Address comments.

Author: yhuai

sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala

@@ -80,9 +80,20 @@ class SQLContext private[sql](
       SQLConf.ALLOW_MULTIPLE_CONTEXTS.key,
       SQLConf.ALLOW_MULTIPLE_CONTEXTS.defaultValue.get)
 
-  SQLContext.assertNoRootSQLContextIsRunning(
-    isRootContext,
-    allowMultipleContexts)
+  // Assert no root SQLContext is running when allowMultipleContexts is false.
+  {
+    if (!allowMultipleContexts && isRootContext) {
+      SQLContext.getInstantiatedContextOption() match {
+        case Some(rootSQLContext) =>
+          val errMsg = "Only one SQLContext/HiveContext may be running in this JVM. " +
+            s"It is recommended to use SQLContext.getOrCreate to get the instantiated " +
+            s"SQLContext/HiveContext. To ignore this error, " +
+            s"set ${SQLConf.ALLOW_MULTIPLE_CONTEXTS.key} = true in SparkConf."
+          throw new SparkException(errMsg)
+        case None => // OK
+      }
+    }
+  }
 
   /**
    * Returns a SQLContext as new session, with separated SQL configurations, temporary tables,
@@ -1258,20 +1269,6 @@ object SQLContext {
     Option(instantiatedContext.get())
   }
 
-  private[sql] def assertNoRootSQLContextIsRunning(
-      isRootContext: Boolean,
-      allowMultipleRootSQLContexts: Boolean): Unit = {
-    if (!allowMultipleRootSQLContexts && isRootContext) {
-      getInstantiatedContextOption() match {
-        case Some(rootSQLContext) =>
-          val errMsg = "Only one SparkContext/HiveContext may be running in this JVM." +
-          s" To ignore this error, set ${SQLConf.ALLOW_MULTIPLE_CONTEXTS.key} = true."
-          throw new SparkException(errMsg)
-        case None => // OK
-      }
-    }
-  }
-
   /**
    * Changes the SQLContext that will be returned in this thread and its children when
    * SQLContext.getOrCreate() is called. This can be used to ensure that a given thread receives

sql/core/src/test/scala/org/apache/spark/sql/MultiSQLContextsSuite.scala

@@ -70,7 +70,7 @@ class MultiSQLContextsSuite extends SparkFunSuite with BeforeAndAfterAll {
         val message = intercept[SparkException] {
           new SQLContext(sparkContext)
         }.getMessage
-        assert(message.contains("Only one SparkContext/HiveContext may be running"))
+        assert(message.contains("Only one SQLContext/HiveContext may be running"))
       }
     } finally {
       sparkContext.stop()