Bitcoin源码阅读笔记二

bitcoinj core篇

AbstractBlockChain.java

弄懂这个抽象类实现的功能对源码的理解非常重要，因为他是bitcoinj主要使用的7大对象之一。AbstractBlockChain的实现需要BlockStore存储所有数据、SPVBlockStore存储在SPV模式下的轻量数据、Block是组成链的必要数据结构、连接Wallet帮助其接收交易数据和通知。AbstractBlockChain有两个子类：BlockChain是一个非常简单的子类，实现了简易的交易验证，帮助SPV模式下的节点高效率的校验header信息而不校验完成的信息，这一点对推广移动应用非常有价值。FullPrunedBlockChain联合FullPrunedBlockStore实现了对block的完整校验。

public abstract class AbstractBlockChain{
	private static final Logger log = LoggerFactory.getLogger(AbstractBlockChain.class);
    protected final ReentrantLock lock = Threading.lock("blockchain");

    /** Keeps a map of block hashes to StoredBlocks. */
    private final BlockStore blockStore;
    
}

BlockStore将会在后面详细介绍，这段代码中值得注意的是ReentrantLock。多线程和并发不是什么新内容，一般使用核心类库Thread构造、启动和操作线程。Java语言中包括了跨线程传达并发性约束的构造Synchronized和volatile。volatile主要用在多个线程感知实例变量被更改了场合，从而使得各个线程获得最新的值。它强制线程每次从主内存中讲到变量，而不是从线程的私有内存中读取变量，从而保证了数据的可见性。相比于volatile, synchronized更重量级一点不仅可以修饰变量还可以修饰方法。并且在保证了可见性的同事也保证了原子性。

java.util.concurrent.lock 中的 Lock 框架是锁定的一个抽象，它允许把锁定的实现作为 Java 类，而不是作为语言的特性来实现。这就为 Lock 的多种实现留下了空间，各种实现可能有不同的调度算法、性能特性或者锁定语义。 ReentrantLock 类实现了 Lock ，它拥有与 synchronized 相同的并发性和内存语义，但是添加了类似锁投票、定时锁等候和可中断锁等候的一些特性。此外，它还提供了在激烈争用情况下更佳的性能。（换句话说，当许多线程都想访问共享资源时，JVM 可以花更少的时候来调度线程，把更多时间用在执行线程上。）

/**
    * Tracks the top of the best known chain.<p>
    *
    * Following this one down to the genesis block produces the story of the economy from the creation of Bitcoin
    * until the present day. The chain head can change if a new set of blocks is received that results in a chain of
    * greater work than the one obtained by following this one down. In that case a reorganize is triggered,
    * potentially invalidating transactions in our wallet.
    */
protected StoredBlock chainHead;

   // TODO: Scrap this and use a proper read/write for all of the block chain objects.
   // The chainHead field is read/written synchronized with this object rather than BlockChain. However writing is
   // also guaranteed to happen whilst BlockChain is synchronized (see setChainHead). The goal of this is to let
   // clients quickly access the chain head even whilst the block chain is downloading and thus the BlockChain is
   // locked most of the time.
   private final Object chainHeadLock = new Object();

   protected final NetworkParameters params;
   private final CopyOnWriteArrayList<ListenerRegistration<NewBestBlockListener>> newBestBlockListeners;
   private final CopyOnWriteArrayList<ListenerRegistration<ReorganizeListener>> reorganizeListeners;
   private final CopyOnWriteArrayList<ListenerRegistration<TransactionReceivedInBlockListener>> transactionReceivedListeners;
   
  class OrphanBlock {
       final Block block;
       final List<Sha256Hash> filteredTxHashes;
       final Map<Sha256Hash, Transaction> filteredTxn;
       OrphanBlock(Block block, @Nullable List<Sha256Hash> filteredTxHashes, @Nullable Map<Sha256Hash, Transaction> filteredTxn) {
           final boolean filtered = filteredTxHashes != null && filteredTxn != null;
           Preconditions.checkArgument((block.transactions == null && filtered)
                                       || (block.transactions != null && !filtered));
           this.block = block;
           this.filteredTxHashes = filteredTxHashes;
           this.filteredTxn = filteredTxn;
       }
   }
   // Holds blocks that we have received but can't plug into the chain yet, eg because they were created whilst we
   // were downloading the block chain.
   private final LinkedHashMap<Sha256Hash, OrphanBlock> orphanBlocks = new LinkedHashMap<Sha256Hash, OrphanBlock>();

孤立块代表接收到但是没有被记入主链的区块，可能由于网络传播的延迟导致同一时间段内发现的两个区块，其中一个更快的传播到了更多的节点处。孤立块的变化情况可以用来反映区块链网络中自私挖掘而情况，因为自私挖掘策略的执行会导致被抛弃的区块数量增加。关于自私挖掘（Selfish Mining）后面我会整理论文专门写一篇来详细的介绍。

removeListener（）和addListener()批量注销和注册三个监听分别是add|removeReorganizeListener、add|removeNewBestBlockListener、add|removeTransactionReceivedListener。