Java堆使用JDBC4PreparedStatement填充

Question

I'm running a Play! 2.3 app that is multithreaded using Akka. Each thread accesses a MySQL database and runs some computations. Every time I run a set of these tasks, the app's memory footprint increases, largely due to JDBC4PreparedStatement objects that never go away. After a few runs, these objects take up over 200MB, and soon add up to an out of memory error Calling Sytem.gc() doesn't lower it at all.

I'm using Hibernate 4.3.0 for persistence, and calling clear() on the EntityManager doesn't help either, so it's not that there are persistent objects being stored anywhere. This answer implies that there may be Statement objects being left open somewhere, but I'm never interfacing directly with Statement objects or Connection objects, just with JPA's EntityManager .

Is this expected behavior for MySQL JDBC or JPA? How can I solve this memory leak? I'm not sure where to look for solutions, being unsure whether this is issue is due to MySQL or JDBC or JPA or Hibernate's implementation of it. Or possibly the way Play! handles JPA.

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EDIT

To be clear, I'm not using Connection or Statement objects, so there isn't anything for me to close. Here's an example, where I get the EntityManager from Play in a transaction, do something, then the transaction commits.

        JPA.withTransaction( () -> {
            Site site= JPA.em().find(Site.class, siteId);
            site.setDomain("www.stackoverflow.com");
        });

Answer 1

This is expected behavior. You will have to tell the mysql connection to return a cursor instead of the select of the full set by setting:

stmt = conn.createStatement(java.sql.ResultSet.TYPE_FORWARD_ONLY,
              java.sql.ResultSet.CONCUR_READ_ONLY);
stmt.setFetchSize(Integer.MIN_VALUE);

Your specific settings could be slight different then this based on your requirements, so some fiddling around might be required.