Kafka 源代码分析之MessageSet

这里分析MessageSet类

MessageSet是一个抽象类,定义了一条log的一些接口和常量,FileMessageSet就是MessageSet类的实现类.一条日志中存储的log完整格式如下

offset+message_size+message

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
  }

}

　　MessageSet定义log的一般接口的格式.