hadoop 学习笔记（第三章 Hadoop分布式文件系统 ）

map->shuffle->reduce

map(k1,v1)--->(k2,v2)

reduce(k2,List<v2>)--->(k2,v3)

传输类型：org.apache.hadoop.io

访问HDFS文件系统

1.java.net.URL 的setURLStreamHandlerFactory() 方法。每个java虚拟机只能调用一次，因此通常在静态方法中调用。如果引用的第三方组件调用过，再次调用会报错。

public class App 
{
    static{
        URL.setURLStreamHandlerFactory(new FsUrlStreamHandlerFactory());
    }

    static InputStream inputStream=null;
    public static void main( String[] args ) throws Exception
    {
        try{
           inputStream=new URL(args[0]).openStream();
           IOUtils.copyBytes(inputStream,System.out,4096,false);
        }finally {
            IOUtils.closeStream(inputStream);
        }
    }
}

2.FileSystem API 读取数据

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

//实际上，FileSystem对象中open()方法返回的是FSDataInputStream对象。其实现了Seekable接口和PositionedReadable接口
public class FSDataInputStream extends DataInputStream implements Seekable, PositionedReadable, ByteBufferReadable, HasFileDescriptor, CanSetDropBehind, CanSetReadahead, HasEnhancedByteBufferAccess, CanUnbuffer {
}
public interface Seekable {
  /**
   * Seek to the given offset from the start of the file.
   * The next read() will be from that location.  Can't
   * seek past the end of the file.
   */
  void seek(long pos) throws IOException;

  /**
   * Return the current offset from the start of the file
   */
  long getPos() throws IOException;

  /**
   * Seeks a different copy of the data.  Returns true if
   * found a new source, false otherwise.
   */
  @InterfaceAudience.Private
  boolean seekToNewSource(long targetPos) throws IOException;
}

public interface PositionedReadable {
  /**
   * Read upto the specified number of bytes, from a given
   * position within a file, and return the number of bytes read. This does not
   * change the current offset of a file, and is thread-safe.
   */
  public int read(long position, byte[] buffer, int offset, int length)
    throws IOException;

  /**
   * Read the specified number of bytes, from a given
   * position within a file. This does not
   * change the current offset of a file, and is thread-safe.
   */
  public void readFully(long position, byte[] buffer, int offset, int length)
    throws IOException;

  /**
   * Read number of bytes equal to the length of the buffer, from a given
   * position within a file. This does not
   * change the current offset of a file, and is thread-safe.
   */
  public void readFully(long position, byte[] buffer) throws IOException;
}

read()和readFully()的区别是readFully()在读取到length之前会阻塞，read()如果读到的小于length，读到多少返回多少。

seek()方法的开销较高，要谨慎使用。

3.写入数据

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

        String localSrc=args[0];
        String dstSrc=args[1];

        InputStream inputStream=new BufferedInputStream(new FileInputStream(localSrc));

        Configuration configuration=new Configuration();
        FileSystem fs=FileSystem.get(URI.create(dstSrc),configuration);

        OutputStream outputStream=fs.create(new Path(dstSrc), new Progressable() {
            @Override
            public void progress() {
                System.out.print(".");
            }
        });

        IOUtils.copyBytes(inputStream,outputStream,4096,true);
    }
}

4.目录与查询

FileSystem提供mkdir方法创建目录。通常不需要，因为create方法写入文件时会自动创建目录

public boolean mkdirs(Path f) throws IOException {
}

FileSystem提供getFileStatus方法返回文件元数据。元数据包括文件的地址，大小，权限等

public abstract FileStatus getFileStatus(Path f) throws IOException;

FIleSystem提供listStatus()方法列出目录中的文件

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

        String uri = args[0];
        Configuration configuration = new Configuration();
        FileSystem fs = FileSystem.get(URI.create(uri), configuration);

        Path[] paths = new Path[args.length];

        for (int i = 0; i < paths.length; i++) {
            paths[i] = new Path(args[i]);
        }

        FileStatus[] status = fs.listStatus(paths);
        Path[] listPaths = FileUtil.stat2Paths(status);

        for (Path path : listPaths) {
            System.out.println(path);
        }
    }
}

FileSystem还提供globStatus方法返回与指定格式匹配的所有FIleStatus

 public FileStatus[] globStatus(Path pathPattern, PathFilter filter) throws IOException ;

FileSystem提供delete方法永久删除文件或目录。如果f是一个空目录，recursive就会被忽略。如果f非空，只有在recursive为true时才会执行删除。

public abstract boolean delete(Path f, boolean recursive) throws IOException;