The boundary string in HttpURLConnection?

W开发者_开发问答hy we have to setup a boundary string? What is the purpose for? any rules on it?

Boundary String, as the name says it is boundary for different parameters in a Httprequest, But I think in java no need to use boundary.
HttpClient, HttpCore by Apache like a lot are there for file handling Httpworks, which will be more easier and efficient