TomP2P 
TomP2P is a P2P library and a distributed hash table (DHT) implementation which provides a decentralized key-value infrastructure for distributed applications. Each peer has a table that can be configured either to be disk-based or memory-based to store its values.
TomP2P stores key-value pairs in a distributed manner. To find the peers to store the data in the distributed hash table, TomP2P uses an iterative routing to find the closest peers. Since TomP2P uses non-blocking communication, a future object is required to keep track of future results. This key concept is used for all the communication (iterative routing and DHT operations, such as storing a value on multiple peers) in TomP2P and it is also exposed in the API. Thus, an operation such as get or put will return immediately and the user can either block the operation to wait for the completion or add a listener that gets notified when the operation completes.
Features
- Java6 DHT implementation with non-blocking IO using Netty.
- XOR-based iterative routing similar to Kademlia.
- Standard DHT operations: put(), get()
- Extended DHT operations and support for custom operations: putIfAbsent(), add(), send(), digest()
- Selective get() using min-max or Bloom filters
- Direct and indirect replication.
- Mesh-based distributed tracker.
- Data protection based on signatures.
- Port forwarding detection and configuration via UPNP and NAT-PMP.
- Runs with IPv6 and IPv4.
- Network operations support the listenable future objects concept.
Code Examples (API v4.1)
//create a peer
Peer peer = new PeerMaker(new Number160(rnd)).setPorts(port).buildAndListen();
//store object
FutureDHT f =peer.put(Number160.createHash(“key”)).setObject(“hello world”).build();
//get object
FutureDHT f = peer.get(Number160.createHash(“key”)).build();
//to get the result, either add listener
f.addListener(...)
//or block
f.await()
//send direct messages to a particular peer
peer.sendDirect().setPeerAddress(peer1).setObject(“test”).build();