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Let threads joining main finish when main ends - #49
A thread that joins the main thread no longer deadlocks when main ends. OpenJDK's java/lang/Thread/UncaughtExceptionsTest.java hung on this: its MainThreadAbruptTermination case starts a non-daemon thread that joins main, and main ends with an uncaught exception.
The VM ran the uncaught exception handler correctly, then waited for the other non-daemon threads before it marked main terminated and woke its joiners. The joining thread waited for main, and main waited for it. Main now exits (Thread.exit, terminated state, joiners woken) before the wait, as HotSpot's launcher detaches main before DestroyJavaVM waits for the remaining threads. An uncaught exception on main still exits with status 1.
Tests:
test/java/MainThreadExit.java matches HotSpot: a non-daemon thread joins main, and main ends with an uncaught exception.
All eight simulator classes in UncaughtExceptionsTest.java match HotSpot's output and exit status; the test passes and goes into passing.txt.
No actionable comments were generated in the recent review.
Summary
Summary by CodeRabbit
Bug Fixes
The main thread now exits before the runtime waits for other non-daemon threads. This ensures uncaught exceptions from the main thread are handled promptly, while the runtime continues waiting for other non-daemon threads before shutting down.
The JVM now exits the main thread before waiting for other non-daemon threads. A new Java test checks a worker’s join on the main thread after an uncaught exception. The corresponding jtreg test is added to the passing-test list.
Changes: Main thread exit ordering
Layer / File(s)
Summary
Exit ordering and join test src/jvm.ts, test/java/MainThreadExit.java, test/jtreg/passing.txt
mainGen exits the main thread before waiting for other non-daemon threads. The new test starts a worker that joins the main thread and prints whether it is alive after the join. The jtreg passing-test list includes UncaughtExceptionsTest.java.
Project516/java-web#29: Adds thread-body handling for uncaught exceptions and Java thread exit, which relates to the main-thread exit behavior changed here.
Project516/java-web#45: Reports that UncaughtExceptionsTest.java hangs because of thread exit handling, which this change also addresses.
Suggested reviewers:project516-review-bot
Merge Risk: Minimal · up to b6bbe
Threads joining main now wake when main ends, which fixes the deadlock described in the PR. No concrete merge-blocking risk remains. One cosmetic edge case remains: during shutdown hooks, main's reported thread state can briefly differ from TERMINATED.
Pre-merge checks: 5
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Explanation
Title check
The title clearly and concisely describes the main change: allowing threads that join the main thread to finish when main terminates.
Description check
The description directly explains the deadlock, the main-thread termination change, the related tests, and the reported validation results.
Docstring Coverage
No eligible functions found in the changed files. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…
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Full details: Docstring Coverage
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A worker waits while main takes flight, The thread exits before the wait. An exception prints its message clear, The worker checks that main is gone. Then shutdown follows, neat and done.
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Files selected for processing: 3
src/jvm.ts
test/java/MainThreadExit.java
test/jtreg/passing.txt
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Additional context used
Learnings: 1
Common learnings
Learnt from: CR
Repo: Project516/java-web PR: 0
File: AGENTS.md:0-0
Timestamp: 2026-11-05T01:39:41.735Z
Learning: Source excerpt:
# java-web
## Direction
- Prove it with tests: programs whose output must match HotSpot exactly, and
OpenJDK's own regression tests where they can run.
Learnt from: CR
Repo: Project516/java-web PR: 0
File: AGENTS.md:0-0
Timestamp: 2026-11-05T01:39:41.735Z
Learning: Source excerpt:
# java-web
## Direction
- Compatibility first. Run JDK classes unmodified and implement what HotSpot
does for them (natives, VM upcalls, init order). Do not patch or replace JDK
classes. When behavior differs from HotSpot, HotSpot is right.
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The PR fixes a deadlock when a non-daemon thread joins the main thread and main ends with an uncaught exception. The fix moves exitThread(t) before the loop that waits for other non-daemon threads, so main becomes joinable (terminated state, joiners woken) before the VM waits for the remaining threads. This matches HotSpot's behavior where the launcher detaches main before DestroyJavaVM waits for other threads. A new test MainThreadExit.java verifies the scenario, and UncaughtExceptionsTest.java now passes.
review-bot, model nvidia/nemotron-3-ultra-550b-a55b:free, verdict approve
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A thread that joins the main thread no longer deadlocks when main ends. OpenJDK's
java/lang/Thread/UncaughtExceptionsTest.javahung on this: itsMainThreadAbruptTerminationcase starts a non-daemon thread that joins main, and main ends with an uncaught exception.The VM ran the uncaught exception handler correctly, then waited for the other non-daemon threads before it marked main terminated and woke its joiners. The joining thread waited for main, and main waited for it. Main now exits (
Thread.exit, terminated state, joiners woken) before the wait, as HotSpot's launcher detaches main beforeDestroyJavaVMwaits for the remaining threads. An uncaught exception on main still exits with status 1.Tests:
test/java/MainThreadExit.javamatches HotSpot: a non-daemon thread joins main, and main ends with an uncaught exception.UncaughtExceptionsTest.javamatch HotSpot's output and exit status; the test passes and goes intopassing.txt.pnpm typecheckandpnpm testpass.