在 Hadoop 中使用 NullWritable 的优势

What are the advantages of using NullWritable for null keys/values over using null texts (i.e. new Text(null)). I see the following from the «Hadoop: The Definitive Guide» book.

NullWritable 是 Writable 的一种特殊类型，因为它具有零长度序列化.无字节被写入流或从流中读取.它用作占位符；例如，在MapReduce，一个键或者一个值在不需要的时候可以声明为NullWritable使用那个位置——它有效地存储了一个常量空值.NullWritable 也可以当您想要存储值列表时，可用作 SequenceFile 中的键，而不是到键值对.它是一个不可变的单例:可以通过调用来检索实例NullWritable.get()

NullWritable is a special type of Writable, as it has a zero-length serialization. No bytes are written to, or read from, the stream. It is used as a placeholder; for example, in MapReduce, a key or a value can be declared as a NullWritable when you don’t need to use that position—it effectively stores a constant empty value. NullWritable can also be useful as a key in SequenceFile when you want to store a list of values, as opposed to key-value pairs. It is an immutable singleton: the instance can be retrieved by calling NullWritable.get()

我不清楚如何使用 NullWritable 写出输出?会不会在开始的输出文件中有一个常量值表示这个文件的key或者value是null，这样MapReduce框架就可以忽略读取nullkeys/值(以 null 为准)?另外，null 文本实际上是如何序列化的?

I do not clearly understand how the output is written out using NullWritable? Will there be a single constant value in the beginning output file indicating that the keys or values of this file are null, so that the MapReduce framework can ignore reading the null keys/values (whichever is null)? Also, how actually are null texts serialized?

谢谢，

文卡特

推荐答案

键/值类型必须在运行时给出，所以任何写或读 NullWritables 的东西都会提前知道它将是处理该类型；文件中没有标记或任何内容.从技术上讲，NullWritables 是读取"的，只是读取"一个 NullWritable 实际上是无操作的.你可以亲眼看到根本没有写或读:

The key/value types must be given at runtime, so anything writing or reading NullWritables will know ahead of time that it will be dealing with that type; there is no marker or anything in the file. And technically the NullWritables are "read", it's just that "reading" a NullWritable is actually a no-op. You can see for yourself that there's nothing at all written or read:

NullWritable nw = NullWritable.get();
ByteArrayOutputStream out = new ByteArrayOutputStream();
nw.write(new DataOutputStream(out));
System.out.println(Arrays.toString(out.toByteArray())); // prints "[]"

ByteArrayInputStream in = new ByteArrayInputStream(new byte[0]);
nw.readFields(new DataInputStream(in)); // works just fine

关于new Text(null)的问题，你可以再试一试:

And as for your question about new Text(null), again, you can try it out:

Text text = new Text((String)null);
ByteArrayOutputStream out = new ByteArrayOutputStream();
text.write(new DataOutputStream(out)); // throws NullPointerException
System.out.println(Arrays.toString(out.toByteArray()));

Text 根本无法使用 null String.

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