how to prepare search-engine

how to prepare search-engine in java/using servlets

Instead of writing a search engine from scratch in java, if it is possible you can use a third party search engine which is compatible with applications written in Java like Inktomi's Inktomi search software(formerly known as Ultraseek). This search engine comes with a Java API to enable integration with Java applications and extend search engine's capabilities.
You can get more information from http://www.inktomi.com/products/search//products/ultraseek/index.html
Hope this helps.