最近工作需要利用到jni开发, 关于返回值这块内容, 稍微有点啰嗦, 结合自己查询的资料和开发的经验, 稍微整理了下.
文章内容分为两块: 简单返回值 和 复杂返回值
内容一. 简单返回值
指的是返回单一基本数据类型的数据.
1. 返回一个值
只需要知道Java基本数据类型与JNI数据类型的映射关系就可以了. 参照下表:
图来源: https://blog.csdn.net/u011974987/article/details/52743495
在jni里, jni的数据类型和C/C++的数据类型可以互相赋值(参考下面代码)
...
double sksk = 10.0;
jdouble rst = 0;
rst = sksk;
return rst;
知道了对应关系, 我们返回对应类型即可, 也可以直接返回值.
`return 1.0; // 要求返回一个jdouble`
`return true; // 要求返回一个jboolean`
是不是感觉so easy?
2. 返回一个字符串
C/C++里面的字符串本质是字符数组, 所以处理是不一样的.
如果字符串内存是动态申请的, 别忘了释放.
char* tmpstr = new char[100];
sprintf(tmpstr , "%s", "this is a test"); // c语言中的字符串
jstring rtstr = env->NewStringUTF(tmpstr); // 转换成jni的jstring
delete[] tmpstr;
return rtstr; // 返回
3. 返回数组: 如果你需要返回多个值, 且是同种类型, 那么考虑下返回一个数组.
double output[3]; // 输出数据
...
jdoubleArray result = env->NewDoubleArray(3); // 返回用数组开辟内存
env->SetDoubleArrayRegion(result, 0, 3, output); // 把输出数据赋值进返回用数组里
return result; // 返回
jni里面每个类型的数组都是一个单独的类型, 比如有jdoubleArray, jbyteArray...
要返回一个数组, 只需要用对应的NewXXXArray函数和SetXXXArrayRegion函数初始化一个数组返回即可.
内容二. 复杂返回值, 返回一个类或者结构体
如果要返回好几个类型的值, 且数据类型各不相同, 这时候是不是想要是可以返回一个 "类 / 结构体" 就好了?
没问题, 继续往下看你就会了.
- 首先, 在.java文件中定义一个类, 用于返回数据.
public class ProcessResult {
public int width;
public int height;
public double H;
public double S;
public double V;
public double[] avgCon;
public int pixelsDataLength;
public byte[] pixelsData;
public ProcessResult() {
width = 0;
height = 0;
}
}
- .cpp的native代码里, 先要找到java里对应的类
jclass rstClass = env->FindClass("com/example/jfznkj/jianshu/ProcessResult"); // 搜索类
jmethodID initFunID = env->GetMethodID(rstClass, "<init>", "()V"); // 获取构造函数
jobject result = env->NewObject(rstClass, initFunID); // 创建一个类的实例
- 找到类后对类的各个字段赋值
jfieldID temp = env->GetFieldID(rstClass, "width", "I");
env->SetIntField(result, temp, width);
temp = env->GetFieldID(rstClass, "height", "I");
env->SetIntField(result, temp, height);
temp = env->GetFieldID(rstClass, "H", "D");
env->SetDoubleField(result, temp, tempH);
temp = env->GetFieldID(rstClass, "S", "D");
env->SetDoubleField(result, temp, tempS);
temp = env->GetFieldID(rstClass, "V", "D");
env->SetDoubleField(result, temp, tempV);
jdoubleArray avgArray = env->NewDoubleArray(25);
env->SetDoubleArrayRegion(avgArray, 0, 25, avg);
temp = env->GetFieldID(rstClass, "avgCon", "[D");
env->SetObjectField(result, temp, avgArray);
temp = env->GetFieldID(rstClass, "pixelsDataLength", "I");
env->SetIntField(result, temp, picDataLength);
jbyteArray picArray = env->NewByteArray(picDataLength);
env->SetByteArrayRegion(picArray, 0, picDataLength, picData);
temp = env->GetFieldID(rstClass, "pixelsData", "[B");
env->SetObjectField(result, temp, picArray);
return result ;
简单的总结, 先搜索到字段地址, 再往里赋准备好的jni数据, 数据如何准备参照 [内容一]
实例各个字段都初始化完毕, 就可以返回一个类拉.
对了, 不管什么类, 都对应jni里都是jobject类型.
最后贴一个我函数的完整代码
// 该函数包含所有的图片处理
// 1. 图片画素数组取出
// 2. hsv计算
// 3. 灰度计算
// 4. 平均浓度计算
// 5. nv21数据类型转换
extern "C" JNIEXPORT jobject JNICALL Java_com_example_jfznkj_jianshu_MainActivity_MuptiplePictureProcessingForSpeedUp
(JNIEnv *env, jobject jobj, jbyteArray data_, jint width, jint height){
jbyte * data = env->GetByteArrayElements(data_, NULL);
// 耀哥灰度
int greyThreshold = 800; //灰度阈值
int diffThreshold = 8; //差值阈值
int startX = width / 4;
int endX = width * 3 / 4;
int startY = height / 4;
int endY = height * 3 / 4;
int grey;
int maxDiff; //r g b 中最大差值
int sumNum = 0; //灰度大于greyThreshold的像素点总数
int tarNum = 0; //r g b 最大差值小于diffThreshold的像素点数;
// hsv计算
double dblH = 0.0, dblS = 0.0, dblV = 0.0;
int colorHArr[361] = {0};
// 浓度计算
int avgConLength = 25;
int avgConStartPos = 6;
double avgConSum[25] = {0.0};
int avgConCnt[25] = {0};
int avgBlockW = width / 5;
int avgBlockH = height / 5;
double avg[25];
int allConLength = 9;
int allConStartPos = avgConStartPos + avgConLength + 1;
double allConSum[9] = {0.0};
int allConCnt[9] = {0};
int allBlockW = width / 3;
int allBlockH = height / 3;
double all[9];
// NV21转换
int nv21Length = width * height * 3 / 2;
jbyte* nvData = new jbyte[nv21Length];
// 图像数据
int picDataLength = width * height * 3;
jbyte* picData = new jbyte[picDataLength];
int tempPicDataIdx = 0;
// 循环
jbyte* pNV12 = nvData;
for (int y = 0; y < height; y++) {
// nv21转换
jbyte* pY = pNV12 + y * width;
jbyte*pCrCb = pNV12 + (width * height) + (y >> 1) * width;
for (int x = 0; x < width; x++) {
//unsigned char colorB = pixels[y * step + x * chan + 0];
//unsigned char colorG = pixels[y * step + x * chan + 1];
//unsigned char colorR = pixels[y * step + x * chan + 2];
unsigned char colorB = data[y * width * 3 + x * 3 + 0];
unsigned char colorG = data[y * width * 3 + x * 3 + 1];
unsigned char colorR = data[y * width * 3 + x * 3 + 2];
// 图片画素数组取出
picData[tempPicDataIdx] = colorB; tempPicDataIdx++;
picData[tempPicDataIdx] = colorG; tempPicDataIdx++;
picData[tempPicDataIdx] = colorR; tempPicDataIdx++;
// hsv
if (y % 3 == 0 && x % 3 == 0) {
double B = (double) colorB;
double G = (double) colorG;
double R = (double) colorR;
double _max = MAXHSV(B, G, R);
double _min = MINHSV(B, G, R);
dblV += _max;
if (std::abs(_max - _min) <= 0.000001) {
dblS += 0;
} else {
dblS += (_max - _min) / _max * 100.0;
}
if (std::abs(_max - _min) <= 0.000001) {
dblH = 0.0;
} else {
if (std::abs(R - _max) <= 0.000001 && G >= B)
dblH = (G - B) / (_max - _min) * 60;
else dblH = (G - B) / (_max - _min) * 60 + 360;
if (std::abs(G - _max) <= 0.000001) dblH = 120 + (B - R) / (_max - _min) * 60;
if (std::abs(B - _max) <= 0.000001) dblH = 240 + (R - G) / (_max - _min) * 60;
}
int nH = (int) dblH;
colorHArr[nH]++;
}
// 浓度计算
int tempAvgIdx = y / avgBlockH * 5 + x / avgBlockW;
avgConSum[tempAvgIdx] += (colorB + colorG + colorR);
avgConCnt[tempAvgIdx] ++;
int tempAllIdx = y / allBlockH * 3 + x / allBlockW;
allConSum[tempAllIdx] += (colorB + colorG + colorR);
allConCnt[tempAllIdx] ++;
// nv21转换
int Y;
int Cb;
int Cr;
Y = (1225 * colorR + 2404 * colorG + 467 * colorB) >> 12;
*pY = NV21_CLAMP_INT32_8(Y);
pY++;
if ((x & 0x00000001) == 0x00000000) {
Cr = (524288 + 2048 * colorR - 1715 * colorG - 333 * colorB) >> 12;
Cb = (524288 - 691 * colorR - 1357 * colorG + 2048 * colorB) >> 12;
*pCrCb = NV21_CLAMP_INT32_8(Cr);
pCrCb++;
*pCrCb = NV21_CLAMP_INT32_8(Cb);
pCrCb++;
}
// 耀哥灰度
if ( x >= startX && x < endX && y >= startY && y < endY && y % 2 == 0 && x % 2 == 0) {
grey = 3 * colorR + 6 * colorG + colorB;//(int)(0.30 * R + 0.59 * G + 0.11 * B);
if (grey > greyThreshold) {
sumNum++;
maxDiff = abs(colorR - colorG);
if (maxDiff > diffThreshold) continue;
maxDiff = abs(colorR - colorB);
if (maxDiff > diffThreshold) continue;
maxDiff = abs(colorB - colorG);
if (maxDiff > diffThreshold) continue;
tarNum++;
}
}
}
}
// 耀哥灰度
double tempGreyValue = 0.0;
if(sumNum > 10) tempGreyValue = (float)tarNum / (float)sumNum;
// hsv
int average = 0;
int hsvCnt = 0;
for (int i = 0; i < 361; i++) {
average += colorHArr[i] * i;
hsvCnt += colorHArr[i];
}
average = average / hsvCnt;
double variance = 0.0;
for (int i = 0; i < 361; i++) {
variance += (i - average) * (i - average) * colorHArr[i];
}
double tempH = variance / hsvCnt; // H
double tempS = dblS / hsvCnt; // S
double tempV = dblV / hsvCnt; // V
// 浓度计算
for (int i = 0; i < avgConLength; i++) {
avg[i] = avgConSum[i] / (double)avgConCnt[i];
}
for (int i = 0; i < allConLength; i++) {
all[i] = allConSum[i] / (double)allConCnt[i];
}
env->ReleaseByteArrayElements(data_, data, 0);
jclass rstClass = env->FindClass("com/example/jfznkj/jianshu/ProcessResult");
jmethodID initFunID = env->GetMethodID(rstClass, "<init>", "()V");
jobject result = env->NewObject(rstClass, initFunID);
jfieldID temp = env->GetFieldID(rstClass, "width", "I");
env->SetIntField(result, temp, width);
temp = env->GetFieldID(rstClass, "height", "I");
env->SetIntField(result, temp, height);
temp = env->GetFieldID(rstClass, "grey", "D");
env->SetDoubleField(result, temp, tempGreyValue);
temp = env->GetFieldID(rstClass, "H", "D");
env->SetDoubleField(result, temp, tempH);
temp = env->GetFieldID(rstClass, "S", "D");
env->SetDoubleField(result, temp, tempS);
temp = env->GetFieldID(rstClass, "V", "D");
env->SetDoubleField(result, temp, tempV);
jdoubleArray avgArray = env->NewDoubleArray(25);
env->SetDoubleArrayRegion(avgArray, 0, 25, avg);
temp = env->GetFieldID(rstClass, "avgCon", "[D");
env->SetObjectField(result, temp, avgArray);
jdoubleArray allArray = env->NewDoubleArray(9);
env->SetDoubleArrayRegion(allArray, 0, 9, all);
temp = env->GetFieldID(rstClass, "allCon", "[D");
env->SetObjectField(result, temp, allArray);
temp = env->GetFieldID(rstClass, "NV21DataLength", "I");
env->SetIntField(result, temp, nv21Length);
jbyteArray nvArray = env->NewByteArray(nv21Length);
env->SetByteArrayRegion(nvArray, 0, nv21Length, nvData);
temp = env->GetFieldID(rstClass, "NV21Data", "[B");
env->SetObjectField(result, temp, nvArray);
temp = env->GetFieldID(rstClass, "pixelsDataLength", "I");
env->SetIntField(result, temp, picDataLength);
jbyteArray picArray = env->NewByteArray(picDataLength);
env->SetByteArrayRegion(picArray, 0, picDataLength, picData);
temp = env->GetFieldID(rstClass, "pixelsData", "[B");
env->SetObjectField(result, temp, picArray);
delete[] nvData;
delete[] picData;
return result;
}
关于jni返回值, 就介绍到这里.
