bitcoinj
A library for working with Bitcoin
...Highly optimized lightweight simplified payment verification (SPV) mode. In this mode, only a small part of the blockchain is downloaded, making bitcoin suitable for usage on constrained devices like smartphones or cheap virtual private servers. A wallet class with encryption, fee calculation, multi-signing, deterministic key derivation, pluggable coin selection/coin control, extensions support and event listeners that let you stay up to date with changes in your balance. Experimental full verification mode, which does the same verification work as Bitcoin Core. ...