object MessageSet {  //消息的常量

  val MessageSizeLength = 4  //消息大小的长度

  val OffsetLength = 8   //offset的长度

  val LogOverhead = MessageSizeLength + OffsetLength   //消息头部的构成

  val Empty = new ByteBufferMessageSet(ByteBuffer.allocate(0)) //空消息.

  /**

   * The size of a message set containing the given messages

   */

  def messageSetSize(messages: Iterable[Message]): Int =

    messages.foldLeft(0)(_ + entrySize(_))  //在最左侧添加消息长度.

  /**

   * The size of a list of messages

   */

  def messageSetSize(messages: java.util.List[Message]): Int = {

    var size = 0

    val iter = messages.iterator

    while(iter.hasNext) {

      val message = iter.next.asInstanceOf[Message]  //设置消息尺寸

      size += entrySize(message)

    }

    size

  }

  /**

   * The size of a size-delimited entry in a message set

   */

  def entrySize(message: Message): Int = LogOverhead + message.size //计算总长度,是头部长度加消息本身长度.

}

/**

 * A set of messages with offsets. A message set has a fixed serialized form, though the container

 * for the bytes could be either in-memory or on disk. The format of each message is

 * as follows:

 * 8 byte message offset number

 * 4 byte size containing an integer N

 * N message bytes as described in the Message class

 */

abstract class MessageSet extends Iterable[MessageAndOffset] { //这个抽象类定义了几个消息接口.

  /** Write the messages in this set to the given channel starting at the given offset byte.

    * Less than the complete amount may be written, but no more than maxSize can be. The number

    * of bytes written is returned */

  def writeTo(channel: GatheringByteChannel, offset: Long, maxSize: Int): Int //写入消息

  /**

   * Provides an iterator over the message/offset pairs in this set

   */

  def iterator: Iterator[MessageAndOffset]   //遍历转换消息

  /**

   * Gives the total size of this message set in bytes

   */

  def sizeInBytes: Int  //字节大小.

  /**

   * Print this message set's contents. If the message set has more than 100 messages, just

   * print the first 100.

   */

  override def toString: String = {

    val builder = new StringBuilder()

    builder.append(getClass.getSimpleName + "(")

    val iter = this.iterator

    var i = 0

    while(iter.hasNext && i < 100) {

      val message = iter.next

      builder.append(message)

      if(iter.hasNext)

        builder.append(", ")

      i += 1

    }

    if(iter.hasNext)

      builder.append("...")

    builder.append(")")

    builder.toString

  }

}