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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.
483
Run your JVM with -noverify or -Xverify:none , which disables all bytecode verification, and write something that would otherwise not be allowed to run.
To crash a JVM, aside from JNI, you need to find a bug in the VM itself. An infinite loop just consumes CPU. Infinitely allocating memory should just cause OutOfMemoryError's in a well built JVM. This would probably cause problems for other threads, but a good JVM still should not crash.
All we have to do is simply write a class that extends the java. lang. Thread class, and provide the logic that we want to perform when the VM is shutting down, inside the public void run() method.
A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode. The JVM is detailed by a specification that formally describes what is required in a JVM implementation.
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.
93
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