OpenJDK 27 · jdk.management.VirtualThreadSchedulerMXBean

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).

Method Summary

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

Inherited from java.lang.management.PlatformManagedObject:: getObjectName

Method Detail

getMountedVirtualThreadCount back to summary

getMountedVirtualThreadCount	back to summary
public 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:int an estimate of the number of virtual threads that are currently mounted by the scheduler; `-1` if not known

public 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:int: an estimate of the number of virtual threads that are currently mounted by the scheduler; -1 if not known

getParallelism back to summary

getParallelism	back to summary
public int getParallelism() Returns the scheduler's target parallelism. Returns:int the scheduler's target parallelism See Also `ForkJoinPool#getParallelism()`

public int getParallelism()

Returns the scheduler's target parallelism.

Returns:int: the scheduler's target parallelism
See Also: ForkJoinPool#getParallelism()

getPoolSize back to summary

getPoolSize	back to summary
public 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:int the current number of platform threads that the scheduler has started but have not terminated; `-1` if not known

public 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:int: the current number of platform threads that the scheduler has started but have not terminated; -1 if not known

getQueuedVirtualThreadCount back to summary

getQueuedVirtualThreadCount	back to summary
public 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:long an estimate of the number of virtual threads that are queued to the scheduler to start or continue execution; `-1` if not known

public 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:long: an estimate of the number of virtual threads that are queued to the scheduler to start or continue execution; -1 if not known

setParallelism back to summary

setParallelism	back to summary
public 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:int the target parallelism level Exceptions 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 supported by the scheduler See Also `ForkJoinPool#setParallelism(int)`

public 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:int: the target parallelism level

Exceptions

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 supported by the scheduler

See Also

ForkJoinPool#setParallelism(int)

public Interface VirtualThreadSchedulerMXBean

Method Summary

Method Detail