JNI 文件遍历

1.引入#include "dirent.h"
2.相关结构体
（1）DIR

struct DIR {
        struct dirent ent;
        struct _WDIR *wdirp;
    };
    typedef struct DIR DIR;

（2）dirent

 struct dirent {
        long d_ino;     /* Always zero */
        unsigned short d_reclen;  /* Structure size */
        size_t d_namlen;    /* Length of name without \0 */
        int d_type;   /* File type */
        char d_name[PATH_MAX];     /* File name */
    };
    typedef struct dirent dirent;

2.c++遍历文件

void showAllFiles(string dir_name) {
    if (dir_name.empty()) {
        LOGE("dir_name is null !");
        return;
    }
    DIR *dir = opendir(dir_name.c_str());
    if (NULL == dir) {
        LOGE("Can not open dir,Check path or permission!");
        return;
    }
    dirent *file;
    while ((file = readdir(dir)) != NULL) {    //类似BufferedReader.readLine(),不过这里返回是dirnet*，不是string
        if (file->d_type == DT_DIR) {   //是目录则递归
            string filePath = dir_name + "/" + file->d_name;
            showAllFiles(filePath);
        } else {                            //是文件则输出文件名
            LOGE("filePath:%s%s", dir_name.c_str(), file->d_name);
        }
    }
    closedir(dir);   //opendir(char* dir)后要closedir(DIR *dir )
}

3.通过比较发现，使用JNI遍历速度特别快，我这里个人测试手机的 根目录"/storage/emulated/0"
JNI不到0.5秒，而JAVA用了3.5秒左右，怪不得招聘时一个个都优先会JNI NDK开发的，性能提升是真的好。
JAVA筛选目录下的所以txt文件

    fun showJavaAllFiels(path:String){
        var file: File = File(path)
        if(file.exists()){
            for(fileitm in file.listFiles()){
                if (fileitm.isFile){
                    if(fileitm.name.endsWith(".txt"))
                        Log.e("native",path+"/"+fileitm.name)
                }else{
                    showJavaAllFiels(path+"/"+fileitm.name)
                }
            }
        }else{
            Log.e("native","file is not exit!")
        }
    }

JNI筛选目录下的所以txt文件

void showAllFiles(string dir_name) {
    if (dir_name.empty()) {
        LOGE("dir_name is null !");
        return;
    }
    DIR *dir = opendir(dir_name.c_str());
    if (NULL == dir) {
        LOGE("Can not open dir,Check path or permission!");
        return;
    }
    dirent *file;
    while ((file = readdir(dir)) != NULL) {
        if (strcmp(file->d_name, ".") == 0 || strcmp(file->d_name, "..") == 0) {
          //  LOGE("ignore . and ..");
            continue;
        }
        if (file->d_type == DT_DIR) {
            string filePath = dir_name + "/" + file->d_name;
            showAllFiles(filePath);
        } else {
            string filename = file->d_name;
            int size = filename.size();
            if(filename.size()>3){
                string endName = filename.substr(size-4,size-1);
                int flag = strcmp(endName.c_str(),".txt");
                if(flag==0){
                    LOGE("filePath:%s/%s", dir_name.c_str(), file->d_name);
                }
            }


        }
    }
    closedir(dir);
}

4.读写txt文件

 fun readTxtFile(path:String){
        var file = File(path)
        if(file.exists()){
            try {
              //读
                //InputStream 是字节输入流的所有类的超类,一般我们使用它的子类,如FileInputStream等.
                //OutputStream是字节输出流的所有类的超类,一般我们使用它的子类,如FileOutputStream等.
                var fileInPutString = FileInputStream(file)
                //InputStreamReader 是字节流通向字符流的桥梁,它将字节流转换为字符流.
                //OutputStreamWriter是字符流通向字节流的桥梁，它将字符流转换为字节流.
                var inputStreamReader = InputStreamReader(fileInPutString)
                //BufferedReader从字符输入流中读取文本，缓冲各个字符
                //BufferedWriter将文本写入字符输出流,缓冲各个字符
                var bufferReader = BufferedReader(inputStreamReader)
                var txt:String =""
                var i= 0
                while (bufferReader.readLine()?.also { txt = it }!=null){
                    i++;
                    Log.e("txt",i.toString()+txt)
                }
                bufferReader.close()
                inputStreamReader.close()
                fileInPutString.close()
                //写
                var newFile = File("/storage/emulated/0/1/new.txt");
                var outPutStream = FileOutputStream(newFile)
                var outPutStreamWrite = OutputStreamWriter(outPutStream)
                var bufferWriter = BufferedWriter(outPutStreamWrite)
                 bufferWriter.write("我 在 write file txt")
                bufferWriter.close()
                outPutStreamWrite.close()
                outPutStream.close()
            }catch (e:IOException){
            }
        }
    }

5.also扩展函数是和let相同，函数体内使用it代表调用对象object，不同点是返回值是当前对象，let返回的是最后一行或指定的return表达式。?.also{}表示object不为null才会执行函数体。