The implementation of exitValue() is not careful enough to check for errors.

JNIEXPORT jint JNICALL
Java_java_lang_ProcessImpl_exitValue(JNIEnv *env, jobject process)
{
    jint exit_code;
    jboolean exc;
    jlong handle = JNU_GetFieldByName(env, &exc, process, "handle", "J").j;
    if (exc) {
        return 0;
    }

    GetExitCodeProcess((void *)handle, &exit_code);
    if (exit_code == STILL_ACTIVE) {
JNU_ThrowByName(env, "java/lang/IllegalThreadStateException",
"process has not exited");
return -1;
    }
    return exit_code;
}

What happens if GetExitCodeProcess fails?
In this case there should be an exception thrown with a human-readable
error description.
Also, in this case exit_code may never be initialized, with the result that the
returned value is whatever random garbage was on the stack at the point of call.