OpenJPA Auto Detach Configuration and Memory Issues

When I was doing some work with OpenJPA, I found there is a possible memory leaking issue when OpenJPA is used for data access without the proper configuration of the parameter of auto detach (openjpa.AutoDetach).

The scenario that the memory issue may happen:

• One EntityManagerFactory and One EntityManager are used throughout the whole session;


• The session contains a lot of queries, they are all read-only queries;


• Most queries are to retrieve database records based on primary keys;


• The records retrieved are approximately evenly distributed across primary keys;


• The database being queried has a large amount of data.

It seems that OpenJPA keeps references to data retrieved from the queries as cache. This prevents JVM garbage collecting them. Eventually, out of memory error happens.

This time, it seems that the call to clear method on EntityManager does not even work. Finally, we added the following properties and it seemed to solve the issue:

openjpa.AutoDetach=nontx-read

In fact, it is not a good practice to use EntityManager as shown in the scenario. Maybe the initial consideration is to re-use the single EntityManager as much as possible to help improve the performance of the session that is created in a batch process.

In general, the EntityManager object should not live in the whole long session. Instead it should be created on demand and closed and disregarded when finishing its unit of work. However, i haven't got time to test how worse the performance can degrade if it is implemented this way.