Hadoop：在本地模式下运行的奇怪的空点异常

Question

我刚开始学习Hadoop，我从一本书中摘录了一个例子。所以我创建了一个本地运行的MapReducer，它可以从NCDC免费数据文件中提取温度。这是一个数据样本：

0143023780999992012010100004+61450+017167FM-12+002799999V0209999C...cut...;

每一个文件（我下载了大约100个文件）都由许多这样的行组成。

我的映射程序执行简单的解析操作，从这些文件中提取温度。整个过程将返回最高温度。

映射器和相关测试：

public class MaxTemperatureMapper extends Mapper<LongWritable,Text,Text,IntWritable> {

@Override
public void map(LongWritable key, Text value, Context context) {
    String record = value.toString();
    String year = record.substring(15,19);
    int airTemperature = extractTemp(record);
    if (isNotValidTemp(record, airTemperature)) return;
    try {
        context.write(new Text(year), new IntWritable(airTemperature));
    } catch (IOException e) {
        e.printStackTrace();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

private boolean isNotValidTemp(String record, int airTemperature) {
    return airTemperature == 9999 || !record.substring(92, 93).matches("[01459]");
}

private int extractTemp(String record) {
    String temp = (record.charAt(87) == '+')
            ? record.substring(88,92)
            : record.substring(87,92);
    return Integer.parseInt(temp);
}

}

public class MaxTemperatureMapperTest {

@Test
public void processRecord() {
    Text value = new Text("0111011120999992012010100004+65450+012217FM-12+000999999V0201301N014019999999N9999999N1+00031-00791099271ADDMA1999999099171MD1810341+9999REMSYN070AAXX  01001 01112 46/// /1314 10003 21079 39917 49927 58034 333 91124;");

    new MapDriver<LongWritable, Text, Text, IntWritable>()
            .withMapper(new MaxTemperatureMapper())
            .withInputValue(value)
            .withOutput(new Text("2012"), new IntWritable(3))
            .runTest();
}

@Test
public void processRecordsFromSuspiciousFile() throws IOException {
    final InputStream is = getClass().getClassLoader().getSystemResource("023780-99999-2012").openStream();
    BufferedReader br = new BufferedReader(new InputStreamReader(is));
    String line;
    Iterator<Integer> ii = Arrays.asList(-114, -120, -65, -45, 1, 4, 6, 6, 10, 16, 18, 29, 32, 17, 7, 16, 22, 8, 8, 20).iterator();
    while ((line = br.readLine()) != null) {
        new MapDriver<LongWritable, Text, Text, IntWritable>()
                .withMapper(new MaxTemperatureMapper())
                .withInputValue(new Text(line))
                .withOutput(new Text("2012"), new IntWritable(ii.next()))
                .runTest();
    }
    br.close();


}
}

减速器及相关测试：

public class MaxTemperatureReducer extends Reducer<Text,IntWritable,Text,IntWritable> {

@Override
public void reduce(Text key, Iterable<IntWritable> values, Context context) {
    int maxValue = Integer.MIN_VALUE;
    for (IntWritable value : values) {
        maxValue = Math.max(value.get(), maxValue);
    }
    try {
        context.write(key, new IntWritable(maxValue));
    } catch (IOException e) {
        e.printStackTrace();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

}

public class MaxTemperatureReducerTest {

@Test
public void processRecord() {

    new ReduceDriver<Text,IntWritable,Text,IntWritable>()
            .withReducer(new MaxTemperatureReducer())
            .withInputKey(new Text("2012"))
            .withInputValues(Arrays.asList(new IntWritable(5), new IntWritable(10)))
            .withOutput(new Text("2012"), new IntWritable(10))
            .runTest();
}
}

最后是驾驶员等级测试：

public class MaxTemperatureDriver extends Configured implements Tool {

@Override
public int run(String[] args) throws Exception {
    if (args.length != 2) {
        System.err.printf("Usage: %s [generic options] <input> <output>\n", getClass().getSimpleName());
        ToolRunner.printGenericCommandUsage(System.err);
        return -1;
    }

    Job job = new Job(getConf(), "Max Temperature");
    job.setJarByClass(getClass());

    FileInputFormat.addInputPath(job, new Path(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));

    job.setMapperClass(MaxTemperatureMapper.class);
    job.setCombinerClass(MaxTemperatureReducer.class);
    job.setReducerClass(MaxTemperatureReducer.class);

    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(Iterable.class);

    return job.waitForCompletion(true) ? 0 : 1;
}

public static void main(String[] args) throws Exception {
    int exitCode = ToolRunner.run(new MaxTemperatureDriver(), args);
    System.exit(exitCode);
}
}

public class MaxTemperatureDriverTest {

@Test
public void test() throws Exception {
    Configuration conf = new Configuration();
    conf.set("fs.default.name", "file:///");
    conf.set("mapred.job.tracker", "local");

    Path input = new Path("file:////home/user/big-data/ncdc/");
    Path output = new Path("output");

    FileSystem fs = FileSystem.getLocal(conf);
    fs.delete(output, true);

    MaxTemperatureDriver driver = new MaxTemperatureDriver();
    driver.setConf(conf);

    int exitCode = driver.run(new String[] { input.toString(), output.toString() });
    assertThat(exitCode, is(0));
}
}

我使用命令行运行整个过程：

$> hadoop doop.MaxTemperatureDriver -fs file:/// -jt local ~/big-data/ncdc/ output

MaxTemperatureDriverTest中的测试，但在这两种情况下，我都得到：

    13/09/21 19:45:13 INFO mapred.MapTask: Processing split: file:/home/user/big-data/ncdc/023780-99999-2012:0+5337
13/09/21 19:45:13 INFO mapred.MapTask: io.sort.mb = 100
13/09/21 19:45:14 INFO mapred.MapTask: data buffer = 79691776/99614720
13/09/21 19:45:14 INFO mapred.MapTask: record buffer = 262144/327680
13/09/21 19:45:14 INFO mapred.LocalJobRunner: Map task executor complete.
13/09/21 19:45:14 WARN mapred.LocalJobRunner: job_local462595973_0001
java.lang.Exception: java.lang.NullPointerException
    at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:354)
Caused by: java.lang.NullPointerException
    at org.apache.hadoop.io.serializer.SerializationFactory.getSerializer(SerializationFactory.java:73)
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.<init>(MapTask.java:970)
    at org.apache.hadoop.mapred.MapTask$NewOutputCollector.<init>(MapTask.java:673)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:756)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:364)
    at org.apache.hadoop.mapred.LocalJobRunner$Job$MapTaskRunnable.run(LocalJobRunner.java:223)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:439)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
    at java.util.concurrent.FutureTask.run(FutureTask.java:138)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    at java.lang.Thread.run(Thread.java:662)

以一种“过于通用”的方式，当试图解析文件“023780-99999-2012”时，它总是返回空指针异常。所以我为它写了一个测试（你可以在映射器测试“进程记录”中看到），但它不返回错误。我也检查了日志，但没有成功。

是否与错误或缺少本地模式参数（线程数、堆内存等）有关？还是密码有问题？

Answer 1

Hadoop完全不知道如何序列化Iterable。如果您真的打算使用Iterable作为输出值类，那么还需要为Iterable指定一个序列化程序。Hadoop使用的典型I/O类型是可写的子类。

更新：我现在知道您打算使用intwriteable作为输出值类。你的问题是这条驱动线：

job.setOutputValueClass(Iterable.class)

应该是

job.setOutputValueClass(IntWritable.class)