HADOOP-13323. Downgrade stack trace on FS load from Warn to debug. Contributed by Steve Loughran.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/FileSystem.java

@@ -2858,7 +2858,15 @@ public abstract class FileSystem extends Configured implements Closeable {
                   ClassUtil.findContainingJar(fs.getClass()), e);
             }
           } catch (ServiceConfigurationError ee) {
-            LOG.warn("Cannot load filesystem", ee);
+            LOG.warn("Cannot load filesystem: " + ee);
+            Throwable cause = ee.getCause();
+            // print all the nested exception messages
+            while (cause != null) {
+              LOG.warn(cause.toString());
+              cause = cause.getCause();
+            }
+            // and at debug: the full stack
+            LOG.debug("Stack Trace", ee);
           }
         }
         FILE_SYSTEMS_LOADED = true;

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/fs/TestFileSystemInitialization.java

@@ -47,16 +47,12 @@ public class TestFileSystemInitialization {
 
   @Test
   public void testMissingLibraries() {
-    boolean catched = false;
     try {
       Configuration conf = new Configuration();
-      FileSystem.getFileSystemClass("s3a", conf);
-    } catch (Exception e) {
-      catched = true;
-    } catch (ServiceConfigurationError e) {
-      // S3A shouldn't find AWS SDK and fail
-      catched = true;
+      Class<? extends FileSystem> fs = FileSystem.getFileSystemClass("s3a",
+          conf);
+      fail("Expected an exception, got a filesystem: " + fs);
+    } catch (Exception | ServiceConfigurationError expected) {
     }
-    assertTrue(catched);
   }
 }