jni进行文件读取

用jni进行文件读写

1、Java代码写native方法如：publicnative String getFromC(String path)；

2、生成头文件：在AndoridStudio的Termial选项下进入到java文件夹 

执行javah native方法所在类的全类名如：javah com.xreal.arsearch.arsearch.NativeDetector

3、将生成的头文件复制到工程的jni目录下（jni目录和jniLibs与java目录同级）

4、在.cpp文件中引入生成的头文件如：

#include"com_xreal_arsearch_arsearch_NativeDetector.h"

5、将头文件中的头文件粘贴到.cpp文件中如：

         JNIEXPORT jint JNICALLJava_com_xreal_arsearch_arsearch_NativeDetector_NativeFileClose
       (JNIEnv*, jobject, jint)；

         删除上面的红字部分

6、（1）JNI读写文件.cpp代码

#include "com_xreal_arsearch_arsearch_NativeDetector.h"
#include <unistd.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <stdlib.h>
#include <fcntl.h>

  char* jstringTostring(JNIEnv*env, jstring jstr)
  {
         char* rtn = NULL;
         jclass clsstring =env->FindClass("java/lang/String");
         jstring strencode =env->NewStringUTF("utf-8");
         jmethodID mid =env->GetMethodID(clsstring, "getBytes","(Ljava/lang/String;)[B");
         jbyteArray barr=(jbyteArray)env->CallObjectMethod(jstr, mid, strencode);
         jsize alen =env->GetArrayLength(barr);
         jbyte* ba =env->GetByteArrayElements(barr, JNI_FALSE);
         if (alen > 0)
         {
                rtn = (char*)malloc(alen+ 1);
                memcpy(rtn, ba, alen);
                rtn[alen] = 0;
         }
        env->ReleaseByteArrayElements(barr, ba, 0);
         return rtn;
  }

   int file_open(const char *filename,intflags)
  {
      int fd;

      fd = open(filename, flags, 0666);
      if (fd == -1)
          return -1;

      return fd;
  }

   int file_read(int fd, unsignedchar *buf, int size)
  {

      return read(fd, buf, size);
  }

   int file_close(int fd)
  {

      return close(fd);
  }

  jstring Java_com_xreal_arsearch_arsearch_NativeDetector_getFromC
   (JNIEnv * env, jobject jobj, jstringjstr){
         char*javaPath=jstringTostring(env,jstr);
         unsigned  char * buf;
         int fd=file_open(javaPath,O_RDONLY);
         int num=file_read(fd,buf,1024);
          return (env)->NewStringUTF(javaPath);

   };

    jint Java_com_xreal_arsearch_arsearch_NativeDetector_NativeFileOpen
     (JNIEnv *env, jobject jobj, jstringfilename, jint flags){
         // const char *filename_char =(env)->GetStringUTFChars(env,filename, NULL);
          char*javaPath=jstringTostring(env,filename);
           return file_open(javaPath,flags);
     }

   jint Java_com_xreal_arsearch_arsearch_NativeDetector_NativeFileRead
    (JNIEnv *env, jobject jobj, jint fd,jbyteArray buf, jint size){
           unsigned char *buf_char= (unsigned char*)((env)->GetByteArrayElements(buf, NULL));
          return file_read(fd,buf_char,  size);
     }

    jint Java_com_xreal_arsearch_arsearch_NativeDetector_NativeFileClose
       (JNIEnv *env, jobject jobj, jintfd){
           return file_close(fd);
       }

（2）java层代码

常用flags：
/*O_RDONLY以只读方式打开文件
O_WRONLY 以只写方式打开文件
O_RDWR 以可读写方式打开文件。上述三种旗标是互斥的，也就是不可同时使用，但可与下列的旗标利用OR(|)运算符组合。
O_CREAT 若欲打开的文件不存在则自动建立该文件。
O_TRUNC 若文件存在并且以可写的方式打开时，此标志位会令文件长度重新清为0，也就是说文件内容清空。
O_APPEND 当读写文件时会从文件尾开始移动，也就是所写入的数据会以附加的方式加入到文件后面。
O_NONBLOCK 以不可阻断的方式打开文件，也就是无论有无数据读取或等待，都会立即返回进程之中。
O_SYNC 以同步的方式打开文件。
O_NOFOLLOW 如果参数pathname所指的文件为一符号连接，则会令打开文件失败。
O_DIRECTORY 如果参数pathname所指的文件并非为一目录，则会令打开文件失败。*/

 

byte[]buf_write=new byte[1024];
byte[] buf_read=new byte[1024];
int fd;

 

int O_RDWR     =     02;
int O_CREAT    =    0100; /* not fcntl */

String path  = Constants.getFotonDirtory();
String fromC = NativeDetector.getInstance().getFromC(path +"test.txt");
//Toast.makeText(this,fromC,0).show();

fd = NativeDetector.getInstance().NativeFileOpen(fromC, O_CREAT  | O_RDWR);
System.out.println("fd_read---->" + fd);
buf_read = new byte[buf_write.length];
int ret_read = NativeDetector.getInstance().NativeFileRead(fd, buf_read,buf_write.length);

System.out.println("读出返回结果" + ret_read);
try {
    String gb2312 = new String(buf_read,"GB2312") + "";
    Toast.makeText(this,gb2312,Toast.LENGTH_SHORT).show();
}  catch (UnsupportedEncodingException e){
    e.printStackTrace();
}
NativeDetector.getInstance().NativeFileClose(fd);

(3)NativeDetector类

package com.xreal.arsearch.arsearch;import org.opencv.core.Mat;/** * Created by Administrator on 2016/8/4 0004. */public class NativeDetector {    static {        System.loadLibrary("JniTest");   //defaultConfig.ndk.moduleName    }    private static NativeDetector singleDetector = null;    public static NativeDetector getInstance() {        if (singleDetector == null) {            singleDetector = new NativeDetector();        }        return singleDetector;    }    public native boolean extractFeatures(long oImage, long descriptors);    public native String getFromC(String path);    public native int NativeFileOpen(String filename, int flags);    public native int NativeFileRead(int fd, byte[] buf, int sizes);    public native long NativeFileSeek(int fd, long Offset, int whence);    public native int NativeFileClose(int fd);}

（4）Application.mk

APP_STL := gnustl_staticAPP_CPPFLAGS := -frtti -fexceptionsAPP_ABI := armeabi armeabi-v7a

（5）Android.mk

LOCAL_PATH := $(call my-dir)include $(CLEAR_VARS)OpenCV_INSTALL_MODULES := onOpenCV_CAMERA_MODULES := offOPENCV_LIB_TYPE :=SHAREDifeq ("$(wildcard $(OPENCV_MK_PATH))","")include ..\..\..\..\native\jni\OpenCV.mkelseinclude $(OPENCV_MK_PATH)endifLOCAL_MODULE := JniTestLOCAL_SRC_FILES := \   E:\MrWangCode\arsearchdemochina\CarHelper-JNIDemo\arsearch\src\main\jni\JniTest.c \   E:\MrWangCode\arsearchdemochina\CarHelper-JNIDemo\arsearch\src\main\jni\NativeDetecor.cpp \   E:\MrWangCode\arsearchdemochina\CarHelper-JNIDemo\arsearch\src\main\jni\PatternDetector.cpp \   E:\MrWangCode\arsearchdemochina\CarHelper-JNIDemo\arsearch\src\main\jni\read.cpp \LOCAL_LDLIBS +=  -lm -lloginclude $(BUILD_SHARED_LIBRARY)