>> Is there any example application in java, specifically on spring that can be embed or call alfresco process service.
Alfresco does provide what they term embedded examples on their download page:
These provide a build for creating your own maven build which you can customize as you need.
You will also be provided with credentials to the Alfresco nexus repository (necessary to build the examples) which also includes the source code (not open source).
>> Since we are in the beginning stage of design an application so the technology stack we choose spring micro service with angular js UI, we are using mysql as database and for authentication & authorization we are using oauth2.0.
The out of box designer, tasklist is an AnguarJS application, however it is tightly coupled with the enterprise REST API. There is no easy way to separate the Designer/Tasklist/Analytics UI from the enterprise REST API.
If you want to create a custom UI, you may want to look into the Alfresco Devopment Framework (which is included with the Process Services license). This is an Angular2 framework that interacts directly with the Process Services Enterprise REST API and can be deployed independently.
>> How to integrate oauth2.0 server users to alfresco BPM service users(Like for example I have my application users, when I click on the BPM Alfresco service it should not ask again to login, it should skip the login step).
There are hooks for your own Spring Security provider available.
If you need any assistance with this project, my company BP3 offers a full suite of consulting and implementation services for Activiti.
Hope this helps,