Question

In one of my past interviews , someone asked me to write a code to crash the JVM. I said System.exit(). Is this correct? Are there any better answers?

clarification: I can include my piece of code during development and deploy. It is not that JVM is already running and I have to write a hacking code to crash the other JVM.

Answer 1

You can use the Unsafe class which is unsafe to use as you might guess.

public static void main(String... args) throws Exception {
    getUnsafe().getByte(0);
}

private static Unsafe getUnsafe() throws NoSuchFieldException, IllegalAccessException {
    Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
    theUnsafe.setAccessible(true);
    return (Unsafe) theUnsafe.get(null);
}

prints

#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007ff1c2f23368, pid=2630, tid=140676351506176
#
# JRE version: 7.0-b147
# Java VM: Java HotSpot(TM) 64-Bit Server VM (21.0-b17 mixed mode linux-amd64 compressed oops)
# Problematic frame:
# V  [libjvm.so+0x82c368]  Unsafe_GetNativeByte+0xa8
#
# Failed to write core dump. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
#
# An error report file with more information is saved as:
# /nfs/peter/IdeaProjects/scratch/hs_err_pid2630.log
#
# If you would like to submit a bug report, please visit:
#   http://bugreport.sun.com/bugreport/crash.jsp
#

I have used this to test when a change has been made to a file. I made the change and crashed the JVM to ensure something else wasn’t flushing or something later. Then I check I saw the update I expected.