Hi Huy,

You may require to have a look on Creating REST APIs using Apex REST below is the link for the same.

https://developer.salesforce.com/page/Creating_REST_APIs_using_Apex_REST

The Force.com REST API lets you use a simple and lightweight API to access Force.com data, using standard OAuth, together with a choice of data flavors – XML and JSON. The REST API is configured out of the box to suit the basic CRUD (create, read, update and delete) operations of data management.

Apex REST augments this functionality and makes it possible for developers to create your own REST-based web services using Apex. It has all of the advantages of the REST architecture, provides the ability to define custom logic and includes automatic argument/object mapping.

This article provides an introduction and overview of the Apex REST functionality, showing you how to take your first steps in creating REST-based web services in Apex. As Apex REST builds on the Force.com REST implementation itself,
the Getting Started with the Force.com REST API article is recommended reading.
Here is a link 
https://developer.salesforce.com/page/Getting_Started_with_the_Force.com_REST_API

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Hi

This example illustrates a sample with external Java application,

https://developer.salesforce.com/page/Apex_Web_Services_and_Callouts

Here is one more generic approach,

https://developer.salesforce.com/page/Integrating_with_the_Force.com_Platform

--Akram