Debugger Support

Enumerating Threads

Virtual threads are just objects in the heap, there may be millions of them. There is no API support for enumerating all virtual threads.

JDI VirtualThreads.allThreads() (and JDWP VirtualMachine/AllThreads) enumerates all active platform threads, virtual threads are not enumerated.

Thread groups

Virtual threads are not active members of a thread group.

JDI ThreadGroupReference::threads (and JDWP ThreadGroupReference/Children) enumerate all active platform threads in a group, virtual threads are not enumerated.

ThreadStart/ThreadEnd events

JDI ThreadStartEvent/ThreadDeathEvents are sent for all threads when enabled. This may impact performance if there are a huge number of virtual threads.

JDWP EventRequest/Set defines a new PlatformThreadsOnly filter that can be used when requesting THREAD_START and THREAD_END events. This allows these events to be filtered for virtual threads so they are not sent to the front-end/debugger.

JDI ThreadStartRequest/ThreadDeathRequest define a new method to control whether thread start/end events are sent for all threads or only platform threads.

IsVirtual

JDI ThreadReference defines isVirtual() to test if a thread is a virtual thread.

JDWP ThreadReference/IsVirtual is the equivalent.

Not Supported

The following are not currently supported for virtual threads:

JDI ThreadReference.stop
JDI ThreadReference.interrupt
JDI ThreadReference.popFrame
JDI ThreadReference.forceEarlyReturn
JDI StackFrame.setValue

JDWP agent options

As a temporary solution to allow existing debuggers work with virtual threads, the JDWP agent will track virtual threads so they can be enumerated for debuggers that want to enumerate all virtual threads. The options that control this behaviour are:

Option Name and Value	Description	Default
enumeratevthreads=y\|n	thread lists include vthreads	n
trackvthreads=some\|all	track some or all vthreads	some
fakevthreadstartevent=y\|n	send fake start event when needed	y

enumeratevthreads control whether or not virtual threads are included in list of threads returned by JDWP VirtualThread/AllThreads command.
trackvthreads controls which virtual threads will be returned when enumeratevthreads=y.
fakevthreadstartevent is independent of the two others. If y, then before sending an event, send a fake THREAD_START event for if the debugger hasn’t already been notified about the virtual thread that the event occurs on. Note currently there is a bug, and if trackvthreads=all, no fake THREAD_START will ever be sent. This is only an issue if the debug agent is started after some virtual threads have been created.

Links

Project Loom Early Access builds

Java Debug Interface (JDI)

JDWP protocol details

JVM Tool Interface

Slides from March 24, 2021 meeting with IntelliJ and Eclipse maintainers

Eclipse Bug 527000 tracks adding support for virtual threads