Repository: apache/kafka
File: core/src/main/scala/kafka/server/KafkaRequestHandler.scala (Scala)

Code before:
  val requestChannel: RequestChannel,
  val apis: ApiRequestHandler,
  time: Time,
  numThreads: Int,
  nodeName: String = "broker"
) extends Logging {
  private val metricsGroup = new KafkaMetricsGroup(this.getClass)

  val threadPoolSize: AtomicInteger = new AtomicInteger(numThreads)
  private val perPoolIdleMeterName = if (nodeName == "broker") {
    "BrokerRequestHandlerAvgIdlePercent"
  } else {
    "ControllerRequestHandlerAvgIdlePercent"
  }
  /* Per-pool idle meter (broker-only or controller-only) */
  private val perPoolIdleMeter = metricsGroup.newMeter(perPoolIdleMeterName, "percent", TimeUnit.NANOSECONDS)
  /* Aggregate meter to track the average free capacity of the request handlers */
  private val aggregateIdleMeter = metricsGroup.newMeter(requestHandlerAvgIdleMetricName, "percent", TimeUnit.NANOSECONDS)

  this.logIdent = s"[data-plane Kafka Request Handler on ${nodeName.capitalize} $brokerId] "
  val runnables = new mutable.ArrayBuffer[KafkaRequestHandler](numThreads)
  for (i <- 0 until numThreads) {
    createHandler(i)
  }

  private def createHandler(id: Int): Unit = synchronized {
    runnables += new KafkaRequestHandler(
      id,
      brokerId,
      aggregateIdleMeter,
      aggregateThreads,
      perPoolIdleMeter,
      threadPoolSize,
      requestChannel,
      apis,
      time,
      nodeName,
    )
    aggregateThreads.getAndIncrement()
    KafkaThread.daemon("data-plane-kafka-request-handler-" + id, runnables(id)).start()
  }

  private def deleteHandler(id: Int): Unit = synchronized {
    runnables.remove(id).stop()
    aggregateThreads.getAndDecrement()
  }

  def resizeThreadPool(newSize: Int): Unit = synchronized {
    val currentSize = threadPoolSize.get
    info(s"Resizing request handler thread pool size from $currentSize to $newSize")
    if (newSize > currentSize) {

Review comment (suggestion):
`synchronized` is not needed since the method is private and it is always called from either the constructor or a `synchronized` method.

Code after:
  val aggregateThreads: AtomicInteger,
  val requestHandlerAvgIdleMetricName: String,
  val brokerId: Int,
  val requestChannel: RequestChannel,
  val apis: ApiRequestHandler,
  time: Time,
  numThreads: Int,
  nodeName: String
) extends Logging {
  // Changing the package or class name may cause incompatibility with existing code and metrics configuration
  private val metricsPackage = "kafka.server"
  private val metricsClassName = "KafkaRequestHandlerPool"
  private val metricsGroup = new KafkaMetricsGroup(metricsPackage, metricsClassName)

  val threadPoolSize: AtomicInteger = new AtomicInteger(numThreads)
  private val perPoolIdleMeterName = if (nodeName == "broker") {
    "BrokerRequestHandlerAvgIdlePercent"
  } else if (nodeName == "controller") {
    "ControllerRequestHandlerAvgIdlePercent"
  } else {
    throw new IllegalArgumentException("Invalid node name:" + nodeName)
  }
  /* Per-pool idle meter (broker-only or controller-only) */
  private val perPoolIdleMeter = metricsGroup.newMeter(perPoolIdleMeterName, "percent", TimeUnit.NANOSECONDS)
  /* Aggregate meter to track the average free capacity of the request handlers */
  private val aggregateIdleMeter = metricsGroup.newMeter(requestHandlerAvgIdleMetricName, "percent", TimeUnit.NANOSECONDS)

  this.logIdent = s"[data-plane Kafka Request Handler on ${nodeName.capitalize} $brokerId] "
  val runnables = new mutable.ArrayBuffer[KafkaRequestHandler](numThreads)
  for (i <- 0 until numThreads) {
    createHandler(i)
  }

  private def createHandler(id: Int): Unit = {
    runnables += new KafkaRequestHandler(
      id,
      brokerId,
      aggregateIdleMeter,
      aggregateThreads,
      perPoolIdleMeter,
      threadPoolSize,
      requestChannel,
      apis,
      time,
      nodeName
    )
    aggregateThreads.getAndIncrement()
    KafkaThread.daemon("data-plane-kafka-request-handler-" + id, runnables(id)).start()
  }

  private def deleteHandler(id: Int): Unit = {