loader if the <code>cl</code> argument is null, return <code>cl</code> otherwise.
+     *
+     * @since 1.4.12
+     */
+    public static ClassLoader systemClassloaderIfNull(ClassLoader cl) {
+        if(cl == null)
+            return ClassLoader.getSystemClassLoader();
+        else
+            return cl;
+    }
+
     /**
      * Compute the number of occurrences a resource can be found by a class loader.
      *
@@ -64,7 +79,6 @@ public Boolean run() {
      * @return
      * @throws IOException
      */
-
     public static Set<URL> getResources(String resource, ClassLoader classLoader) throws IOException {
         // See LBCLASSIC-159
         Set<URL> urlSet = new HashSet<URL>();