java - 为什么 SequenceFile 被截断了？

Question

我正在学习Hadoop，这个问题困扰了我一段时间。基本上，我正在将一个 SequenceFile 写入磁盘，然后将其读回。但是，每次读取时都会得到一个EOFException。更深层次的观察发现，在写入序列文件时，它被过早截断，并且总是在写入索引 962 之后发生，并且文件总是固定大小为 45056 字节。

我在 MacBook Pro 上使用 Java 8 和 Hadoop 2.5.1。事实上，我在另一台运行 Java 7 的 Linux 机器上尝试了相同的代码，但同样的事情发生了。

我可以排除 writer/reader 没有正确关闭。我尝试使用带有显式 writer.close() 的旧式 try/catch(如代码所示)，并使用较新的 try-with-resource 方法。两者都不起作用。

我们将不胜感激任何帮助。

以下是我使用的代码:

public class SequenceFileDemo {

private static final String[] DATA = { "One, two, buckle my shoe",
    "Three, four, shut the door",
    "Five, six, pick up sticks",
    "Seven, eight, lay them straight",
    "Nine, ten, a big fat hen" };

public static void main(String[] args) throws Exception {
    String uri = "file:///Users/andy/Downloads/puzzling.seq";
    Configuration conf = new Configuration();
    FileSystem fs = FileSystem.get(URI.create(uri), conf);

    Path path = new Path(uri);      
    IntWritable key = new IntWritable();
    Text value = new Text();

    //API change
    try {
        SequenceFile.Writer writer = SequenceFile.createWriter(conf, 
            stream(fs.create(path)),
            keyClass(IntWritable.class),
            valueClass(Text.class));

        for ( int i = 0; i < 1024; i++ ) {
            key.set( i);
            value.clear();
            value.set(DATA[i % DATA.length]);

            writer.append(key, value);
            if ( (i-1) %100 == 0 ) writer.hflush();
            System.out.printf("[%s]\t%s\t%s\n", writer.getLength(), key, value);
        }

        writer.close();

    } catch (Exception e ) {
        e.printStackTrace();
    }


    try {
        SequenceFile.Reader reader = new SequenceFile.Reader(conf, 
                SequenceFile.Reader.file(path));
        Class<?> keyClass = reader.getKeyClass();
        Class<?> valueClass = reader.getValueClass();

        boolean isWritableSerilization = false;
        try {
            keyClass.asSubclass(WritableComparable.class);
            isWritableSerilization = true;
        } catch (ClassCastException e) {

        }

        if ( isWritableSerilization ) {
            WritableComparable<?> rKey = (WritableComparable<?>) ReflectionUtils.newInstance(keyClass, conf);
            Writable rValue = (Writable) ReflectionUtils.newInstance(valueClass, conf);
            while(reader.next(rKey, rValue)) {
                System.out.printf("[%s] %d %s=%s\n",reader.syncSeen(), reader.getPosition(), rKey, rValue);
            }
        } else {
            //make sure io.seraizliatons has the serialization in use when write the sequence file
        }

        reader.close();
    } catch(IOException e) {
        e.printStackTrace();
    }
}

}

Answer 1

我确实发现了这个错误，这是因为你从来没有关闭在 Writer.stream(fs.create(path)) 中创建的流。

出于某种原因，关闭不会向下传播到您刚刚在那里创建的流。我想这是一个错误，但我现在懒得在 Jira 中查找它。

解决问题的一种方法是简单地使用 Writer.file(path) 代替。

显然，您也可以直接关闭创建流。在下面找到我更正的示例:

    Path path = new Path("file:///tmp/puzzling.seq");

    try (FSDataOutputStream stream = fs.create(path)) {
        try (SequenceFile.Writer writer = SequenceFile.createWriter(conf, Writer.stream(stream),
                Writer.keyClass(IntWritable.class), Writer.valueClass(NullWritable.class))) {

            for (int i = 0; i < 1024; i++) {
                writer.append(new IntWritable(i), NullWritable.get());
            }
        }
    }

    try (SequenceFile.Reader reader = new SequenceFile.Reader(conf, Reader.file(path))) {
        Class<?> keyClass = reader.getKeyClass();
        Class<?> valueClass = reader.getValueClass();

        WritableComparable<?> rKey = (WritableComparable<?>) ReflectionUtils.newInstance(keyClass, conf);
        Writable rValue = (Writable) ReflectionUtils.newInstance(valueClass, conf);
        while (reader.next(rKey, rValue)) {
            System.out.printf("%s = %s\n", rKey, rValue);
        }

    }