The bitcoinj library is easy to use for Bitcoin wallet and transaction functions for both native Java and Android applications. Although there are certain features missing, it seems mature enough to be included in a Bitcoin walled app or service.
Sometimes the source code leaves a bit to be desired in structure and readability: anonymous inner classes and other deep nesting blocks sometimes makes it difficult to follow; inheritance is often used where composition would have been be better; the Collections classes could have been used over arrays in many places. All of this might come back to haunt the developers later, for now they seem to be plowing on.
At least the basics are straight forward. The following code will read a test walled from disk, or create a new one if it does not already exist. The TestNet3 block chain and network is used. Since the bitcoinj library relies heavily on the Google Guava (com.google.common) classes, there are frequent artifacts of the threading and callback handling showing up. In this example, we want the code to block and wait, therefore the extra await-functions are required.
To get some free test coins, run the following code, wait for the prompt which shows the next receiving address, and head over to faucet.xeno-genesis.com to ask them to send some money there. It should show up as received within a few seconds. Your wallet now contains some coins.
Since the test network is a real network with real miners, it’s good etiquette to return your test coins to the pool for others to use once you’re done with them. The following code takes care of that, returning them to the TP Faucet default return address “n2eMqTT929pb1RDNuqEnxdaLau1rxy3efi”. You can return all your coins, or just a fraction if you want to experiment more. This will also wait a few seconds for the callback confirmation.
Finally, it’s worth noting that bitcoinj is a “live” library, in development and with the latest update available through Gradle. To make this work, there’s a few settings and dependencies to take care of. The logging framework used by bitcoinj is SL4J, and an actual implementation library (e.g. “sl4j-simple”) is also need. It can be downloaded, or included as a Gradle build dependency as seen below.
Then, your source code directory structure might not match the default Gradle “main”, “test” structure. My current structure keeps all source code under the directory “src”, so I have specified that.
Gradle integrates OK with Eclipse, but be careful with the “refresh” option. It tends to insist on changing the classpath setting of the project, so the packages disappear. It’s a good idea to keep the .classpath setting file under version control.