将字符串的arraylist从Native java返回到JNI [英] Returning an arraylist of string from Native java to JNI

问题描述

           ArrayList<String> myArraylist;          

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

如何在JNI端获取上述方法中的每个项目并推送到向量并从JNI返回到JNI层中的其他CPP调用。

How to get the each items from the above method in JNI side and Push to vector and return from the JNI to other CPP calls in the JNI layer.

推荐答案

将ArrayList转换为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);
  }
}

将ArrayList从JNI推送回Java

如果不在JNI中修改此列表，那么最好的策略是在本机代码中的某处保留对它的全局引用。如果您稍微修改它，请保持此 jobject 始终保持最新（您可能需要方法 java_util_ArrayList_add 或 java_util_ArrayList_set ）。

If you don't modify this list in JNI, then the best strategy would be to simply keep a global reference to it somewhere in your native code. If you modify it a little, keep this jobject always up-to-date (you will probably need the methods java_util_ArrayList_add or java_util_ArrayList_set).

如果您选择从 scratch 向量重建列表，您将展开上述方法：

If you choose to reconstruct the list from scratch the vector, you will unwind the above method:

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;
}

无论如何，在使用Jni时要小心线程，始终附加在本机线程被销毁之前，你的本机线程和分离。

At any rate, be careful with threads when you work with Jni, always attach your native threads and detach before the native thread is destroyed.

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