• An implementation of micropayment channels was added. There have been many bugfixes and improvements since the first announcement. This feature allows you to set up a 1:1 payment relationship with a remote server and after a short setup process send very tiny payments, very rapidly. It's suitable for metered billing applications. An article, "Working with micropayments" explains how to use it. This work was a joint effort between Matt and myself.
• A simple sublibrary has been added that provides async IO based client/server classes that transmit length prefixed protocol buffers.
• Thanks to Matija Mazi, some classes have been added that implement the BIP 32 deterministic wallet algorithm. Note that these classes are not yet used elsewhere in the system and full deterministic wallet support is therefore not available, however, a low level API is available for experimentation. That API is very likely to change in future releases so don't get too attached to it.
• Thanks to Gary Rowe, we have integrated a new Maven plugin that checks the SHA1 hashes of downloaded dependencies against a hard-coded list. This means that even if an upstream Maven repository or developer were to be compromised, library dependencies could not be switched out for corrupted versions without someone noticing. For 0.10 the dependency hashes were just initialised based on what was already downloaded. In future, reproducible builds of upstream dependencies and auditing of changes would provide better security. You can and should use Gary's plugin in your own projects to defend against a possible compromise of the bitcoinj repository.
• Callback handling has been much improved. Each event listener can have an Executor specified which takes responsibility for running the callback. If you don't specify one they run by default on a single background thread, the "user thread", instead of the origin framework threads. This means your callbacks no longer need to be thread safe as they're always run serially. You can also change the default executor if you would like to control the thread on which callbacks run, for example to marshal them into your GUI toolkit thread automatically. This fixes some of the most painful parts of the pre-0.10 API, for instance that transaction confidence listeners were not allowed to re-enter the library.
• Exception handling has also improved. You can assign a global Thread.UncaughtExceptionHandler which receives any exceptions thrown on the user thread (i.e. by your own event listeners), as well as any internal exceptions thrown by network threads (like inability to parse a message sent by a remote peer). Because your listeners now run on a separate thread by default, you can no longer accidentally cause internal data corruption or prevent other callbacks from running by leaking exceptions out of your callbacks; a subtle knife-edge in the previous API.
• Support for automatic wallet key rotation has been added.
• We now require Bloom-capable (0.8+) peers by default and will disconnect from older nodes. This avoids accidental bandwidth saturation on mobile devices.
• The wallet now accepts timelocked transactions if it created them itself.
• The wallet can be told to empty itself out, in which case the fee will be subtracted from the total amount instead of added. This simplifies the common case of wanting to send your entire balance whilst still including a fee.
• Some JNI peers for event listeners were added. Auto-generated JNI bindings are experimental and not yet merged in to the mainline codebase: for now they are available as part of a separate project on github. This work allows you to access the bitcoinj API using relatively natural looking C++ code and an embedded JVM.
• You can now register custom PeerFilterProvider implementors to add things to Bloom filters that aren't necessarily in wallets.
• We have begun adding nullity annotations to the API. Combined with a strong static analysis engine like FindBugs or the IntelliJ Inspector, you can find cases where you aren't handling possible null pointers. Note that you should configure your static analysis system to understand the Guava Preconditions assertions, as otherwise you will get false positives.
• You can now control how much information Wallet toString() dumps contain more precisely. Extensions can contribute to a wallets debug dump as well, and transaction data is now optional.
• Documentation: The getting started tutorial and PingService example were rewritten. New articles were added that cover optimising chain sync and using the library from non-Java languages. Existing articles were also extended and refreshed.
• Many bug fixes and new methods. You should upgrade as soon as possible to get the bug fixes, in particular, one that could cause transactions inside the same block to be incorrectly re-ordered when using Bloom filtering (which can affect the wallet). The library code now has more internal annotations to help static analysis engines, and several bugs were fixed as a result of that.

API Changes

• The ScriptBuilder class now takes TransactionSignature objects, these wrap a raw ECDSA signature and the SIGHASH flags together, with utility methods to work with them.
• The Locks class has been renamed to Threading. The thread on which callbacks run has been changed, see above.
• The WalletEventListener.onKeyAdded method became onKeysAdded and now takes a list, to make processing of bulk adds more efficient.
• BitcoinURIParseException is now checked so you can't forget to handle bogus URIs.
• The Wallet.toString(..) method has additional parameters now so you can control what is included in the dump.