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。