I have many documents located on my disk and I want to build a search engine to search through them. I know Google Desktop Search or Bing Desktop Search could do that. But I want to know if there's some SDK/API to do that so I can do some customization.
What I want to achieve, is that I can provide a document and the local search engine will return all the documents similar to it.
In general there are Lucene and Solr that can help to solve search related needs in Java (I guess you are using Java based on the tag GWT).
But I don't know how to do a search by example with these tools. I think you have to extract the relevant information of the document to construct a search based on it.