BitCoinJ之Hello World示例程序

简介

比特币(BitCoin)是一种基于区块链(BlockChain)技术的数字化货币. 本文介绍了使用基于Java语言的BitCoinJ API实现一个简单的收取和支付比特币的示例程序.

开发环境

本示例使用BitCoinJ(https://bitcoinj.github.io/) API, 目前的发布版本是0.14.3, 其JAR包可以从官网下载, 也可以通过如下的Maven定义在项目POM中引用.

<dependencies>
   <dependency>
     <groupId>org.bitcoinj</groupId>
     <artifactId>bitcoinj-core</artifactId>
     <version>0.14.3</version>
     <scope>compile</scope>
   </dependency>
 </dependencies>

收取比特币

收取比特币的示例代码如下

public class BitCoinHelloWorld implements WalletCoinsReceivedEventListener {

    public static void main(String[] args) {
        BitCoinHelloWorld demo = new BitCoinHelloWorld();

        demo.run();
    }

    public void run() {
        try {
            init();

            System.out.println("Waiting for coins...");

            while (true) {
                Thread.sleep(20);
            }
        } catch (BlockStoreException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void init() throws BlockStoreException {
        NetworkParameters params  = TestNet3Params.get();

        ECKey key = new ECKey();
        System.out.println("We created a new key:\n" + key);

        Address addressFromKey = key.toAddress(params);
        System.out.println("Public Address generated: " + addressFromKey);

        System.out.println("Private key is: " + key.getPrivateKeyEncoded(params).toString());

        Wallet wallet = new Wallet(params);
        wallet.importKey(key);

        File blockFile = new File("/tmp/bitcoin-blocks");
        SPVBlockStore blockStore = new SPVBlockStore(params, blockFile);

        BlockChain blockChain = new BlockChain(params, wallet, blockStore);
        PeerGroup peerGroup = new PeerGroup(params, blockChain);
        peerGroup.addPeerDiscovery(new DnsDiscovery(params));
        peerGroup.addWallet(wallet);

        System.out.println("Start peer group");
        peerGroup.start();

        System.out.println("Downloading block chain");
        peerGroup.downloadBlockChain();
        System.out.println("Block chain downloaded");

        wallet.addCoinsReceivedEventListener(this);
    }

    @Override
    public void onCoinsReceived(final Wallet wallet, final Transaction transaction, Coin prevBalance, Coin newBalance) {
        final Coin value = transaction.getValueSentToMe(wallet);

        System.out.println("Received tx for " + value.toFriendlyString() + ": " + transaction);

        System.out.println("Previous balance is " + prevBalance.toFriendlyString());

        System.out.println("New estimated balance is " + newBalance.toFriendlyString());

        System.out.println("Coin received, wallet balance is :" + wallet.getBalance());

        Futures.addCallback(transaction.getConfidence().getDepthFuture(1), new FutureCallback<TransactionConfidence>() {
            public void onSuccess(TransactionConfidence result) {
                System.out.println("Transaction confirmed, wallet balance is :" + wallet.getBalance());
            }

            public void onFailure(Throwable t) {
                t.printStackTrace();
            }
        });
    }
}

该示例程序首先调用init方法进行初始化, 然后进入一个等待循环, 当有比特币到来时, onCoinsReceived方法就被触发.

各个步骤的具体分析如下

选择运行环境

比特币应用可以在三种不同的环境中运行: 正式流通网络, 测试流通网络以及本地开发环境. 初始化的第一步是通过设置一个NetworkParameters变量来选择运行环境, 以下代码使用测试流通网络

NetworkParameters params  = TestNet3Params.get();

获取地址和设置钱包对象

以下代码首先创建一个可用于接受比特币的地址, 并将其导入相应的钱包对象中.

ECKey key = new ECKey();
System.out.println("We created a new key:\n" + key);

Address addressFromKey = key.toAddress(params);
System.out.println("Public Address generated: " + addressFromKey);

System.out.println("Private key is: " + key.getPrivateKeyEncoded(params).toString());

Wallet wallet = new Wallet(params);
wallet.importKey(key);

接入流通网络并下载比特币区块

以下代码接入流通网络并下载比特币区块

File blockFile = new File("/tmp/bitcoin-blocks");
SPVBlockStore blockStore = new SPVBlockStore(params, blockFile);

BlockChain blockChain = new BlockChain(params, wallet, blockStore);
PeerGroup peerGroup = new PeerGroup(params, blockChain);
peerGroup.addPeerDiscovery(new DnsDiscovery(params));
peerGroup.addWallet(wallet);

System.out.println("Start peer group");
peerGroup.start();

System.out.println("Downloading block chain");
peerGroup.downloadBlockChain();
System.out.println("Block chain downloaded");

需要注意的是peerGroup.downloadBlockChain();这一步可能会运行很长时间.

设置事件响应

以下代码设置当比特币到来时的事件响应

wallet.addCoinsReceivedEventListener(this);

比特币到来事件响应

当比特币到来时onCoinsReceived方法就会触发, 注意该方法的newBalance参数提供的是钱包中金额的估计值,其实际金额要等到交易被网络确认后才会提现在wallet.getBalance()的返回值中, 如以下代码所示

Futures.addCallback(transaction.getConfidence().getDepthFuture(1), new FutureCallback<TransactionConfidence>() {
    public void onSuccess(TransactionConfidence result) {
        System.out.println("Transaction confirmed, wallet balance is :" + wallet.getBalance());
    }

    public void onFailure(Throwable t) {
        t.printStackTrace();
    }
});

支付比特币

支付比特币的过程相对简单, 首选设置要支付的金额.

final Coin amountToSend = Coin.valueOf(10, 0);

其次设置接收方的地址

Address toAddress = Address.fromBase58(params, "n2eMqTT929pb1RDNuqEnxdaLau1rxy3efi");

然后发送该支付交易

final Wallet.SendResult sendResult = wallet.sendCoins(peerGroup, toAddress, amountToSend);

并设置交易完成后的事件响应

sendResult.broadcastComplete.addListener(new Runnable() {
  @Override
  public void run() {
      System.out.println("Coins Sent! Transaction hash is " + sendResult.tx.getHashAsString());
  }
}, MoreExecutors.sameThreadExecutor());

总结

本文给出了一个基于BitCoinJ的比特币收发示例程序, 并对BitCoinJ的编程模式以及其事件响应机制做了初步介绍.