HADOOP-12313. NPE in JvmPauseMonitor when calling stop() before start(). Contributed by Gabor Liptak.

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -1463,6 +1463,9 @@ Release 2.8.0 - UNRELEASED
     HADOOP-11677. Add cookie flags for logs and static contexts.
     (nijel via wheat9)
 
+    HADOOP-12313. NPE in JvmPauseMonitor when calling stop() before start().
+    (Gabor Liptak via wheat9)
+
 Release 2.7.3 - UNRELEASED
 
   INCOMPATIBLE CHANGES

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/JvmPauseMonitor.java

@@ -78,12 +78,16 @@ public class JvmPauseMonitor {
     Preconditions.checkState(monitorThread == null,
         "Already started");
     monitorThread = new Daemon(new Monitor());
-    monitorThread.start();
+    if (shouldRun) {
+      monitorThread.start();
+    } else {
+      LOG.warn("stop() was called before start() completed");
+    }
   }
   
   public void stop() {
     shouldRun = false;
-    if (monitorThread != null) {
+    if (isStarted()) {
       monitorThread.interrupt();
       try {
         monitorThread.join();