jni有关知识点总结
一、c/c++语言：
1.关于动态链接库的搜索问题：
LIBRARY_PATH is used by gcc before compilation to search for directories containing libraries that need to be linked to your program.
LD_LIBRARY_PATH is used by your program to search for directories containing the libraries after it has been successfully compiled and linked.
二、java中的数据类型与c语言中的数据类型转化问题
1.java中的long类型为8个字节而c语言中的long类型为4个字节，所以应当在java中使用int来对应c语言中的long或者unsigned long类型
下面给一个简单示例：
1 JNIEXPORT jint JNICALL example 2 (JNIEnv * env, jclass obj, jstring address, jshort port, 3 jstring username, jstring password, jlong channel, 4 jintArray start_time, jintArray end_time, jstring path){ 5 6 const char* addr = env->GetStringUTFChars(address, NULL); 7 const char* uname = env->GetStringUTFChars(username, NULL); 8 const char* pwd = env->GetStringUTFChars(password, NULL); 9 char* p = (char*)env->GetStringUTFChars(path, NULL); 10 jint* start = env->GetIntArrayElements(start_time, NULL); 11 jint* end = env->GetIntArrayElements(end_time, NULL); 12 int code = download(addr, (WORD)port, uname, pwd, (DWORD)channel, (DWORD*)start, (DWORD*)end, p); 13 env->ReleaseStringUTFChars(address, addr); 14 env->ReleaseStringUTFChars(username, uname); 15 env->ReleaseStringUTFChars(password, pwd); 16 env->ReleaseStringUTFChars(path, p); 17 env->ReleaseIntArrayElements(start_time, start, 0); 18 env->ReleaseIntArrayElements(end_time, end, 0); 19 20 return code; 21 }
上例中的DWORD就是unsigned long的别名。我们使用的是jintArray.
三、jni加载动态库在windows和linux平台上的不同规则
对于System.loadLibrary("nvr");在windows上加载的是nvr.dll文件而在linux上加载的是libnvr.so文件
四、对于jni方法中返回值为jintArray类型的处理方法
// 新建一个数组
jintArray arr = env->NewIntArray(2);
// 获取数组第一个元素的地址
jint* el = env->GetIntArrayElements(arr, NULL);
// 给数组中的元素赋值
*el = result[0];
*(el+1) = result[1];
最后把数组返回即可return arr;
