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：

JNIEXPORT jint JNICALL example
  (JNIEnv * env, jclass obj, jstring address, jshort port, 
  jstring username, jstring password, jlong channel, 
  jintArray start_time, jintArray end_time, jstring path){

      const char* addr = env->GetStringUTFChars(address, NULL);
      const char* uname = env->GetStringUTFChars(username, NULL);
      const char* pwd = env->GetStringUTFChars(password, NULL);
      char* p = (char*)env->GetStringUTFChars(path, NULL);
      jint* start = env->GetIntArrayElements(start_time, NULL);
      jint* end = env->GetIntArrayElements(end_time, NULL);
      int code = download(addr, (WORD)port, uname, pwd, (DWORD)channel, (DWORD*)start, (DWORD*)end, p);
        env->ReleaseStringUTFChars(address, addr);
        env->ReleaseStringUTFChars(username, uname);
        env->ReleaseStringUTFChars(password, pwd);
        env->ReleaseStringUTFChars(path, p);
        env->ReleaseIntArrayElements(start_time, start, 0);
        env->ReleaseIntArrayElements(end_time, end, 0);

        return code;
  }

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;