HDFS文件读写操作（基础基础超基础）

环境

• OS: Ubuntu 16.04 64-Bit
• JDK: 1.7.0_80 64-Bit
• Hadoop: 2.6.5

原理

《权威指南》有两张图，下次po上来好好聊一下

实测

读操作

1. 创建在hadoop目录下myclass（放.java和.class文件）和input目录
2. 在input目录下建立quangle.txt文件，并写入内容
3. 将本地文件上传到hdfs的相应文件夹（笔者此处为/class4）中：
   
   hadoop fs -copyFromLocal quangle.txt /class4/quangle.txt
4. 配置hadoop-env.sh文件，添加HADOOP_CLASSPATH变量指向myclass
5. 在myclass中建立FileSystemCat.java代码文件：
6. 编译代码
   
   javac -classpath ../share/hadoop/common/hadoop-common-2.6.5.jar FileSystemCat.java
7. 由编译代码读HDFS文件
   
   hadoop FileSystemCat /class4/quangle.txt

import java.io.InputStream;

import java.net.URI;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.*;
import org.apache.hadoop.io.IOUtils;

public class FileSystemCat {
    public static void main(String[] args) throws Exception {
        String uri = args[0];
        Configuration conf = new Configuration();
        FileSystem fs = FileSystem.get(URI.create(uri), conf);
        InputStream in = null;
        try {
            in = fs.open(new Path(uri));
            IOUtils.copyBytes(in, System.out, 4096, false);
        } finally {
            IOUtils.closeStream(in);
        }
    }
}

写操作

本地文件读入HDFS中

步骤几乎与写操作一致，主要看如何调用API

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.net.URI;

// 以下调用到API均在hadoop-common-2.6.5.jar中
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.util.Progressable;

public class LocalFile2Hdfs {
  public static void main(String[] args) throws Exception {
    String local = args[0];	// 源文件地址
    String uri = args[1];	// 目标文件位置参数

    FileInputStream in = null;
    OutputStream out = null;
    Configuration conf = new Configuration();
    try {
      // 获取读入文件数据
      in = new FileInputStream(new File(local));

      // 获取目标文件信息
      FileSystem fs = FileSystem.get(URI.create(uri), conf);
      out = fs.create(new Path(uri), new Progressable() {
        // 显示进度，每次将64KB数据包写入datanode后打印一次
        public void progress() {
          System.out.println("*");
        }
      });

      in.skip(100);
      byte[] buffer = new byte[20];

      // 读去字符到buffer，再写入Path中
      int bytesRead = in.read(buffer);
      if(bytesRead >= 0) {
        out.write(buffer, 0, bytesRead);
      }
    } finally {
      IOUtils.closeStream(in);
      IOUtils.closeStream(out);
    }
  }
}

从HDFS上读文件并写入本地

代码如下：

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.net.URI;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IOUtils;

public class Hdfs2LocalFile {
    public static void main(String[] args) throws Exception {

        String uri = args[0];
        String local = args[1];

        FSDataInputStream in = null;
        OutputStream out = null;
        Configuration conf = new Configuration();
        try {
            FileSystem fs = FileSystem.get(URI.create(uri), conf);
            in = fs.open(new Path(uri));
            out = new FileOutputStream(local);

            byte[] buffer = new byte[20];
            in.skip(100);
            int bytesRead = in.read(buffer);
            if (bytesRead >= 0) {
                out.write(buffer, 0, bytesRead);
            }
        } finally {
            IOUtils.closeStream(in);
            IOUtils.closeStream(out);
        }
    }
}