The way to keep an ObjectDB database encrypted is simply to store it on an encrypted drive. Modern operating systems support encryption of complete drives, directories or files, and an ObjectDB database is an ordinary file.
If you are going to encrypt the ObjectDB database file, then for maximum performance use an SSD hard drive with built in hardware encryption ability. If your hard drive does not support hardware encryption prefer using a CPU that supports the AES instruction set.
ObjectDB - Fast Object Database for Java (JPA/JDO)