Archive for the ‘Programming’ Category

How to call/invoke external DLL library method/function from Java code?

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.
Hereunder you can analyse a simple Java program which uses a sample .dll. You can download my java code and .dll library sources later.

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

Downloads:

  • 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;
		}
	}

}

How to properly run java console application with a doubleclick (Windows)

You’ve developed a java application which runs fine in your IDE but now you want to show your application to the outer world. Running a java program by setting up windows command window and then typing java -jar myProgram.jar paramA paramB .. may be a bit confusing for an everyday user, so the simplier means to do it are quite desirable.

A simple way to achieve this is to use a batch (.bat) file.

Let’s assume you have a console java application which takes one parameter – a configuration filepath and name. You can create a simple script file which will run this application in console for you after a doubleclick.

You should:

1. Create a text file like this :

@echo off
 set jarpath="JavaApp.jar"
 java -jar %jarpath% %CD%\Config.txt
 PAUSE

%CD% is a pseudo-variable which holds the working directory. It’s useful when you eg. want to load a config file located in the same folder as your .bat file.
PAUSE displays a localized version of “Press any key to continue…” message.

2. Save it as .bat file.
3. The program is ready for double click run!

How to send data from OutputStream to a String in Java?

The solution is relatively simple but it got me a while until I’ve found a solution on the net.
So:
You have to use a ByteArrayOutputStream, a constructor of String and know the character encoding used by the method which you get your OutputStream from.
A little code:
ByteArrayOutputStream os = new ByteArrayOutputStream();
aClass.outputStreamMethod(os);
String aString = new String(os.toByteArray(),"UTF-8");
And that’s all:) Pay attention to the character encoding.