1、Java String 转 C++ String / C++ char*

//C++string 需要导入 #include <string>头文件
// jstring jValue 是从java层传入，或者通过javaBean对象的属性值获取
std::string strValue;
const char *cstr = env->GetStringUTFChars(jValue, NULL); //转成C++ char*
strValue = std::string(cstr);
jniEnv->ReleaseStringUTFChars(jValue, cstr);

// 例如 javabean 中有dataPath 属性，现获取javaBean对象的dataPath值，转成C++ string
#include <jni.h>
#include <string>
/*
** obj : java Bean 的对象
**/
std::string ShapeParamsJniData::GetDataPathValue(jobject obj) {

    int attach = 0;
    JNIEnv *jniEnv = GetAttachEnv(&attach);
    if (jniEnv == NULL) {
        return 0;
    }
    jstring jValue = static_cast<jstring>(jniEnv->GetObjectField(obj,ShapeJniData::dataPathFieldId));
    std::string strValue;
    if (jValue != nullptr) {
        const char *cstr = jniEnv->GetStringUTFChars(jValue, NULL); //转成C++ char*
        strValue = std::string(cstr);
        jniEnv->ReleaseStringUTFChars(jValue, cstr);
    }
    if (attach) {
        DetachEnv();
    }

    return strValue;
}

2、 java 浮点型数组 转 C++ 浮点型数组

/**
* dataObj 是java Bean对象，m_matsFieldId 是属性 float[] mats;
**/
jfloatArray matArr = static_cast<jfloatArray>(env->GetObjectField(dataObj, m_matsFieldId));
 
 jint matLen = env->GetArrayLength(matArr);
 //在C/C++中创建一个缓存区，然后将Java中的原始数组拷贝到缓冲区中去
//strData 是C++ 结构体的声明 ，strData.m_mat4 是结构体 float[]类型成员
 env->GetFloatArrayRegion(matArr,0,matLen,strData.m_mat4); 

3、 Java对象数组 -> C++ 结构体指针数组

// obj 是javaBean 对象，touchPointsFieldId是该对象的 类型是 object[]
jobjectArray pointArr = static_cast<jobjectArray>(env->GetObjectField(obj,touchPointsFieldId));

// 通过GetArrayLength方法获取数组大小
jsize size = env->GetArrayLength(pointArr); 

// pTouchPoint ，C++ 的结构体指针，
//注意：使用new申请的空间需要在方法外 delete[]
TouchPoint* pTouchPoint = new TouchPoint[size]; 

for (int i = 0; i < size; i++){
       // 通过 GetObjectArrayElement 获取元素
        jobject touchPointObj = env->GetObjectArrayElement(pointArr,i);
       // GetAllValue 方法是封装 转 属性 值的
        TouchPoint touchPoint = TouchPointJniData::GetAllValue(touchPointObj); 
        pTouchPoint[i] = touchPoint;
  }

4、 Java List -> C++ Map

extern "C"
JNIEXPORT void JNICALL
Java_com_engine_operate_edit_initListId(JNIEnv *env, jclass clazz) {
   
    jclass clz = env->FindClass("java/util/ArrayList");
    ListJniData::clz = static_cast<jclass>(env->NewGlobalRef(clz));

    ListJniData::listInitMethodId = env->GetMethodID(clz,"<init>", "()V");
    ListJniData::listAddMethodId = env->GetMethodID(clz,"add", "(Ljava/lang/Object;)Z");
    ListJniData::listGetMethodId = env->GetMethodID(clz,"get", "(I)Ljava/lang/Object;");
    ListJniData::listSizeMethodId = env->GetMethodID(clz,"size", "()I");
}

void SelectLimitJniData::GetTypeParamListValue(jobject obj, SelectLimit *selectLimit)  {

    int attach = 0;
    JNIEnv *env = GetAttachEnv(&attach);
    if (env == NULL){
        return;
    }
    map<PenType, float> minMaps;
    map<PenType, float> maxMaps;

    jobject listParams = env->GetObjectField(obj,SelectLimitJniData::penTypeParamListFieldId);
    if (listParams != nullptr){
        jint size = env->CallIntMethod(listParams,ListJniData::listSizeMethodId);
        for (int i = 0; i < size; i++){
           jobject param = env->CallObjectMethod(listParams,ListJniData::listGetMethodId,i);
            if (param == nullptr){
                continue;
            }
            jint type = env->GetIntField(param,SelectLimitJniData::penTypeFieldId);
            PenType penType = static_cast<PenType>(type);
            jfloat maxPenSize = env->GetFloatField(param,SelectLimitJniData::maxPenSizeFieldId);
            jfloat minPenSize = env->GetFloatField(param,SelectLimitJniData::minPenSizeFieldId);

            minMaps[penType] = minPenSize;
            maxMaps[penType] = maxPenSize;
        }
        selectLimit->minSizeMap = minMaps;
        selectLimit->maxSizeMap = maxMaps;
    }

    if (attach){
        DetachEnv();
    }
}