gistfile1.txt

import java.lang.reflect.AccessibleObject;
import java.lang.reflect.Field;

public class Play {
    public static void main(String[] args) throws Exception {
        AddressExtractor ae = new AddressExtractor();
        Field pvField = AddressExtractor.class.getDeclaredField("pointerValue");
        Field type = Field.class.getDeclaredField("type");
        type.setAccessible(true);
        type.set(pvField, Object.class);

        // Setting is relatively safe:
        Dummy dummy = new Dummy();
        pvField.set(ae, dummy);
        System.out.println(Long.toHexString(ae.pointerValue));

        // Getting is NOT!
        ae.pointerValue = 0xDEADBEEF;

        // Used to be this in example:
        //System.out.println(pvField.get(ae) == foo);

        // It is the same as getting Object in the local.
        // ...and if GC happens in the middle:
        Object o = pvField.get(ae);
        System.gc(); // SEGV, because "o" is not actually the object
        System.out.println(o == dummy);

        System.in.read();
    }
}

class AddressExtractor {
    public long pointerValue;
}

class Dummy {
    public long value;
}

/*
$ ~/Install/jdk8u232/bin/java Play
b002c00c
#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007f85878c563c, pid=3437, tid=0x00007f85854af700
#
# JRE version: OpenJDK Runtime Environment (8.0) (build 1.8.0-builds.shipilev.net-openjdk-jdk8-b24-20191015-jdk8u232-b09:jdk8u232-ga)
# Java VM: OpenJDK 64-Bit Server VM (25.71-b24-20191015-jdk8u232-09:jdk8u232-ga mixed mode linux-amd64 compressed oops)
# Problematic frame:
# V  [libjvm.so+0x91363c]  PSRootsClosure<false>::do_oop(oopDesc**)+0x1c
#
# 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:
# /home/shade/temp/reflect/hs_err_pid3437.log
#
# If you would like to submit a bug report, please visit:
#   http://bugreport.java.com/bugreport/crash.jsp
#
Aborted (core dumped)

$ ~/Install/jdk11.0.6-ea/bin/java Play
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by Play (file:/home/shade/temp/reflect/) to field java.lang.reflect.Field.type
WARNING: Please consider reporting this to the maintainers of Play
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
Exception in thread "main" java.lang.IllegalArgumentException: Can not set long field AddressExtractor.pointerValue to Dummy
	at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:167)
	at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:171)
	at java.base/jdk.internal.reflect.UnsafeLongFieldAccessorImpl.set(UnsafeLongFieldAccessorImpl.java:102)
	at java.base/java.lang.reflect.Field.set(Field.java:780)
	at Play.main(Play.java:14)

*/