MongoDB to Java integration
Hibernate 4.0 has OGM which supports MongoDB you can see an example of Hibernate with MongoDB here Porting Seam Hotel Booking Example to OGM
You can choose Morphia. It is the most stable one over other ORM tools for Mongodb.
Some of the other tools are :
You can find the full list in the MongoDB Third Party Frameworks and Libs part.
DataNucleus JPA provides full persistence to MongoDB using real JPA (as opposed to Spring-Data, Morphia etc which have their own APIs, hence you don't have direct portability - but that may not be your prime concern); in the case of DataNucleus it provides JPA2 (with some preview JPA2.1 features). It also provides JPA persistence to HBase, RDBMS, NeoDatis, Excel, ODF, XML, JSON, LDAP and some others in case you ever need portability to those.