Making an object reference null from JNI code

Here is an abstraction of my problem. I want to set an object reference from the java code to null in a JNI function. For example:

// -- java code --
String s="this is a new string";
func(s);                        //passing the object to the JNI function.
System.out.println(s);

This should print null

Firstly, I am passing that object as jobject in the JNI function and then using DeleteGlobalRef to remove the reference. The JNI function is given below.

JNIEXPORT void JNICALL
Java_func_test(JNIEnv *env, jobject this, jobject todel)
{
    (*env)->DeleteGlobalRef(env,todel);
}

This makes the JVM to crash with the following message.

> To suppress the following error report, specify this argument
> after -XX: or in .hotspotrc: SuppressErrorAt=/oopStorage.cpp:697
>
> A fatal error has been detected by the Java Runtime Environment:
>
> Internal Error (src/hotspot/share/gc/shared/oopStorage.cpp:697), pid=59362, tid=59363
> assert(block != __null) failed: JNI global: invalid release 0x00007f66ed673788
>
> JRE version: OpenJDK Runtime Environment (14.0) (slowdebug build 14-internal+0-adhoc.manavjeet.jdk14)
> Java VM: OpenJDK 64-Bit Server VM (slowdebug 14-internal+0-adhoc.manavjeet.jdk14, mixed mode, tiered, compressed
> oops, g1 gc, linux-amd64)
> Problematic frame:
> V [libjvm.so+0xf684eb] OopStorage::release(oopDesc* const*)+0x49
>
> Core dump will be written. Default location: Core dumps may be processed with "/usr/share/apport/apport %p %s %c %d %P %E" (or
> dumping to
> /home/manavjeet----/core.59362)
>
> An error report file with more information is saved as:
> /home/manavjeet/---/hs_err_pid59362.log
>
> If you would like to submit a bug report, please visit:
> https://bugreport.java.com/bugreport/crash.jsp
> Current thread is 59363 Dumping core ... fish: “./java-slowdebug helloworld” terminated by signal SIGABRT (Abort)

By this I understand this is horribly wrong. Can anyone please point out the mistake and suggest the correct way to set the object reference in java to null.

Java is pass by value. The call func(s) copies the reference s and gives it to func, which cannot change the original. null is not an object; it's a special reference. That means func can't set s to null, because it can't change s, only the object behind it. Just being a native method does not give func magical powers to break the rules of the language. The JNI is mostly for interfacing with functionality outside the language. Unless you find a way to peer into the caller's bytecode and mutate its stack frame, you can't do this.

Also, DeleteGlobalRef does not work this way. It can only free references made by NewGlobalRef, which you may call to remember state between calls to native functions. todel here is a local reference, created anew for each call to the native method. I believe you could DeleteLocalRef it, but that would not do anything.

<!-- language: c -->

JNIEXPORT void JNICALL
Java_func_test(JNIEnv *env, jobject this, jobject todel)
{
    (*env)->DeleteLocalRef(env, todel);
}

This should at least not crash, but it still can't modify the reference the caller has; it's basically equivalent to

<!-- language: java -->

void func(Object todel) {
    todel = null;
}

which does nothing. Again: Java copies the reference upon the call to func. If the caller has their own copy of the reference you're trying to delete, no matter how hard you try to delete your copy of the reference, the caller's copy just won't change.

If you really, really wanted to, you could box the reference into another object and mutate that:

<!-- language: java -->

public class Box<T> {
    public T ref;
    public Box() { this(null); }
    public Box(T ref) { this.ref = ref; }
    public String toString() {
        return "Box(" + System.identityHashCode(ref) + ": " + ref + ")";
    }
}

<T> void func(Box<T> box) { box.ref = null; }
// as I said; native methods are not magic
// rewriting func with the JNI does not make it more powerful; just more inscrutable

void test() {
    var s = new Box<>("this is a new string");
    func(s);
    System.out.println(s); // "Box(0: null)"
}

but... why would you?