How to get around problem that you can't pass env between threads?

Question

I get all the typical parameters in a JNI method call, but I need to display callback messages using these parameters from another thread. The main problem is saving the JNIEnv pointer for future use. How do you do that?
EX: - my C program that is turned into a dll:
static JNIEnv *m_env;
static jobject m_jobj;
first_method(JNIEnv* env, jobject jobj, ...) {
// I want to save the env and jobj globally
// so it can be used in callback
m_env = env;
m_jobj = env->NewGlobalRef(jobj);
// start the thread that generates callbacks
// callback method - called by another thread
second_method(...) {
// use the global variables to call a Java method
jclass clazz = m_env->GetObjectClass(m_jobj);
jmethodID mid = m_env->GetMethodID(clazz, ...);
jstring msg = m_env->NewStringUTF("A message");
m_env->CallVoidMethod(m_jobj, mid, msg);

Answer 1

You have to register each thread (there is a method.) Then there is another method (or maybe the same one) that returns the env for the thread.