Using DBOutputFormat to write data to Mysql causes IOException - mysql

Question

Recently, I am learning MapReduce and use it to write data to MySQL database. There are two ways to do so, DBOutputFormat and SQOOP. I tried the first one (refer to here), but encountered a problem, following is the error:
...
16/05/25 09:36:53 INFO mapred.LocalJobRunner: 3 / 3 copied.
16/05/25 09:36:53 INFO mapred.LocalJobRunner: reduce task executor complete.
16/05/25 09:36:53 WARN output.FileOutputCommitter: Output Path is null in cleanupJob()
16/05/25 09:36:53 WARN mapred.LocalJobRunner: job_local1404930626_0001
java.lang.Exception: java.io.IOException
at org.apache.hadoop.mapred.LocalJobRunner$Job.runTasks(LocalJobRunner.java:462)
at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:529)
Caused by: java.io.IOException
at org.apache.hadoop.mapreduce.lib.db.DBOutputFormat.getRecordWriter(DBOutputFormat.java:185)
at org.apache.hadoop.mapred.ReduceTask$NewTrackingRecordWriter.<init>(ReduceTask.java:540)
at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:614)
at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:389)
at org.apache.hadoop.mapred.LocalJobRunner$Job$ReduceTaskRunnable.run(LocalJobRunner.java:319)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)
16/05/25 09:36:54 INFO mapreduce.Job: Job job_local1404930626_0001 failed with state FAILED due to: NA
16/05/25 09:36:54 INFO mapreduce.Job: Counters: 38
File System Counters
FILE: Number of bytes read=32583
FILE: Number of bytes written=796446
FILE: Number of read operations=0
FILE: Number of large read operations=0
FILE: Number of write operations=0
HDFS: Number of bytes read=402
HDFS: Number of bytes written=0
HDFS: Number of read operations=18
HDFS: Number of large read operations=0
HDFS: Number of write operations=0
...
while I manually use JDBC to connect and insert data, it turns out to be successful. And I notice that the map/reduce task executors are complete, but it encounters the IOException. So I guess the problem is database-related.
My code is here. Appriciated if some one could help me to figure out what is the problem.
Thanks in advance!