12.7: Accessing Bitcoind with Java

NOTE: This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.

Interacting with the bitcoind directly and using command-line curl can get simple if you understand how it works, but there's a project JavaBitcoindRpcClient that provides the functionality in a Java-API level, making it even easier to interact with your Bitcoin Server.

Setup Dependency

If you use Maven in your Java project, you can include the dependency:

<dependency>
  <groupId>wf.bitcoin</groupId>
  <artifactId>JavaBitcoindRpcClient</artifactId>
  <version>0.9.13</version>
</dependency>

Or if you use Gradle:

compile 'wf.bitcoin:JavaBitcoindRpcClient:0.9.13'

Build Your Connection

To use JavaBitcoindRpcClient, you need to create a BitcoindRpcClient instance. The arguments in the URL are username, password, IP address and port. You should know this information from your work with curl . As you'll recall, the IP address 127.0.0.1 and port 18332 should be correct for the standard testnet setup described in this documents, while you can extract the user and password from ~/.bitcoin/bitcoin.conf.

  BitcoindRpcClient rpcClient = new BitcoinJSONRPCClient("http://bitcoinrpc:d8340efbcd34e312044c8431c59c792c@127.0.0.1:18332");

MAINNET VS TESTNET: The port would be 8332 for a mainnet setup.

If rpcClient is successfully initialized, you'll be able to send off RPC commands.

Later, when you're all done with your bitcoind connection, you should close it:

rpcClient.stop();

Making your first RPC Call

In order to use an RPC method using JavaBitcoindRpcClient, you'll find that the BitcoindRpcClient provides most of the functionality that can be accessed through bitcoin-cli or curl.

For example, to execute the getmininginfo, you should use the getMiningInfo() method:

MiningInfo info = rpcClient.getMiningInfo();
System.out.println("Mining Information");
System.out.println("------------------");
System.out.println("Chain......: " + info.chain());
System.out.println("Blocks.....: " + info.blocks());
System.out.println("Difficulty.: " + info.difficulty());
System.out.println("Hash Power.: " + new BigDecimal(info.networkHashps()).toPlainString());

The output for this line should be similar to this:

Mining Information
------------------
Chain......: test
Blocks.....: 1254920
Difficulty.: 1.0
Hash Power.: 6585163152453.466796875

Creating an Address

You can create a new address attaching a specific label to it, as well as dump the private key for a specific address:

  String address = rpcClient.getNewAddress("Learning-Bitcoin-from-the-Command-Line");
  System.out.println("New Address: " + address);

  String privKey = rpcClient.dumpPrivKey(address);
  System.out.println("Priv Key: " + privKey);

Output:

New Address: mpsFtZ8qTJPRGZy1gaaUw37fHeUSPLkzzs
Priv Key: cTy2AnmAALsHokYzJzTdsUBSqBtypmWfmSNYgG6qQH43euUZgqic

Sending Transactions

You can easily send a transaction using the method sendToAddress().

String sendToAddress = rpcClient.sendToAddress("mgnNsZj6tPzpd7JwTTidUKnGoDTkcucLT5", 1);
System.out.println("Send: " + sendToAddress);

This program will output a transaction id, for example:

a2d2f629d6666ca6e440169a322850cd9d133f637f7a02a02a0a7477bc5687d4

In case you want to adjust the transaction fee, you can use the setTxFee method before sending the output:

rpcClient.setTxFee(new BigDecimal(0.001).setScale(3, BigDecimal.ROUND_DOWN));

Listening to Transactions or Blocks

You may want to write applications that keep listening the Blockchain, and execute a specific code when something happens, such as a transaction that involves an address in your wallet, or even the generation of a new block in the network. To do that, JavaBitcoindRpcClient provides support to BitcoinAcceptor, where you can attach listeners in the network.

Example:

  BitcoinAcceptor acceptor = new BitcoinAcceptor(rpcClient, blockHash, 6, new BitcoinPaymentListener() {

      @Override
      public void transaction(Transaction tx) {
          System.out.println("Transaction: " + tx);

      }

      @Override
      public void block(String block) {
          System.out.println("Block: " + block);

      }
  });
  acceptor.run();

Every time some transaction is sent, or a new block is generated, you should see a similar output in your console:

Transaction: {account=Tests, address=mhopuJzgmTwhGfpNLCJ9CRknugY691oXp1, category=receive, amount=5.0E-4, label=Tests, vout=1, confirmations=0, trusted=false, txid=361e8fcff243b74ebf396e595a007636654f67c3c7b55fd2860a3d37772155eb, walletconflicts=[], time=1513132887, timereceived=1513132887, bip125-replaceable=unknown}

Block: 000000004564adfee3738314549f7ca35d96c4da0afc6b232183917086b6d971

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