Threads and lifecycle question 12 of 180 junior
What happens when a thread throws an exception nobody catches?
- The whole JVM exits with that exception
- Correct: Only that thread dies; its uncaught exception handler runs and other threads carry on
- The exception is rethrown in the thread that called
start() - The thread restarts from the beginning of
run()
The exception unwinds run(), the thread terminates, and the JVM hands the exception to the thread's
uncaught exception handler. The lookup goes from the thread's own handler, set with
setUncaughtExceptionHandler, to its ThreadGroup, which delegates to
Thread.getDefaultUncaughtExceptionHandler(), and if nobody set one, the stack trace is printed to
System.err. Every other thread, including main, keeps running, and the JVM exits only when the last
non-daemon thread finishes.
That is exactly why it is dangerous. A consumer thread that dies on one malformed message stops consuming forever, the queue behind it fills, and the only evidence is a stack trace in a log nobody reads. The process looks healthy.
Two habits fix it. Loops that must survive a bad input catch at the top of the loop body and carry on.
And main installs a default handler that logs properly, so a dead thread is at least visible:
Thread.setDefaultUncaughtExceptionHandler((thread, error) ->
log.error("thread {} died", thread.getName(), error));In a thread pool the rules change: execute reaches this handler, while submit stores the exception in
the Future, where it stays silent until someone calls get().