12_7_Accessing_Bitcoind_with_Java.md

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 Java

To install Java on the VPS Server, you are able to use the apt-get command. We will also use Apache Maven to manage the dependencies, so we will install it together.

$ apt-get install openjdk-9-jre-headless maven

You can verify your Java installation:

$ java -version
openjdk version "9-internal"
OpenJDK Runtime Environment (build 9-internal+0-2016-04-14-195246.buildd.src)
OpenJDK 64-Bit Server VM (build 9-internal+0-2016-04-14-195246.buildd.src, mixed mode)

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'

If you want a sample project and some instructions on how to run it on the server that we just created, you can refer to the Bitcoind Java Sample Project.

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, using the same method names. For more details about the commands that you are able to execute and what to expect back, you should refer to 3.2: Knowing Your Bitcoin Setup.

For example, to execute the getmininginfo command to get the block information and the difficulty on the network, 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 on your wallet attaching a specific label to it, as well as dump the private key for a specific address. For more information about the wallet setup, you can check 3.3: Setting Up Your Wallet.

  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(). For more information about sending transactions, you can check 4: Sending Bitcoin Transactions.

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