JDBM2 provides HashMap and TreeMap which are backed by disk storage.
'org.fusesource.jdbm:jdbm:2.0.0'
<dependency>
<groupId>org.fusesource.jdbm</groupId>
<artifactId>jdbm</artifactId>
<version>2.0.0</version>
</dependency>
<dependency org="org.fusesource.jdbm" name="jdbm" rev="2.0.0"/>
"org.fusesource.jdbm", "jdbm", "2.0.0"