HADOOP-105 Give warning for deprecated single BSON output file

Author: visualzhou

core/src/main/java/com/mongodb/hadoop/BSONFileOutputFormat.java

@@ -41,6 +41,8 @@ public RecordWriter<K, V> getRecordWriter( final TaskAttemptContext context ) th
         Configuration conf = context.getConfiguration();
         if(conf.get("mapred.output.file") != null){
             outPath = new Path(conf.get("mapred.output.file"));
+            LOG.warn("WARNING: mapred.output.file is deprecated since it only allows one reducer. "
+                    + "Do not set mapred.output.file. Every reducer will generate data to its own output file.");
         }else{
             outPath = getDefaultWorkFile(context, ".bson");
         }

core/src/main/java/com/mongodb/hadoop/mapred/BSONFileOutputFormat.java

@@ -38,6 +38,8 @@ public RecordWriter<K, V> getRecordWriter(FileSystem ignored, JobConf job, Strin
         Path outPath;
         if(job.get("mapred.output.file") != null){
             outPath = new Path(job.get("mapred.output.file"));
+            LOG.warn("WARNING: mapred.output.file is deprecated since it only allows one reducer. "
+                    + "Do not set mapred.output.file. Every reducer will generate data to its own output file.");
         }else{
             outPath = getPathForCustomFile(job, name);
         }