hadoop +可写接口+ readFields 在reducer 中抛出异常

Question

我有一个简单的映射缩减程序，其中我的映射和缩减基元如下所示：

map(K,V) = (Text, OutputAggregator)
reduce(Text, OutputAggregator) = (Text,Text)

重要的一点是，从我的映射函数中，我发出了一个 OutputAggregator 类型的对象，它是我自己的实现 Writable 接口的类。但是，我的减少失败，但出现以下异常。更具体地说，readFieds() 函数抛出异常。有什么线索吗？我使用hadoop 0.18.3

10/09/19 04:04:59 INFO jvm.JvmMetrics: Initializing JVM Metrics with processName=JobTracker, sessionId=
10/09/19 04:04:59 WARN mapred.JobClient: Use GenericOptionsParser for parsing the arguments. Applications should implement Tool for the same.
10/09/19 04:04:59 INFO mapred.FileInputFormat: Total input paths to process : 1
10/09/19 04:04:59 INFO mapred.FileInputFormat: Total input paths to process : 1
10/09/19 04:04:59 INFO mapred.FileInputFormat: Total input paths to process : 1
10/09/19 04:04:59 INFO mapred.FileInputFormat: Total input paths to process : 1
10/09/19 04:04:59 INFO mapred.JobClient: Running job: job_local_0001
10/09/19 04:04:59 INFO mapred.MapTask: numReduceTasks: 1
10/09/19 04:04:59 INFO mapred.MapTask: io.sort.mb = 100
10/09/19 04:04:59 INFO mapred.MapTask: data buffer = 79691776/99614720
10/09/19 04:04:59 INFO mapred.MapTask: record buffer = 262144/327680
Length = 10
10
10/09/19 04:04:59 INFO mapred.MapTask: Starting flush of map output
10/09/19 04:04:59 INFO mapred.MapTask: bufstart = 0; bufend = 231; bufvoid = 99614720
10/09/19 04:04:59 INFO mapred.MapTask: kvstart = 0; kvend = 10; length = 327680
gl_books
10/09/19 04:04:59 WARN mapred.LocalJobRunner: job_local_0001
java.lang.NullPointerException
 at org.myorg.OutputAggregator.readFields(OutputAggregator.java:46)
 at org.apache.hadoop.io.serializer.WritableSerialization$WritableDeserializer.deserialize(WritableSerialization.java:67)
 at org.apache.hadoop.io.serializer.WritableSerialization$WritableDeserializer.deserialize(WritableSerialization.java:40)
 at org.apache.hadoop.mapred.Task$ValuesIterator.readNextValue(Task.java:751)
 at org.apache.hadoop.mapred.Task$ValuesIterator.next(Task.java:691)
 at org.apache.hadoop.mapred.Task$CombineValuesIterator.next(Task.java:770)
 at org.myorg.xxxParallelizer$Reduce.reduce(xxxParallelizer.java:117)
 at org.myorg.xxxParallelizer$Reduce.reduce(xxxParallelizer.java:1)
 at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.combineAndSpill(MapTask.java:904)
 at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.sortAndSpill(MapTask.java:785)
 at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.flush(MapTask.java:698)
 at org.apache.hadoop.mapred.MapTask.run(MapTask.java:228)
 at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:157)
java.io.IOException: Job failed!
 at org.apache.hadoop.mapred.JobClient.runJob(JobClient.java:1113)
 at org.myorg.xxxParallelizer.main(xxxParallelizer.java:145)
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
 at java.lang.reflect.Method.invoke(Unknown Source)
 at org.apache.hadoop.util.RunJar.main(RunJar.java:155)
 at org.apache.hadoop.mapred.JobShell.run(JobShell.java:54)
 at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
 at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:79)
 at org.apache.hadoop.mapred.JobShell.main(JobShell.java:68)

Answer 1

发布有关自定义代码的问题时：发布相关代码段。所以第 46 行和前面几行 & 的内容after 确实会有所帮助...:)

但是这可能会有所帮助：

编写自己的可写类时的陷阱是 Hadoop 一遍又一遍地重用该类的实际实例。在调用 readFields 之间，您不会获得闪亮的新实例。

因此，在 readFields 方法开始时，您必须假设您所在的对象充满了“垃圾”，并且必须在继续之前清除。

我给您的建议是实现一个“clear()”方法，该方法完全擦除当前实例并将其重置为创建该实例且构造函数完成后的状态。当然，您将该方法称为键和值的 readFields 中的第一件事。

华泰

Answer 2

除了 Niels Basjes 的回答之外：只需在空构造函数中初始化您的成员变量（您必须提供该构造函数，否则 Hadoop 无法初始化您的对象），例如：

public OutputAggregator() {
    this.member = new IntWritable();
    ...
}

假设 this.member 的类型为 <代码>IntWritable。