what's this error???C++ to java ,help!!!!

//this's jni function in dll;

JNIEXPORT jstring JNICALL Java_vc_CallVc_getActiveWindowTitle//get title
(JNIEnv * jn, jclass)
{
             jstring str;
            const char *strTitle     =GetStringUTFChars( str , 0);
           ::GetWindowText(::GetForegroundWindow(),(LPSTR)strTitle,MAX_PATH);
             char  srtMsg[MAX_PATH];
            sprintf(srtMsg,this dialog title is:%s,strTitle);
            AfxMessageBox(strTitle);
            jn->ReleaseStringChars(str,0);
            return  (jstring)strTitle;//error  ???
}

//me demo dialog function for vc++2005


void CJniSparkDlg::OnBnClickedCancel6()
{
    // TODO: Add your control notification handler code here
    JNIEnv *a;
    jclass c;
    Java_vc_CallVc_screenCameraShot(a,c);
}