对象反序列化readFields时报Error: java.lang.NullPointerException

场景：在使用自定义对象序列化时，报错：

<2015-09-29 14:59:15,578> <INFO > <org.apache.hadoop.mapreduce.Job> - Task Id : attempt_1441852661551_0672_r_000002_2, Status : FAILED(Job.java:printTaskEvents:1462)

Error: java.lang.NullPointerException

at com.wbkit.cobub.models.UserActionDay.readFields(UserActionDay.java:86)

at org.apache.hadoop.io.serializer.WritableSerialization$WritableDeserializer.deserialize(WritableSerialization.java:71)

at org.apache.hadoop.io.serializer.WritableSerialization$WritableDeserializer.deserialize(WritableSerialization.java:42)

at org.apache.hadoop.mapreduce.task.ReduceContextImpl.nextKeyValue(ReduceContextImpl.java:146)

at org.apache.hadoop.mapreduce.task.ReduceContextImpl.nextKey(ReduceContextImpl.java:121)

at org.apache.hadoop.mapreduce.lib.reduce.WrappedReducer$Context.nextKey(WrappedReducer.java:307)

at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:170)

at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:627)

at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:389)

at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:163)

at java.security.AccessController.doPrivileged(Native Method)

at javax.security.auth.Subject.doAs(Subject.java:415)

at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1671)

at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:158)

自定义对象序列化应用分析：

所有的Writable实现都必须有一个默认的构造函数，以便MapReduce框架可以对它们进行实例化，然后再调用readFields()函数查看（填充）各个字段的值。

解决办法：

在自定义序列化对象中，即使在默认构造中也对对象的成员变量进行初始化。

参考:

详细请参见 Hadoop权威指南(第三版) 4.3.3小节；