Hadoop 为 java.nio.ByteBuffer 的 keytype 抛出 ClassCastException

发布于 2024-12-27 00:47:39 字数 1197 浏览 5 评论 0原文

我使用“hadoop-0.20.203.0rc1.tar.gz”进行集群设置。每当我设置 job.setMapOutputKeyClass(ByteBuffer.class);

并运行作业时，我都会收到以下异常：

    12/01/13 15:09:00 INFO mapred.JobClient: Task Id : attempt_201201131428_0005_m_000001_2, Status : FAILED
java.lang.ClassCastException: class java.nio.ByteBuffer
        at java.lang.Class.asSubclass(Class.java:3018)
        at org.apache.hadoop.mapred.JobConf.getOutputKeyComparator(JobConf.java:776)
        at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.<init>(MapTask.java:958)
        at org.apache.hadoop.mapred.MapTask$NewOutputCollector.<init>(MapTask.java:673)
        at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:755)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:369)
        at org.apache.hadoop.mapred.Child$4.run(Child.java:259)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:396)
        at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1059)
        at org.apache.hadoop.mapred.Child.main(Child.java:253)

我还注意到 ByteBuffer 是可比较的而不是可写的，这有什么区别吗？如果需要任何其他信息，请告诉我。

原文

I am using "hadoop-0.20.203.0rc1.tar.gz" for my cluster setup.
Whenever I set job.setMapOutputKeyClass(ByteBuffer.class);

and run the job I get following Exception:

    12/01/13 15:09:00 INFO mapred.JobClient: Task Id : attempt_201201131428_0005_m_000001_2, Status : FAILED
java.lang.ClassCastException: class java.nio.ByteBuffer
        at java.lang.Class.asSubclass(Class.java:3018)
        at org.apache.hadoop.mapred.JobConf.getOutputKeyComparator(JobConf.java:776)
        at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.<init>(MapTask.java:958)
        at org.apache.hadoop.mapred.MapTask$NewOutputCollector.<init>(MapTask.java:673)
        at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:755)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:369)
        at org.apache.hadoop.mapred.Child$4.run(Child.java:259)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:396)
        at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1059)
        at org.apache.hadoop.mapred.Child.main(Child.java:253)

Also i have noticed that ByteBuffer is Comparable and not Writable is that make any diffrence?
Let me know if any additional information needed.

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白日梦 2025-01-03 00:47:39

这是抛出异常的地方。这是代码。

public RawComparator getOutputKeyComparator() {
    Class<? extends RawComparator> theClass = getClass("mapred.output.key.comparator.class",
        null, RawComparator.class);
    if (theClass != null)
        return ReflectionUtils.newInstance(theClass, this);
    return WritableComparator.get(getMapOutputKeyClass().asSubclass(WritableComparable.class));
}

如果 JobConf 上未定义 mapred.output.key.comparator.class 属性，则该键必须实现 WritableComparable 接口。 ByteBuffer 类没有实现 WritableComparable 接口，因此出现异常。

顺便说一句，WritableComparable 接口是 Writable 和 Comparable 类的子接口。

This is where the exception is being thrown. Here is the code from SVN.

public RawComparator getOutputKeyComparator() {
    Class<? extends RawComparator> theClass = getClass("mapred.output.key.comparator.class",
        null, RawComparator.class);
    if (theClass != null)
        return ReflectionUtils.newInstance(theClass, this);
    return WritableComparator.get(getMapOutputKeyClass().asSubclass(WritableComparable.class));
}

If mapred.output.key.comparator.class property is not defined on JobConf then the key must implement the WritableComparable interface. ByteBuffer class doesn't implement the WritableComparable interface, so the exception.

BTW, WritableComparable interface is sub-interface of Writable and Comparable classes.

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