为什么sequencefile会被截断?

6qqygrtg  于 2021-05-30  发布在  Hadoop
关注(0)|答案(3)|浏览(415)

我在学习 Hadoop 这个问题让我困惑了一段时间。基本上我在写一本书 SequenceFile 到磁盘,然后读回。然而,每次我得到一个 EOFException 阅读时。深入研究会发现,在写入序列文件时,它会被过早地截断,而且总是在写入索引962之后发生,并且文件的固定大小总是为45056字节。
我正在MacBookPro上使用Java8和Hadoop2.5.1。事实上,我在Java7下的另一台linux机器上尝试了相同的代码,但同样的情况也发生了。
我可以排除书写器/读取器未正确关闭的可能性。我尝试使用旧样式的try/catch和显式writer.close(),如代码所示,还使用了较新的try-with-resource方法。两者都不起作用。
任何帮助都将不胜感激。
下面是我使用的代码:

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();
    }
}

}
yiytaume

yiytaume1#

我认为您缺少writer.close()在write循环之后。在你开始阅读之前,这应该是最后的冲水。

ohfgkhjo

ohfgkhjo2#

多亏了托马斯。
归根结底,如果作者创造了“拥有”的不是流。在创建writer时,如果传入writer.file(path)选项,writer“拥有”内部创建的底层流,并在调用close()时将其关闭。但是,如果我们传入writer.stream(astream),writer会假定其他人是该流的响应,并且在调用close()时不会关闭它。简言之,这不是一个错误,只是我不太了解它。

wtzytmuj

wtzytmuj3#

实际上,我发现了这个错误,这是因为您从未关闭中创建的流 Writer.stream(fs.create(path)) .
由于某些原因,关闭不会传播到您刚才在那里创建的流。我想这是一个bug,但我现在懒得在jira中查找它。
解决问题的一个方法就是简单地使用 Writer.file(path) 相反。
显然,您也可以显式地关闭create流。下面是我的更正示例:

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);
        }

    }

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