Flume学习——BasicChannelSemantics

public class MemoryChannel extends BasicChannelSemantics

public abstract class BasicChannelSemantics extends AbstractChannel

public abstract class AbstractChannel implements Channel, LifecycleAware, Configurable

AbstractChannel只在实现了LifecycleAware,NamedComponent和Configurable中的一些接口，除了对部分方法进行了同步，没什么特殊。

BasicChannelSemantics是实现事务机制的关键之一

 * <p>
 * An implementation of basic {@link Channel} semantics, including the
 * implied thread-local semantics of the {@link Transaction} class,
 * which is required to extend {@link BasicTransactionSemantics}.
 * </p>
 */

BasicChannelSemantics实现了基础的Channel语法，包括了Transaction的thread-local语法。

首先BasicChannelSemantics类持有一个ThreadLocal对象，它维护了一个BasicTransactionSemantics对象。BasicTransationSementics是一个abstract class,提供了Transaction接口的基础实现。

这些每个线程都包括了一个唯一的Transaction对象，保证了事务的隔离性。

  private ThreadLocal<BasicTransactionSemantics> currentTransaction
      = new ThreadLocal<BasicTransactionSemantics>();

在线程中获取当前线程中的事务通过getTransaction方法，它会调用BasicChannelSemantics中定义的的抽象方法createTransaction()来获取BasicTransactionSemantics的实例。

/**
   * <p>
   * Initializes the channel if it is not already, then checks to see
   * if there is an open transaction for this thread, creating a new
   * one via <code>createTransaction</code> if not.
   * @return the current <code>Transaction</code> object for the
   *     calling thread
   * </p>
   */
  @Override
  public Transaction getTransaction() {

    if (!initialized) {
      synchronized (this) {
        if (!initialized) {
          initialize();
          initialized = true;
        }
      }
    }

    BasicTransactionSemantics transaction = currentTransaction.get();
    if (transaction == null || transaction.getState().equals(
            BasicTransactionSemantics.State.CLOSED)) {
      transaction = createTransaction();
      currentTransaction.set(transaction);
    }
    return transaction;
  }

首先，Channel使用了延迟初始化的机制。只有在Channel第一次被调用getTransaction()时，它的initialize()方法才被调用。

当currentTransaction中并不包含一个BasicTransactionSemantics对象的时候，或者当前的transaction对象已经处理CLOSED状态的时候，它就调用createTransaction方法来获取一个Transaction,并设置给thread-local的currentTransaction。

至此，Transaction的thread-local机制已实现。

那么Channel是如何利用thread-local的Transaction对象来实现消息存取的事务呢?它只是确保事务已开启，然后将消息的存取功能代理给本线程的transaction对象。

  @Override
  public void put(Event event) throws ChannelException {
    BasicTransactionSemantics transaction = currentTransaction.get();
    Preconditions.checkState(transaction != null,
        "No transaction exists for this thread");
    transaction.put(event);
  }

  @Override
  public Event take() throws ChannelException {
    BasicTransactionSemantics transaction = currentTransaction.get();
    Preconditions.checkState(transaction != null,
        "No transaction exists for this thread");
    return transaction.take();
  }