Which embedded DB written in Java for a simple key/value store? [closed]

There seem to be a couple of ports of Google's LevelDB into Java:

• Dain LevelDB Port (pure Java)
• Dain LevelDB (JNI)

Then there is a whole list of embedded Java databases here:

• Embedded java databases
• Java Embedded Databases Comparison

For your use case I would recommend MapDB (http://www.mapdb.org)

It matches your requirements:

• written in Java
• embeddable - single jar with no dependencies
• not SQL - gives you maps that are persisted to disk
• open source (Apache 2 licence)
• easy to backup (few files)

and has other nice features like transactions, concurrency and performance.

Chronicle-Map is a new nice player on this field.

• It is off-heap residing (with ability for being persisted to disk by means of memory-mapped files) Map implementation
• Super-fast -- sustains millions of queries/updates per second, i. e. each query has sub-microsecond latency on average
• Supports concurrent updates (assumed to be a drop-in replacement of ConcurrentHashMap)
• Special support of property maps you mentioned, if the set of properties is fixed within the collection -- allows to update specific properties of the value without any serialization/deserialization of the whole value (20 fields). This feature is called data value generation in Chronicle/Lang project.
• And many more...