Originally filed as a pull request https://github.com/wiredtiger/wiredtiger/pull/2904 by github user jindongh:

The updated ex_thread test may crash without this fix, because JNI wrapper tried to free session->lang_private after it returned session with connection.close().

After java thread A opens the session, it will allocate memory and assign it to session->lang_private. When java thread A closes the session, it will call connection.close(session) first, and then try to free session->lang_private. The problem is: after thread A calls connection.close() and before it frees lang_private, another thread B may reuse this session and close it, then thread A will find lang_private is set to NULL by thread B and it crashes.

So we need to free lang_private before we call connection.close in the JNI wrapper.