org.apache.hadoop.fs-BlockLocation

---

工具类吧

---

package org.apache.hadoop.fs;

import org.apache.hadoop.io.*;
//IO包下的类还没涉及到。遇到一个分析一个。
import java.io.*;

/*
 * A BlockLocation lists hosts, offset and length
 * of block. 
 * 
 */
//记录block的元数据信息，如所在host，长度和偏移量
public class BlockLocation implements Writable {
//针对集群块位置的类
  static {               // register a ctor
    WritableFactories.setFactory
      (BlockLocation.class,
       new WritableFactory() {
         public Writable newInstance() { return new BlockLocation(); }
       });
  }
//注册了一个Writable子类BlockLocation的工厂。内部类。详细可看http://book.2cto.com/201305/21915.html
  private String[] hosts; //hostnames of datanodes
  //节点的主机名数组
  private String[] names; //hostname:portNumber of datanodes
  //节点的名称数组。名称的格式。
  private String[] topologyPaths; // full path name in network topology
  //节点在网络拓扑结构中的地址
  private long offset;  //offset of the of the block in the file
  //块在文件中的偏移量
  private long length;
  //块长度

  /**
   * Default Constructor
   */
  public BlockLocation() {
    this(new String[0], new String[0],  0L, 0L);
  }
//默认构造方法
  /**
   * Constructor with host, name, offset and length
   */
  public BlockLocation(String[] names, String[] hosts, long offset, 
                       long length) {
    if (names == null) {
      this.names = new String[0];
    } else {
      this.names = names;
    }
    if (hosts == null) {
      this.hosts = new String[0];
    } else {
      this.hosts = hosts;
    }
    this.offset = offset;
    this.length = length;
    this.topologyPaths = new String[0];
  }
//根据名称主机偏移量长度初始化一个块对象
  /**
   * Constructor with host, name, network topology, offset and length
   */
  public BlockLocation(String[] names, String[] hosts, String[] topologyPaths,
                       long offset, long length) {
    this(names, hosts, offset, length);
    if (topologyPaths == null) {
      this.topologyPaths = new String[0];
    } else {
      this.topologyPaths = topologyPaths;
    }
  }
//根据.........
  /**
   * Get the list of hosts (hostname) hosting this block
   */
  public String[] getHosts() throws IOException {
    if ((hosts == null) || (hosts.length == 0)) {
      return new String[0];
    } else {
      return hosts;
    }
  }
//获得块的主机
  /**
   * Get the list of names (hostname:port) hosting this block
   */
  public String[] getNames() throws IOException {
    if ((names == null) || (names.length == 0)) {
      return new String[0];
    } else {
      return this.names;
    }
  }
//。。。。
  /**
   * Get the list of network topology paths for each of the hosts.
   * The last component of the path is the host.
   */
  public String[] getTopologyPaths() throws IOException {
    if ((topologyPaths == null) || (topologyPaths.length == 0)) {
      return new String[0];
    } else {
      return this.topologyPaths;
    }
  }
  //。。。。。
  /**
   * Get the start offset of file associated with this block
   */
  public long getOffset() {
    return offset;
  }
  //。。。。。
  /**
   * Get the length of the block
   */
  public long getLength() {
    return length;
  }
  //。。。。。
  /**
   * Set the start offset of file associated with this block
   */
  public void setOffset(long offset) {
    this.offset = offset;
  }
//。。。。。
  /**
   * Set the length of block
   */
  public void setLength(long length) {
    this.length = length;
  }
//。。。。。
  /**
   * Set the hosts hosting this block
   */
  public void setHosts(String[] hosts) throws IOException {
    if (hosts == null) {
      this.hosts = new String[0];
    } else {
      this.hosts = hosts;
    }
  }
//。。。。。
  /**
   * Set the names (host:port) hosting this block
   */
  public void setNames(String[] names) throws IOException {
    if (names == null) {
      this.names = new String[0];
    } else {
      this.names = names;
    }
  }
//。。。。。
  /**
   * Set the network topology paths of the hosts
   */
  public void setTopologyPaths(String[] topologyPaths) throws IOException {
    if (topologyPaths == null) {
      this.topologyPaths = new String[0];
    } else {
      this.topologyPaths = topologyPaths;
    }
  }
//。。。。。
  /**
   * Implement write of Writable
   */
  public void write(DataOutput out) throws IOException {
    out.writeLong(offset);
    out.writeLong(length);
    out.writeInt(names.length);
    for (int i=0; i < names.length; i++) {
      Text name = new Text(names[i]);
      name.write(out);
    }
    out.writeInt(hosts.length);
    for (int i=0; i < hosts.length; i++) {
      Text host = new Text(hosts[i]);
      host.write(out);
    }
    out.writeInt(topologyPaths.length);
    for (int i=0; i < topologyPaths.length; i++) {
      Text host = new Text(topologyPaths[i]);
      host.write(out);
    }
  }
  //把块信息写到输出流中。用到了Writable子类Long,Int,Text的Write方法。序列化
  /**
   * Implement readFields of Writable
   */
  public void readFields(DataInput in) throws IOException {
    this.offset = in.readLong();
    this.length = in.readLong();
    int numNames = in.readInt();
    this.names = new String[numNames];
    for (int i = 0; i < numNames; i++) {
      Text name = new Text();
      name.readFields(in);
      names[i] = name.toString();
    }
    int numHosts = in.readInt();
    for (int i = 0; i < numHosts; i++) {
      Text host = new Text();
      host.readFields(in);
      hosts[i] = host.toString();
    }
    int numTops = in.readInt();
    Text path = new Text();
    for (int i = 0; i < numTops; i++) {
      path.readFields(in);
      topologyPaths[i] = path.toString();
    }
  }
  //把块信息从输入流中读出来。....。反序列化
  public String toString() {
    StringBuilder result = new StringBuilder();
    result.append(offset);
    result.append(',');
    result.append(length);
    for(String h: hosts) {
      result.append(',');
      result.append(h);
    }
    return result.toString();
  }
  //。。。。。。
}