Introduction
Java’s JVM allows you to do many smart things but sometimes you may be forced to directly use external library or writing code in pure java would be very time-consuming comparing it with something more low-leveled. I stumbled upon such a problem and had to use a .dll driver from a hardware provider. Of course I googled it and didn’t find much info, and what I’ve found mostly concerned about JNI (Java Native Interface). I would like to tell you about JNA which is fast and simple to use in aforementioned task.
So, how does it work? The only thing you have to do is to
download and import JNA (Java Native Access) to your project and write a simple class.
Java code:
package jnahelloworldtest;
import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.NativeLong;
import com.sun.jna.Platform;
import com.sun.jna.*;
/** Simple example of native library declaration and usage. */
public class Main {
public interface simpleDLL extends Library {
simpleDLL INSTANCE = (simpleDLL) Native.loadLibrary(
(Platform.isWindows() ? "simpleDLL" : "simpleDLLLinuxPort"), simpleDLL.class);
// it's possible to check the platform on which program runs, for example purposes we assume that there's a linux port of the library (it's not attached to the downloadable project)
byte giveVoidPtrGetChar(Pointer param); // char giveVoidPtrGetChar(void* param);
int giveVoidPtrGetInt(Pointer param); //int giveVoidPtrGetInt(void* param);
int giveIntGetInt(int a); // int giveIntGetInt(int a);
void simpleCall(); // void simpleCall();
}
public static void main(String[] args) {
simpleDLL sdll = simpleDLL.INSTANCE;
sdll.simpleCall(); // call of void function
int a = 3;
int result1 = sdll.giveIntGetInt(a); // calling function with int parameter&result
System.out.println("giveIntGetInt("+a+"): " + result1);
String testStr = "ToBeOrNotToBe";
Memory mTest = new Memory(testStr.length()+1); // '+1' remember about extra byte for \0 character!
mTest.setString(0, testStr);
String testReturn = mTest.getString(0); // you can see that String got properly stored in Memory object
System.out.println("String in Memory:"+testReturn);
Memory intMem = new Memory(4); // allocating space
intMem.setInt(0, 666); // setting allocated memory to an integer
Pointer intPointer = intMem.getPointer(0);
int int1 = sdll.giveVoidPtrGetInt(Pointer.NULL); // passing null, getting default result
System.out.println("giveVoidPtrGetInt(null):" + int1); // passing int stored in Memory object, getting it back
int int2 = sdll.giveVoidPtrGetInt(intMem);
//int int2 = sdll.giveVoidPtrGetInt(intPointer); causes JVM crash, use memory object directly!
System.out.println("giveVoidPtrGetInt(666):" + int2);
byte char1 = sdll.giveVoidPtrGetChar(Pointer.NULL); // passing null, getting default result
byte char2 = sdll.giveVoidPtrGetChar(mTest); // passing string stored in Memory object, getting first letter
System.out.println("giveVoidPtrGetChar(null):" + (char)char1);
System.out.println("giveVoidPtrGetChar('ToBeOrNotToBe'):" + (char)char2);
}
}
Mappings table
Not every native type maps directly to Java type, furthermore a class representing pointers had to be introduced. You can read a handy type mapping table below:
| Native Type |
Size |
Java Language Type |
Common Windows Types |
| char |
8-bit integer |
byte |
BYTE, TCHAR |
| short |
16-bit |
short |
short WORD |
| wchar_t |
16/32-bit character |
char |
WCHAR, TCHAR |
| int |
32-bit integer |
int |
DWORD |
| int |
boolean value |
boolean |
BOOL |
| long |
32/64-bit integer |
NativeLong |
LONG |
| long long, __int64 |
64-bit integer |
long |
|
| float |
32-bit FP |
float |
|
| double |
64-bit FP |
double |
|
| char* |
C string |
String |
LPTCSTR |
| void* |
pointer |
Pointer |
LPVOID, HANDLE, LPXXX |
- DLLVisual Studio source project – ready to use Visual Studio 2010 project. You can alter it to continue experimenting with invoking DLL methods of library attached to the Netbeans project.
- Netbeans project – ready to use NetBeans 6 project using example code explained in this article with appropriate jars and DLL library attached
- JNA library jars – link to official JNA libraries download page
Useful links:
DLL library C++ functions source :
#include "simpleDLL.h"
#include <stdexcept>
using namespace std;
namespace simpleDLLNS
{
char simpleDLL::giveVoidPtrGetChar(void* param)
{
if(param != 0)
{
char* paramChrPtr = (char*)param;
return *paramChrPtr;
}
else
{
return 'x';
}
}
int simpleDLL::giveIntGetInt(int a)
{
return 2*a;
}
void simpleDLL::simpleCall(void)
{
int x = 3;
return;
}
int simpleDLL::giveVoidPtrGetInt(void* param)
{
if(param!=0)
{
int* x = (int*)param;
return *x;
}
else
{
return -1;
}
}
}