Return arraylist string from native java to jni

Question

           ArrayList<String> myArraylist;          

           public ArrayList<String> getData(){
               myArraylist = new ArrayList<String>();          
               myArraylist.add("1267982563");
               myArraylist.add("2345678");
               myArraylist.add("5432789");
               return myArraylist;
           }

How to get each element from the above method in the JNI side and click on the vector and return from JNI to other CPP calls in the JNI layer.

Answer 1

Convert ArrayList to std :: vector <std :: string>:

static jclass java_util_ArrayList;
static jmethodID java_util_ArrayList_;
jmethodID java_util_ArrayList_size;
jmethodID java_util_ArrayList_get;
jmethodID java_util_ArrayList_add;
static thread_local JNIEnv* env;

void init() {
  java_util_ArrayList      = static_cast<jclass>(env->NewGlobalRef(env->FindClass("java/util/ArrayList")));
  java_util_ArrayList_     = env->GetMethodID(java_util_ArrayList, "<init>", "(I)V");
  java_util_ArrayList_size = env->GetMethodID (java_util_ArrayList, "size", "()I");
  java_util_ArrayList_get  = env->GetMethodID(java_util_ArrayList, "get", "(I)Ljava/lang/Object;");
  java_util_ArrayList_add  = env->GetMethodID(java_util_ArrayList, "add", "(Ljava/lang/Object;)Z");
}

std::vector<std::string> java2cpp(jobject arrayList) {
  jint len = env->CallIntMethod(arrayList, java_util_ArrayList_size);
  std::vector<std::string> result;
  result.reserve(len);
  for (jint i=0; i<len; i++) {
    jstring element = static_cast<jstring>(env->CallObjectMethod(arrayList, java_util_ArrayList_get, i));
    const char* pchars = env->GetStringUTFChars(element, nullptr);
    result.emplace_back(pchars);
    env->ReleaseStringUTFChars(element, pchars);
    env->DeleteLocalRef(element);
  }
}

Push ArrayList from JNI back to Java

JNI, - . , jobject (, , java_util_ArrayList_add java_util_ArrayList_set).

scratch , :

jobject cpp2java(std::vector<std::string> vector) {
  jobject result = env->NewObject(java_util_ArrayList, java_util_ArrayList_, vector.size());
  for (std::string s: vector) {
    jstring element = env->NewStringUTF(s.c_str());
    env->CallBooleanMethod(result, java_util_ArrayList_add, element);
    env->DeleteLocalRef(element);
  }
  return result;
}

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