Lucene.Net中 FSDirectory存储方式下一个 Document是如何得到的

防止忘记的最好的方法就是记下来。

这是一段最简单的搜索代码：

public void Search()
{
var dir=FSDirectory.Open(new DirectoryInfo("xxx"));
var searcher = new IndexSearcher(dir, true);
var query = new TermQuery(new Term("Title", "jinzhao"));
var tops=searcher.Search(query,100);
foreach(var top in tops)
{
var doc=searcher.Doc(top);
Output(doc);
}
}

红色的一句话就返回了一个完整document，是search内部的IndexReader（Lucene.Net.Index.IndexReader）返回的document，方法如下：

public abstract Document Document(int n, FieldSelector fieldSelector);

下面是这个类的实现：



他们的关系如下：

MultiReader和ParallelReader维护了IndexReader的一个集合（这些IndexReader可能由下面几重实现，但是不包含SegmentReader），封装了访问多个reader的方式，原理就是lucene里最常见的偏移的方式；

DirectoryReader等除SegmentReader外模拟的是一个目录，就像索引文件夹一样，它维护了一组SegmentReader的实现，原理如上；

SegmentReader是读取文档的最小单位它不再维护任何子的IndexReader，接收到ID后就会读取通过public sealed class FieldsReader 读取这个文档的字段（Lucene的核心就是文档，一个文档由若干字段组成），这里加载方式有立即加载、立即加载指定字段、懒加载等其它几种，方法如下：

public /*internal*/ Document Doc(int n, FieldSelector fieldSelector)
{
SeekIndex(n);
long position = indexStream.ReadLong();
fieldsStream.Seek(position);

Document doc = new Document();
int numFields = fieldsStream.ReadVInt();
for (int i = 0; i < numFields; i++)
{
int fieldNumber = fieldsStream.ReadVInt();
FieldInfo fi = fieldInfos.FieldInfo(fieldNumber);
FieldSelectorResult acceptField = fieldSelector == null?FieldSelectorResult.LOAD:fieldSelector.Accept(fi.name);

byte bits = fieldsStream.ReadByte();
System.Diagnostics.Debug.Assert(bits <= FieldsWriter.FIELD_IS_COMPRESSED + FieldsWriter.FIELD_IS_TOKENIZED + FieldsWriter.FIELD_IS_BINARY);

bool compressed = (bits & FieldsWriter.FIELD_IS_COMPRESSED) != 0;
bool tokenize = (bits & FieldsWriter.FIELD_IS_TOKENIZED) != 0;
bool binary = (bits & FieldsWriter.FIELD_IS_BINARY) != 0;
//TODO: Find an alternative approach here if this list continues to grow beyond the
//list of 5 or 6 currently here.  See Lucene 762 for discussion
if (acceptField.Equals(FieldSelectorResult.LOAD))
{
AddField(doc, fi, binary, compressed, tokenize);
}
else if (acceptField.Equals(FieldSelectorResult.LOAD_FOR_MERGE))
{
AddFieldForMerge(doc, fi, binary, compressed, tokenize);
}
else if (acceptField.Equals(FieldSelectorResult.LOAD_AND_BREAK))
{
AddField(doc, fi, binary, compressed, tokenize);
break; //Get out of this loop
}
else if (acceptField.Equals(FieldSelectorResult.LAZY_LOAD))
{
AddFieldLazy(doc, fi, binary, compressed, tokenize);
}
else if (acceptField.Equals(FieldSelectorResult.SIZE))
{
SkipField(binary, compressed, AddFieldSize(doc, fi, binary, compressed));
}
else if (acceptField.Equals(FieldSelectorResult.SIZE_AND_BREAK))
{
AddFieldSize(doc, fi, binary, compressed);
break;
}
else
{
SkipField(binary, compressed);
}
}

return doc;
}

标红的是一个IndexInput的实现，它是具体读取的方法，实现一般在存储类中以嵌套公开的方式实现，比如此处例子的实现如下：

public /*protected internal*/class SimpleFSIndexInput : BufferedIndexInput, System.ICloneable
{

protected internal class Descriptor : System.IO.BinaryReader
{
// remember if the file is open, so that we don't try to close it
// more than once
protected internal volatile bool isOpen;
internal long position;
internal long length;

public Descriptor(/*FSIndexInput enclosingInstance,*/ System.IO.FileInfo file, System.IO.FileAccess mode)
: base(new System.IO.FileStream(file.FullName, System.IO.FileMode.Open, mode, System.IO.FileShare.ReadWrite))
{
isOpen = true;
length = file.Length;
}

public override void Close()
{
if (isOpen)
{
isOpen = false;
base.Close();
}
}

~Descriptor()
{
try
{
Close();
}
finally
{
}
}
}

可以看到最后字段由System.IO.BinaryReader到文件中读取。

完。