Very similar to including Java Strings:
The passed object, WebSiteVisit:

package helloworld;

/**
*
* @author thomas
*/
public class WebSiteVisit {
 /** URL of the web page that linked user to this one */
 public String refererURL;
 public String pageVisited;
 public int ipAddress;
 IPAddress ipAddr;

 /** Creates a new instance of WebSiteVisit */
 public WebSiteVisit() {
 }

 public WebSiteVisit(String referer, String page, int ip) {
     this.refererURL = referer;
     this.ipAddress = ip;
     this.pageVisited = page;
     this.ipAddr = new IPAddress(ip);
 }
}

The embedded object, IPAddress:

package helloworld;

/**
*
* @author thomas
*/
public class IPAddress {
  public int byte1, byte2, byte3, byte4;
  /** Creates a new instance of IPAddress */
  public IPAddress() {
  }
  public IPAddress(int address) {
      byte1 = (address & 0xff000000) >> 24;
      byte2 = (address & 0x00ff0000) >> 16;
      byte3 = (address & 0x0000ff00) >> 8;
      byte4 =  address & 0x000000ff;
  }

}

The C source to the dynamic library, including the IPAddress object access.

JNIEXPORT void JNICALL Java_helloworld_Main_nativePrintNumber
 (JNIEnv *env, jobject obj, jint ji)
{
   jfieldID fid;
   jint version;
   jclass c;
   c = (*env)->GetObjectClass(env, obj);
   fid = (*env)->GetFieldID(env, c, "version_number", "I");
   version = (*env)->GetIntField(env, obj, fid);
   (*env)->SetIntField(env, obj, fid, ji*ji);
   printf("::: %d, %dn", fid, version);
   hello_number(ji);
}
JNIEXPORT void JNICALL Java_helloworld_Main_nativePrintObject
 (JNIEnv *env, jobject ths, jobject wsv)
{
   jfieldID fid;
   jint ipAddress;
   jstring jPageVisited;
   char *pageVisited;
   jstring jRefererURL;
   char *refererURL;
   jclass c;
   jobject ipObject;
   jclass ipObjectClass;
   jint byte1, byte2, byte3, byte4;
  
   c = (*env)->GetObjectClass(env, wsv);

   .
   .
   .
  
   fid = (*env)->GetFieldID(env, c, "ipAddr", "Lhelloworld/IPAddress;");
   if(!fid) {
       return;
   }
   ipObject = (*env)->GetObjectField(env, wsv, fid);
   ipObjectClass = (*env)->GetObjectClass(env, ipObject);
  
   fid = (*env)->GetFieldID(env, ipObjectClass, "byte1", "I");
   byte1 = (*env)->GetIntField(env, ipObject, fid);
  
   fid = (*env)->GetFieldID(env, ipObjectClass, "byte2", "I");
   byte2 = (*env)->GetIntField(env, ipObject, fid);

   fid = (*env)->GetFieldID(env, ipObjectClass, "byte3", "I");
   byte3 = (*env)->GetIntField(env, ipObject, fid);
  
   fid = (*env)->GetFieldID(env, ipObjectClass, "byte4", "I");
   byte4 = (*env)->GetIntField(env, ipObject, fid);

   printf("wsv.ipAddr = %d.%d.%d.%dns", byte1, byte2, byte3, byte4);
}