How to get JNIEnv from swig?

%{
#include <stdint.h>
#include "jni.h"
/**
 *  A stash area embedded in each allocation to hold java handles
 */
struct Jalloc {
  jbyteArray jba;
  jobject ref;
};

static JavaVM *cached_jvm = 0;

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved)
{
  cached_jvm = jvm;
  return JNI_VERSION_1_2;
}

static JNIEnv * JNU_GetEnv() {
  JNIEnv *env;
  jint rc = (*cached_jvm)->GetEnv(cached_jvm, (void **)&env, JNI_VERSION_1_2);
  if (rc == JNI_EDETACHED)
     av_log(NULL, AV_LOG_ERROR, "current thread not attached\n");
  if (rc == JNI_EVERSION)
    av_log(NULL, AV_LOG_ERROR, "jni version not supported\n");
  return env;
}
%}