[hadoop2.7.1]I/O之SequenceFile最新API编程实例（写入、读取）

写操作

根据上一篇的介绍，在hadoop2.x之后，hadoop中的SequenceFile.Writer将会逐渐摒弃大量的createWriter（）重载方法，而整合为更为简洁的createWriter()方法，除了配置参数外，其他的参数统统使用SequenceFile.Writer.Option来替代，具体有：

新的API里提供的option参数：

FileOption
FileSystemOption
StreamOption
BufferSizeOption
BlockSizeOption
ReplicationOption
KeyClassOption
ValueClassOption
MetadataOption
ProgressableOption
CompressionOption

这些参数能够满足各种不同的需要，参数之间不存在顺序关系，这样减少了代码编写工作量，更为直观，便于理解，下面先来看看这个方法，后边将给出一个具体实例。

• createWriter

  public staticorg.apache.hadoop.io.SequenceFile.WritercreateWriter(Configurationconf,
                                                      org.apache.hadoop.io.SequenceFile.Writer.Option...opts)
                                                               throws IOException

  Create a new Writer with the given options.
  Parameters:
  conf - the configuration to use
  opts - the options to create the file with
  Returns:
  a new Writer
  Throws:
  IOException

权威指南第四版中提供了一个SequenceFileWriteDemo实例：

// cc SequenceFileWriteDemo Writing a SequenceFile
import java.io.IOException;
import java.net.URI;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.Text;

// vv SequenceFileWriteDemo
public class SequenceFileWriteDemo {
  
  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 IOException {
    String uri = args[0];
    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();
    SequenceFile.Writer writer = null;
    try {
      writer = SequenceFile.createWriter(fs, conf, path,
          key.getClass(), value.getClass());
      
      for (int i = 0; i < 100; i++) {
        key.set(100 - i);
        value.set(DATA[i % DATA.length]);
        System.out.printf("[%s]\t%s\t%s\n", writer.getLength(), key, value);
        writer.append(key, value);
      }
    } finally {
      IOUtils.closeStream(writer);
    }
  }
}
// ^^ SequenceFileWriteDemo

对于上面实例中的createWriter()方法用整合之后的最新的方法来改写一下，代码如下：

package org.apache.hadoop.io;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.SequenceFile.Writer;
import org.apache.hadoop.io.SequenceFile.Writer.FileOption;
import org.apache.hadoop.io.SequenceFile.Writer.KeyClassOption;
import org.apache.hadoop.io.SequenceFile.Writer.ValueClassOption;
import org.apache.hadoop.io.Text;

public class THT_testSequenceFile2 {

	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 IOException {
		// String uri = args[0];
		String uri = "file:///D://B.txt";
		Configuration conf = new Configuration();
		Path path = new Path(uri);

		IntWritable key = new IntWritable();
		Text value = new Text();
		SequenceFile.Writer writer = null;
		SequenceFile.Writer.FileOption option1 = (FileOption) Writer.file(path);
		SequenceFile.Writer.KeyClassOption option2 = (KeyClassOption) Writer.keyClass(key.getClass());
		SequenceFile.Writer.ValueClassOption option3 = (ValueClassOption) Writer.valueClass(value.getClass());
		
		try {
			
			writer = SequenceFile.createWriter( conf, option1,option2,option3,Writer.compression(CompressionType.RECORD));
			
			for (int i = 0; i < 10; i++) {
				key.set(1 + i);
				value.set(DATA[i % DATA.length]);
				System.out.printf("[%s]\t%s\t%s\n", writer.getLength(), key,
						value);
				writer.append(key, value);
			}
		} finally {
			IOUtils.closeStream(writer);
		}
	}
}

运行结果如下：

2015-11-06 22:15:05,027 INFO  compress.CodecPool (CodecPool.java:getCompressor(153)) - Got brand-new compressor [.deflate]
[128]	1	One, two, buckle my shoe
[173]	2	Three, four, shut the door
[220]	3	Five, six, pick up sticks
[264]	4	Seven, eight, lay them straight
[314]	5	Nine, ten, a big fat hen
[359]	6	One, two, buckle my shoe
[404]	7	Three, four, shut the door
[451]	8	Five, six, pick up sticks
[495]	9	Seven, eight, lay them straight
[545]	10	Nine, ten, a big fat hen

生成的文件：

读操作

新的API里提供的option参数：

FileOption -表示读哪个文件
InputStreamOption
StartOption
LengthOption -按照设置的长度变量来决定读取的字节
BufferSizeOption
OnlyHeaderOption

根据最新的API直接上源码：

package org.apache.hadoop.io;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.SequenceFile.Reader;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.util.ReflectionUtils;

public class THT_testSequenceFile3 {

	public static void main(String[] args) throws IOException {
		//String uri = args[0];
		String uri = "file:///D://B.txt";
		Configuration conf = new Configuration();
		Path path = new Path(uri);
		SequenceFile.Reader.Option option1 = Reader.file(path);
		SequenceFile.Reader.Option option2 = Reader.length(174);//这个参数表示读取的长度
		
		SequenceFile.Reader reader = null;
		try {
			reader = new SequenceFile.Reader(conf,option1,option2);
			Writable key = (Writable) ReflectionUtils.newInstance(
					reader.getKeyClass(), conf);
			Writable value = (Writable) ReflectionUtils.newInstance(
					reader.getValueClass(), conf);
			long position = reader.getPosition();
			while (reader.next(key, value)) {
				String syncSeen = reader.syncSeen() ? "*" : "";
				System.out.printf("[%s%s]\t%s\t%s\n", position, syncSeen, key,
						value);
				position = reader.getPosition(); // beginning of next record
			}
		} finally {
			IOUtils.closeStream(reader);
		}
	}
}

我这儿设置了一个读取长度的参数，只读到第174个字节那，所以运行结果如下：

2015-11-06 22:53:00,602 INFO  compress.CodecPool (CodecPool.java:getDecompressor(181)) - Got brand-new decompressor [.deflate]
[128]	1	One, two, buckle my shoe
[173]	2	Three, four, shut the door