今天我们看看Nutch网页抓取，所用的几种数据结构：
主要涉及到了这几个类：FetchListEntry，Page，
首先我们看看FetchListEntry类：
public final class FetchListEntry implements Writable, Cloneable
实现了Writable, Cloneable接口，Nutch许多类实现了Writable, Cloneable。
自己负责自己的读写操作其实是个很合理的设计方法，分离出来反倒有很琐碎
的感觉。
看看里面的成员变量：

public static final String DIR_NAME = "fetchlist";//要写入磁盘的目录
private final static byte CUR_VERSION = 2;//当前的版本号
private boolean fetch;//是否抓取以便以后更新
private Page page;//当前抓取的页面
private String[] anchors;//抓取到的该页面包含的链接

我们看看如何读取各个字段的，也就是函数
public final void readFields(DataInput in) throws IOException
读取version 字段，并判断如果版本号是否大约当前的版本号，则抛出版本不匹配的异常，
然后读取fetch 和page 字段。
判断如果版本号大于1，说明anchors已经保存过了，读取anchors，否则直接赋值一个空的字符串
代码如下：

    byte version = in.readByte();                 // read version
    if (version > CUR_VERSION)                    // check version
      throw new VersionMismatchException(CUR_VERSION, version);

    fetch = in.readByte() != 0;                   // read fetch flag

    page = Page.read(in);                         // read page

    if (version > 1) {                            // anchors added in version 2
      anchors = new String[in.readInt()];         // read anchors
      for (int i = 0; i < anchors.length; i++) {
        anchors[i] = UTF8.readString(in);
      }
    } else {
      anchors = new String[0];
    }
 

同时还提供了一个静态的读取各个字段的函数，并构建出FetchListEntry对象返回：

public static FetchListEntry read(DataInput in) throws IOException {
    FetchListEntry result = new FetchListEntry();
    result.readFields(in);
    return result;
}

写得代码则比较易看,分别写每个字段：

public final void write(DataOutput out) throws IOException {
    out.writeByte(CUR_VERSION);                   // store current version
    out.writeByte((byte)(fetch ? 1 : 0));         // write fetch flag
    page.write(out);                              // write page
    out.writeInt(anchors.length);                 // write anchors
    for (int i = 0; i < anchors.length; i++) {
      UTF8.writeString(out, anchors[i]);
    }
  }

其他的clone和equals函数实现的也非常易懂。
下面我们看看Page类的代码：
public class Page implements WritableComparable, Cloneable
和FetchListEntry一样同样实现了Writable, Cloneable接口，我们看看Nutch的注释，我们就非常容易知道各个字段的意义了：

/*********************************************
 * A row in the Page Database.
 * <pre>
 *   type   name    description
 * ---------------------------------------------------------------
 *   byte   VERSION  - A byte indicating the version of this entry.
 *   String URL      - The url of a page.  This is the primary key.
 *   128bit ID       - The MD5 hash of the contents of the page.
 *   64bit  DATE     - The date this page should be refetched.
 *   byte   RETRIES  - The number of times we've failed to fetch this page.
 *   byte   INTERVAL - Frequency, in days, this page should be refreshed.
 *   float  SCORE   - Multiplied into the score for hits on this page.
 *   float  NEXTSCORE   - Multiplied into the score for hits on this page.
 * </pre>
 *
 * @author Mike Cafarella
 * @author Doug Cutting
 *********************************************/

各个字段：

private final static byte CUR_VERSION = 4;
  private static final byte DEFAULT_INTERVAL =
    (byte)NutchConf.get().getInt("db.default.fetch.interval", 30);

  private UTF8 url;
  private MD5Hash md5;
  private long nextFetch = System.currentTimeMillis();
  private byte retries;
  private byte fetchInterval = DEFAULT_INTERVAL;
  private int numOutlinks;
  private float score = 1.0f;
  private float nextScore = 1.0f;

同样看看他是如何读取自己的各个字段的，其实代码加上本来提供的注释，使很容易看懂的，不再详述：

ublic void readFields(DataInput in) throws IOException {
    byte version = in.readByte();                 // read version
    if (version > CUR_VERSION)                    // check version
      throw new VersionMismatchException(CUR_VERSION, version);

    url.readFields(in);
    md5.readFields(in);
    nextFetch = in.readLong();
    retries = in.readByte();
    fetchInterval = in.readByte();
    numOutlinks = (version > 2) ? in.readInt() : 0; // added in Version 3
    score = (version>1) ? in.readFloat() : 1.0f;  // score added in version 2
    nextScore = (version>3) ? in.readFloat() : 1.0f;  // 2nd score added in V4
  }

写各个字段也很直接：

public void write(DataOutput out) throws IOException {
    out.writeByte(CUR_VERSION);                   // store current version
    url.write(out);
    md5.write(out);
    out.writeLong(nextFetch);
    out.write(retries);
    out.write(fetchInterval);
    out.writeInt(numOutlinks);
    out.writeFloat(score);
    out.writeFloat(nextScore);
  }

我们顺便看看提供方便读写Fetch到的内容的类FetcherOutput：这个类通过委托前面介绍的两个类的读写，提供了Fetche到的
各种粒度结构的读写功能，代码都比较直接，不再详述。
下次我们看看parse-html插件，看看Nutch是如何提取html页面的。

  public static final String DIR_NAME = "content";
  private final static byte VERSION = 1;
  private String url;
  private String base;
  private byte[] content;
  private String contentType;
  private Properties metadata;

super.readFields(in);                         // check version

    url = UTF8.readString(in);                    // read url
    base = UTF8.readString(in);                   // read base

    content = WritableUtils.readCompressedByteArray(in);

    contentType = UTF8.readString(in);            // read contentType

    int propertyCount = in.readInt();             // read metadata
    metadata = new Properties();
    for (int i = 0; i < propertyCount; i++) {
      metadata.put(UTF8.readString(in), UTF8.readString(in));
    }

public final void write(DataOutput out) throws IOException {
    super.write(out);                             // write version

    UTF8.writeString(out, url);                   // write url
    UTF8.writeString(out, base);                  // write base

    WritableUtils.writeCompressedByteArray(out, content); // write content

    UTF8.writeString(out, contentType);           // write contentType
    
    out.writeInt(metadata.size());                // write metadata
    Iterator i = metadata.entrySet().iterator();
    while (i.hasNext()) {
      Map.Entry e = (Map.Entry)i.next();
      UTF8.writeString(out, (String)e.getKey());
      UTF8.writeString(out, (String)e.getValue());
    }
  }