Context Propagation ThreadLocal with Virtual Threads (Loom)

[marlonpatrick]

Hey folks,

I have an application micronaut 4.4.8 running on Java 21.

I created a new rest endpoint using @ExecuteOn(TaskExecutors.VIRTUAL) and enabled JVM flag -Djdk.traceVirtualThreadLocals=true.

When I do this, I noticed an use of ThreadLocal from micronaut:

VirtualThread[#69,virtual-executor3]/runnable@ForkJoinPool-1-worker-3
    java.base/java.lang.ThreadLocal.setInitialValue(ThreadLocal.java:236)
    java.base/java.lang.ThreadLocal.get(ThreadLocal.java:194)
    java.base/java.lang.ThreadLocal.get(ThreadLocal.java:172)
    io.netty.util.internal.InternalThreadLocalMap.slowGet(InternalThreadLocalMap.java:130)
    io.netty.util.internal.InternalThreadLocalMap.get(InternalThreadLocalMap.java:117)
    io.netty.util.concurrent.FastThreadLocal.get(FastThreadLocal.java:136)
    io.micronaut.core.propagation.ThreadContext.get(ThreadContext.java:66)
    io.micronaut.core.propagation.PropagatedContextImpl.propagate(PropagatedContextImpl.java:183)
    io.micronaut.core.propagation.PropagatedContext.lambda$wrap$3(PropagatedContext.java:210)
    java.base/java.util.concurrent.ThreadPerTaskExecutor$TaskRunner.run(ThreadPerTaskExecutor.java:314)
    java.base/java.lang.VirtualThread.run(VirtualThread.java:309)

VirtualThread[#69,virtual-executor3]/runnable@ForkJoinPool-1-worker-3
    java.base/java.lang.ThreadLocal.set(ThreadLocal.java:253)
    io.netty.util.internal.InternalThreadLocalMap.slowGet(InternalThreadLocalMap.java:133)
    io.netty.util.internal.InternalThreadLocalMap.get(InternalThreadLocalMap.java:117)
    io.netty.util.concurrent.FastThreadLocal.get(FastThreadLocal.java:136)
    io.micronaut.core.propagation.ThreadContext.get(ThreadContext.java:66)
    io.micronaut.core.propagation.PropagatedContextImpl.propagate(PropagatedContextImpl.java:183)
    io.micronaut.core.propagation.PropagatedContext.lambda$wrap$3(PropagatedContext.java:210)
    java.base/java.util.concurrent.ThreadPerTaskExecutor$TaskRunner.run(ThreadPerTaskExecutor.java:314)
    java.base/java.lang.VirtualThread.run(VirtualThread.java:309)

Is this use of ThreadLocal a big problem? There is anything I can do to prevent this?

[dstepanov]

Why would you want to prevent it?

[re-thc]

ThreadLocal isn't quite useful if we run on virtual threads and lots of short lived "threads" are created? The "caching" becomes ineffective.

[dstepanov]

We don't use it for caching; we use it to propagate context