理解Lucene（一）
1、什么是lucene？
People new to Lucene often mistake it for a ready-to-use application like a
file-search program, a web crawler, or a web site search engine. That isn’t what
Lucene is: Lucene is a software library, a toolkit if you will, not a full-featured
search application. It concerns itself with text indexing and searching, and it
does those things very well. Lucene lets your application deal with business rules
specific to its problem domain while hiding the complexity of indexing and
searching implementation behind a simple-to-use API. You can think of Lucene
as a layer that applications sit on top of，正如下图所示
Figure
2、Lucene能做什么？
Lucene allows you to add indexing and searching capabilities to your applications
(these functions are described in section 1.3). Lucene can index and make searchable
any data that can be converted to a textual format.
This means you can use Lucene to index and
search data stored in files: web pages on remote web servers, documents stored in
local file systems, simple text files, Microsoft Word documents, HTML or PDF
files, or any other format from which you can extract textual information.
3、什么是Indexing和Index
This is where indexing comes in: To search large amounts of text
quickly, you must first index that text and convert it into a format that will let you
search it rapidly, eliminating the slow sequential scanning process. This conversion
process is called indexing, and its output is called an index.
具体到Lucene，Index的含义:
You can think of an index as a data structure that allows fast random access to
words stored inside it. The concept behind it is analogous to an index at the end
of a book, which lets you quickly locate pages that discuss certain topics. In the
case of Lucene, an index is a specially designed data structure, typically stored
on the file system as a set of index files.
appendix B,有Index file的结构说明
4、什么是Searching
Searching is the process of looking up words in an index to find documents where
they appear
Searching的品质由Pricision和Recall描述。
Recall measures how well the search system finds relevant documents,
whereas precision measures how well the system filters out the irrelevant
documents.
评价Searching的其他因素：
We already mentioned speed and the ability to quickly
search large quantities of text. Support for single and multiterm queries, phrase
queries, wildcards, result ranking, and sorting are also important, as is a friendly
syntax for entering those queries.
5、简单的例子：搜索一个目录树中的txt，很小的例子，但可以对Lucene的整体有个了解，略
通过例子可以看出Lucene和具体业务是分开的，它只用到了很少的LuceneApi
6、注意：
Note that the Hits object contains only references to the underlying documents.
In other words, instead of being loaded immediately upon search, matches are
loaded from the index in a lazy fashion—only when requested with the hits.
doc(int) call.
Hits只是包含documents的索引，匹配项从索引加载到Search是延迟加载的――仅当调用
Hits.doc()时
