I want to build a JavaEE GED application using alfresco and activiti Workflow engine which manages and monitors the mail of an organization (Letter, Fax, mail etc.). Since I am
I can suggest two solutions using Using the REST API or CMIS with a library like Apache Chemistry
Apache Chemistry is very well documented and you can find good pointers on the official website for pretty much any thing you would want to do !
If you want to get a session from your Alfresco Repository for example follow this post : How to retrieve 'repository root' id/children from CMIS repository?
Alternatively, you can find a lot of resources around for interacting with alfresco using REST (either the new REST API, or old restful endpoints). Check the platform integrations portal from the official docs, it would be a very good starting point.