I used 4 maps.

MetaData Map

This map uses the file name as the key and the value is a FileMetaData object. The FileMetaData object has things like lastmodificationtime and number of bytes.

Chunk Map

This has a String key and a byte[] value. The String Key is the fileName + "#" + block number. Every file is stored as a set of blocks. Each block has an entry in the chunk map. The chunks are fixed size. The size is currently defined in GridOutputStream as a constant. 

Directory Map

This stores Lucene Directory objects. The key is the directory name and the value is a Set of Strings. Each string in the set is a file name used by this directory.

Lock Map

This uses a String key which is a combination of the Directory name and the file name. The value is irrelevant, I use a Boolean. If an entry exists in the map for a file then someone has locked it. Thus, the locking protocol just attempts to insert a 'lock' in the map for a given file. This is only possible if there is no current entry. Unlock simply removes the entry for a file. Locks can be given a lease behavior by simply adding a TTL evictor on this Map. You could set the default TTL to be 10 minutes so worst case a locked file would be automatically unlocked after 10 minutes. Possibly an improvement here would be to 'touch' the lock every N reads or writes of data to the file and use a LAST_ACCESS_TIME eviction policy. This would likely prevent the lock being removed/invalidated if it's still in use.

No near cache is used. I used my wxsutils library to interact with the grid because it simplifies the application programming to just simple put/remove static operators.