Do errors thrown within UncaughtExceptionHandler get swallowed?

Question

Thread.UncaughtExceptionHandler states that when the method which handles uncaught exceptions itself throws an exception, that exception will be ignored:

void uncaughtException(Thread t, Throwable e):

Method invoked when the given thread terminates due to the given uncaught exception.

Any exception thrown by this method will be ignored by the Java Virtual Machine.

However when I tested it, the JVM did not ignore the exceptions handled by the uncaught exception handler`:

public static void main(final String args[]) {
    Thread.currentThread().setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
        @Override
        public void uncaughtException(Thread arg0, Throwable arg1) {
            throw new java.lang.RuntimeException("e2");
        }
    });
    throw new RuntimeException("e1");
}

Eclipse Console output (JRE 1.7):

Exception: java.lang.RuntimeException thrown from the UncaughtExceptionHandler in thread "main"

Another oddity I found out is that the output I get isn't coming from System.err. It seems to be from another stream altogether. I verified this by redirecting System.err to System.out, but I'm still getting "red" output:

public static void main(final String[] args) {
    System.setErr(System.out);
    System.out.println(System.err == System.out);
    System.err.println("this is black color");
    try {
        throw new Error("test stacktrace color");
    } catch (Throwable e) {
        e.printStackTrace();
    }
    try {
        Thread.sleep(2500);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    Thread.currentThread().setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
        @Override
        public void uncaughtException(Thread t, Throwable e) {
            throw new RuntimeException("from handler");
        }
    });
    throw new RuntimeException("from main");
}

The output (bolded signifies red color):

true

this is black color

java.lang.Error: test stacktrace color at asf.df.main(df.java:13)

Exception: java.lang.RuntimeException thrown from the UncaughtExceptionHandler in thread "main"

What's the explanation for these phenomenons?

What happens to errors thrown within UncaughtExceptionHandler? What's the expected (documented or guaranteed) behavior?

Answer 1

HotSpot JVM prints the exceptions thrown from the UncaughtExceptionHandler. See JavaThread::exit

    if (HAS_PENDING_EXCEPTION) {
      ResourceMark rm(this);
      jio_fprintf(defaultStream::error_stream(),
            "\nException: %s thrown from the UncaughtExceptionHandler"
            " in thread \"%s\"\n",
            pending_exception()->klass()->external_name(),
            get_thread_name());
      CLEAR_PENDING_EXCEPTION;
    }

JVM prints these exceptions itself directly on stderr regardless of the System.err state - whether it was redirected or not.

Well, this kind of warning does not affect the application - in this sense the exception is "ignored". But you are right, this behavior is not obvious. Javadoc is misleading and is better to be fixed.

Answer 2

The exceptions are ignored and processing continues when thrown from a non-main thread.

If it is thrown in main the error code returned is non-zero.

The unhandled exceptions are logged via syserr.

public static void main(final String[] args) {

    final Thread myThread = new Thread(new Runnable() {

        @Override
        public void run() {
            Thread.currentThread()
                .setUncaughtExceptionHandler(new UncaughtExceptionHandler() {

                    @Override
                    public void uncaughtException(final Thread t, final Throwable e) {

                        System.out.println("In child UncaughtExceptionHandler at " + java.time.Instant.now());

                        throw new RuntimeException("From child thread UncaughtExceptionHandler"
                                + java.time.Instant.now());

                    }
                });
            throw new RuntimeException("from runnable");
        }
    });

    Thread.currentThread()
    .setUncaughtExceptionHandler(new UncaughtExceptionHandler() {

        @Override
        public void uncaughtException(final Thread t, final Throwable e) {

                System.out.println("In main UncaughtExceptionHandler " + java.time.Instant.now());

                throw new RuntimeException("From main thread UncaughtExceptionHandler" + java.time.Instant.now());

        }
    });

    myThread.start();

    LockSupport.parkNanos(TimeUnit.SECONDS.toNanos(2));

    System.out.println("After child thread: " + java.time.Instant.now());

    //Will result in a non-zero return code
    throw new RuntimeException("from main");
}

Output:

In child UncaughtExceptionHandler at 2014-07-19T04:10:46.184Z

Exception: java.lang.RuntimeException thrown from the UncaughtExceptionHandler in thread "Thread-0" After child thread: 2014-07-19T04:10:48.197Z In main UncaughtExceptionHandler 2014-07-19T04:10:48.197Z

Exception: java.lang.RuntimeException thrown from the UncaughtExceptionHandler in thread "main"