There are two possible solutions.
You can switch from embedded mode to client-server mode. Unlike embedded mode, client server mode supports accessing a database concurrently from multiple processes (embedded mode supports concurrently only within a JVM, i.e. multithreading, but not accessing the database from different processes / JVMs).
Alternatively you may try a new undocumented feature of ObjectDB, embedded-server mode. Enable it just by adding the <embedded-server> element to your objectdb.conf file:
<connection port="6136" max="0" />
<data path="$objectdb/db" />
<embedded-server enabled="true" />
When enabled, embedded mode runs automatic server at the background, and any attempt to access the database while it is open in embedded mode is automatically routed to that server in client-server mode.
ObjectDB - Fast Object Database for Java (JPA/JDO)