I'm looking for designing a small eCommerce website for my business. I ll be using Spring and Java stuffs in the backend. And the website should be device responsive. I'm very confused to decide which CMS to use. Please someone help me.
E-Commerce: if it's a shop, you might want to look into BroadLeaf Commerce: it's not a CMS but an open source shop system, but built on the technology stack you're looking for. http://www.broadleafcommerce.org/
Other than that: I used to work with OpenCms for many years (since 2005), and you could integrate it with Spring (which is what we did, see my answer to this question here: What java based CMS can manage existing pages' content? and Integration of Java application with OpenCms)
However, if I'd built a new site with a CMS on Java I'd probably go with Magnolia CMS, because the documentation and community is more mature despite that it's newer than OpenCms. http://www.magnolia-cms.com/ Spring integration: https://wiki.magnolia-cms.com/display/WIKI/Magnolia+Blossom
Regarding the architecture: Magnolia uses Jackrabbit as a JCR-compatible repository, while OpenCms uses its own approach, with the downside imho that they store all content files inside a MySQL database instead of on the file system. That doesn't bring much benefit but only disadvantages.
Regarding your requirement for responsive design: it doesn't matter which CMS you choose, because what you will render as the frontend (whether it's html5, json, xml) doesn't really matter for the choice of the CMS. I'd say, choose the CMS or E-Commerce system which has the best functional features that match with your set of requirements, and take a look at the UI, which one you'll find most appealing.
Regarding the architecture and underlying technology, the three above should all do well.
== Update 20.05.2016 ==
Just to confirm my above suggestion that I made three years ago: after having worked with OpenCms for around 10 years, I switched to Magnolia entirely 2 years ago and not regretting it. IMHO it's the best open source WebCMS out there that can actually compete with the big players. Community and documentation is excellent. I use the free community edition. btw: I don't work for Magnolia International Ltd nor any of their partners, this is just my personal opinion and experience.