VirtualThreadSchedulerMXBean (Java SE 25 & JDK 25)
Description: declaration: module: jdk.management, package: jdk.management, interface: VirtualThreadSchedulerMXBean
Keywords: jdk.management.VirtualThreadSchedulerMXBean interface

# Interface VirtualThreadSchedulerMXBean

All Superinterfaces:
  `PlatformManagedObject`

public interface VirtualThreadSchedulerMXBean extends PlatformManagedObject
Management interface for the JDK's virtual thread scheduler.

`VirtualThreadSchedulerMXBean` supports monitoring of the virtual thread scheduler's target parallelism, the platform threads used by the scheduler, and the number of virtual threads queued to the scheduler. It also supports dynamically changing the scheduler's target parallelism.

The management interface is registered with the platform `MBeanServer`. The `ObjectName` that uniquely identifies the management interface within the `MBeanServer` is: "jdk.management:type=VirtualThreadScheduler".

Direct access to the MXBean interface can be obtained with `ManagementFactory.getPlatformMXBean(Class)`.

Since:
  24

- ## Method Summary

  Modifier and Type
  Method
  Description
  `int`
  `getMountedVirtualThreadCount()`
  Returns an estimate of the number of virtual threads that are currently mounted by the scheduler; `-1` if not known.
  `int`
  `getParallelism()`
  Returns the scheduler's target parallelism.
  `int`
  `getPoolSize()`
  Returns the current number of platform threads that the scheduler has started but have not terminated; `-1` if not known.
  `long`
  `getQueuedVirtualThreadCount()`
  Returns an estimate of the number of virtual threads that are queued to the scheduler to start or continue execution; `-1` if not known.
  `void`
  `setParallelism(int size)`
  Sets the scheduler's target parallelism.

  ### Methods declared in interface PlatformManagedObject

  `getObjectName`

- ## Method Details

  - ### getParallelism

    int getParallelism()
    Returns the scheduler's target parallelism.
    Returns:
      the scheduler's target parallelism
    See Also:
      - `ForkJoinPool.getParallelism()`

  - ### setParallelism

    void setParallelism(int size)
    Sets the scheduler's target parallelism.

    Increasing the target parallelism allows the scheduler to use more platform threads to carry virtual threads if required. Decreasing the target parallelism reduces the number of threads that the scheduler may use to carry virtual threads.

    API Note:
      If virtual threads are mounting and unmounting frequently then downward adjustment of the target parallelism will likely come into effect quickly.
    Implementation Note:
      The JDK's virtual thread scheduler is a `ForkJoinPool`. Target parallelism defaults to the number of available processors. The minimum target parallelism is 1, the maximum target parallelism is 32767.
    Parameters:
      `size` - the target parallelism level
    Throws:
      `IllegalArgumentException` - if size is less than the minimum, or greater than the maximum, supported by the scheduler
      `UnsupportedOperationException` - if changing the target parallelism is not suppored by the scheduler
    See Also:
      - `ForkJoinPool.setParallelism(int)`

  - ### getPoolSize

    int getPoolSize()
    Returns the current number of platform threads that the scheduler has started but have not terminated; `-1` if not known.

    The count includes the platform threads that are currently carrying virtual threads and the platform threads that are not currently carrying virtual threads. The thread count may be greater than the scheduler's target parallelism.

    Implementation Note:
      The JDK's virtual thread scheduler is a `ForkJoinPool`. The pool size is the number of worker threads.
    Returns:
      the current number of platform threads that the scheduler has started but have not terminated; `-1` if not known

  - ### getMountedVirtualThreadCount

    int getMountedVirtualThreadCount()
    Returns an estimate of the number of virtual threads that are currently mounted by the scheduler; `-1` if not known.

    The number of mounted virtual threads is equal to the number of platform threads carrying virtual threads.

    Implementation Note:
      This method may overestimate the number of virtual threads that are mounted.
    Returns:
      an estimate of the number of virtual threads that are currently mounted by the scheduler; `-1` if not known

  - ### getQueuedVirtualThreadCount

    long getQueuedVirtualThreadCount()
    Returns an estimate of the number of virtual threads that are queued to the scheduler to start or continue execution; `-1` if not known.
    Implementation Note:
      This method may overestimate the number of virtual threads that are queued to execute.
    Returns:
      an estimate of the number of virtual threads that are queued to the scheduler to start or continue execution; `-1` if not known