jclass envcls = env->FindClass("android/os/Environment"); //获得类引用
 if (envcls == nullptr) return 0;
 
 //找到对应的类，该类是静态的返回值是File
 jmethodID id = env->GetStaticMethodID(envcls, "getExternalStorageDirectory", "()Ljava/io/File;");
 
 //调用上述id获得的方法，返回对象即File file=Enviroment.getExternalStorageDirectory()
 //其实就是通过Enviroment调用 getExternalStorageDirectory()
 jobject fileObj = env->CallStaticObjectMethod(envcls,id,"");
 
 //通过上述方法返回的对象创建一个引用即File对象
 jclass flieClass = env->GetObjectClass(fileObj); //或得类引用
 //在调用File对象的getPath()方法获取该方法的ID，返回值为String 参数为空
 jmethodID getpathId = env->GetMethodID(flieClass, "getPath", "()Ljava/lang/String;");
 //调用该方法及最终获得存储卡的根目录
 jstring pathStr = (jstring)env->CallObjectMethod(fileObj,getpathId,"");
 
 path = env->GetStringUTFChars(pathStr,NULL);