src/jdk.management/share/classes/jdk/management/VirtualThreadSchedulerMXBean.java

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package jdk.management;

import java.lang.management.ManagementFactory;
import java.lang.management.PlatformManagedObject;
import java.util.concurrent.ForkJoinPool;
import javax.management.MBeanServer;
import javax.management.ObjectName;

/**
 * Management interface for the JDK's {@linkplain Thread##virtual-threads virtual thread}
 * scheduler.
 *
 * <p> {@code VirtualThreadSchedulerMXBean} supports monitoring of the virtual thread
 * scheduler's target parallelism, the {@linkplain Thread##platform-threads 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.
 *
 * <p> The management interface is registered with the platform {@link MBeanServer
 * MBeanServer}. The {@link ObjectName ObjectName} that uniquely identifies the management
 * interface within the {@code MBeanServer} is: "jdk.management:type=VirtualThreadScheduler".
 *
 * <p> Direct access to the MXBean interface can be obtained with
 * {@link ManagementFactory#getPlatformMXBean(Class)}.
 *
 * @since 24
 */
public interface VirtualThreadSchedulerMXBean extends PlatformManagedObject {

    /**
     * {@return the scheduler's target parallelism}
     *
     * @see ForkJoinPool#getParallelism()
     */
    int getParallelism();

    /**
     * Sets the scheduler's target parallelism.
     *
     * <p> Increasing the target parallelism allows the scheduler to use more platform
     * threads to <i>carry</i> virtual threads if required. Decreasing the target parallelism
     * reduces the number of threads that the scheduler may use to carry virtual threads.
     *
     * @apiNote If virtual threads are mounting and unmounting frequently then downward
     * adjustment of the target parallelism will likely come into effect quickly.
     *
     * @implNote The JDK's virtual thread scheduler is a {@link ForkJoinPool}. Target
     * parallelism defaults to the number of {@linkplain Runtime#availableProcessors()
     * available processors}. The minimum target parallelism is 1, the maximum target
     * parallelism is 32767.
     *
     * @param size the target parallelism level
     * @throws IllegalArgumentException if size is less than the minimum, or
     *         greater than the maximum, supported by the scheduler
     * @throws UnsupportedOperationException if changing the target
     *         parallelism is not suppored by the scheduler
     *
     * @see ForkJoinPool#setParallelism(int)
     */
    void setParallelism(int size);

    /**
     * {@return the current number of platform threads that the scheduler has started
     * but have not terminated; {@code -1} if not known}
     *
     * <p> The count includes the platform threads that are currently <i>carrying</i>
     * 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.
     *
     * @implNote The JDK's virtual thread scheduler is a {@link ForkJoinPool}. The pool
     * size is the {@linkplain ForkJoinPool#getPoolSize() number of worker threads}.
     */
    int getPoolSize();

    /**
     * {@return an estimate of the number of virtual threads that are currently
     * <i>mounted</i> by the scheduler; {@code -1} if not known}
     *
     * <p> The number of mounted virtual threads is equal to the number of platform
     * threads carrying virtual threads.
     *
     * @implNote This method may overestimate the number of virtual threads that are mounted.
     */
    int getMountedVirtualThreadCount();

    /**
     * {@return an estimate of the number of virtual threads that are queued to
     * the scheduler to start or continue execution; {@code -1} if not known}
     *
     * @implNote This method may overestimate the number of virtual threads that are
     * queued to execute.
     */
    long getQueuedVirtualThreadCount();
}