JNI 数据类型用法

1.数组array类型

JNIEXPORT float JNICALL Java_com_example_demo_DEMO_sample(JNIEnv * env,jobject,jintArray data)

jint *intdata；
intdata= env->GetIntArrayElements(data, NULL);
if (intdata == NULL) {
return 0;
}
int len_data = env->GetArrayLength(data);


....

env->ReleaseIntArrayElements(data, intdata, 0);

2.string类型

JNIEXPORT int JNICALL Java_com_example_demo_DEMO_sample(JNIEnv* env, jobject, jstring Path)
const char *nativeString = env->GetStringUTFChars(Path, 0);
...		
env->ReleaseStringUTFChars(Path, nativeString);
LOGD("Method Description: %s", "loaded haar files...");

3.NIO 获取地址

c++:
JNIEXPORT float JNICALL Java_com_example_demo_DEMO_sample(JNIEnv * env,jobject instance,object  directBuffer,jint len_array)
int len_data=len_array;

//    unsigned double* buff;
//    buff = (unsigned char*) env->GetDirectBufferAddress(directBuffer);
unsigned char * buff= (unsigned char*)env->GetDirectBufferAddress(directBuffer);


java:
java函数参数：sample（ByteBuffer buffer）;
....
ByteBuffer buffer = ByteBuffer .allocaterDirect(1024);
String str="iiiiiiiiii";
buffer.put(str.getBytes());
this.sample(buffer);

一定要记得jobject instance，上次就是因为这个，找半天bug

4.新建byte缓存

unsigned char* buffer = (unsigned char*) malloc(1024);
jobject directBuffer;
directBuffer = env->NewDirectByteBuffer(buffer, 1024);

调用 Java 方法所需参数 : 调用 Java 方法需要 JNIEnv *env 参数 和 对应的 jobject instance Java 类参数 ;

① JNIEnv *env : JNI 环境 , 注意子线程的 JNI 环境需要获取 , 主线程的 JNI 环境可以直接从 Native 层实现的 Java 方法中获取 ;

② jobject instance : 在 Native 层的 Java 对象 ;

参考
https://www.dynamsoft.com/codepool/android-ndk-optimize-camera2-barcode.html
https://code.i-harness.com/ja-jp/q/acdfef
https://www.itread01.com/p/3467.html
https://ocw.cs.pub.ro/courses/_media/osp/lectures/lecture-ndk-2.pdf
https://blog.csdn.net/shulianghan/article/details/104644627