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Enumerating Threads

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

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JDI VirtualThreads.allThreads (

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and JDWP VirtualMachine/AllThreads

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) continue to enumerate all platform threads.

Thread groups

Virtual threads are not active members of a thread group. 

JDI ThreadGroupReference::threads

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(and JDWP ThreadGroupReference/Children

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Thread start/end events

• New JDI ThreadStartRequest/ThreadEndRequest removeVirtualThreadFilter method to virtual events for virtual threads, filter is opt-out

) continue to enumerate all platform threads in the group.

ThreadStart/ThreadEnd events

JDWP EventRequest/Set defines a new modifier VirtualThreadsExclude that can be used when request event kinds THREAD_START and THREAD_END to filer those events for virtual threads.

JDI ThreadStartRequest/ThreadDeathRequest define a new method removeVirtualThreadFilter that can be used to remove the filtering of

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thread start/end events for virtual threads

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. This may be changed so that JDI clients opt-out rather than opt-in

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Test if a thread is a virtual thread

• New JDI ThreadReference::isVirtual

• New JDWP ThreadReference/IsVirtual

for these events.

Not Supported

The following are current not supported

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for virtual threads:

• JDI ThreadReference::stop / JDWP ThreadReference/Stop
• JDI ThreadReference::interrupt / JDWP ThreadReference/Interrupt

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Force early return, set locals, .... what else?

Debuggers and tools using the Java Debug Interface API observe the following differences:

1. com.sun.jdi.VirtualMachine::allThreads enumerates platform threads only, the list does not include mirrors for virtual threads.
2. Virtual threads are not live threads in a thread group so ThreadGroupReference::threads only lists platform threads.
3. ThreadStartRequest/ThreadDeathRequest request notifications of thread start/termination of platform threads by default. Tools have to opt-in to get start/death events for virtual threads (by invoking the removeVirtualThreadFilter method on the event request.

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• ForceEarlyReturn
• Set locals

JDWP agent options (temporary)

Option Name and Value           Description                       Default
---------------------           -----------                       -------

trackvthreads=some|all           track some or all vthreads       some
enumeratevthreads=y|n           thread lists include vthreads     y
fakevthreadstartevent=y|n       send fake start event when needed y

1. enumeratevthreads control whether or not vthreads are included in VM.allThreads and ThreadGroupReference.children.

2. trackvthreads determines what vthreads will be returned if enumeratevthreads=y, so it doesn’t carry any meaning to the user if enumeratevthreads=n, but will still impact with the debug agent does internally

3. 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 vthread the event occurred on. Intellij needs this. 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 vthreads have been created (a mode I haven’t even tested yet, and I don’t think is commonly used). (edited)

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