How To Call A Third Party DLL ?

Question

Hi All,
I do have a dll file written in C/C++. I need to call it from java.
I do have all the function declarations & descriptions, but not the source of the dll.
I want to call those methods in java. I know that I have to use JNI for this. But dont know how to do it ?
please guide me to do it .
Thanks in Advance.

Answer 1

Matthijs@ wrote:
Just speaking for myself:
It is a bad idea to learn C/C++ using JNI.I'm not trying to learn C++ using JNI. Instead, I have an existing DLL written in C++, and I want to call it from Java. Simple as that. Why do I want to call it from Java? Because I have a lot of existing Java code that I need to integrate with this DLL.
If you already know C/C++ then it should be rather obvious how to do what you stated.I don't know C++, so I guess it's not obvious.
To use JNI you need to know C or C++.
There are alternative wrappers (which actually use JNI dynamically) that have the goal of letting you do it without C/C++.
However the complexity of that comes from understanding that one is still interfacing with a native method which often has a C/C++ structure (figuratively but possibly literally as well.) As the complexity of the native API the chance that someone with no C/C++ experience figuring out the correct way to use it goes down.
You can research jnative (also called JNA) for this approach.
Not however that questions about JNA (or other wrappers) on this forum/site are unlikely to get answers. There might be other sites where that is different. On the other hand JNI will almost always get answers.
As a general programming concept JNI exists to provide a layer between disparate layers. Thus again it should be obvious that one will in fact need to write JNI code to access a native library.Sure. Never said I didn't expect to write JNI code. The thing that bothers me is that the one good resource I found on JNI spends I don't know how many pages explaining how to write the native code with method names like Java_Class_x etc. In my and the OP's scenario the DLL can't be modified, so the immediate question that comes to mind is: if the methods have to be called Java_etc, how can I make this work with my DLL? Of course, perhaps if you sit down and think about it, you might figure out that you can write C++ code that wraps the DLL, and then use JNI to call that code. However, this might not be so easy to figure out for people that are not programming wizards like yourself, and nonetheless have a need to integrate Java with C++. Although I am no wizard, I'm no beginner either (in Java, at least) but I didn't think of it until after reading the relevant chapter (I admit that I then had a slap-forehead "of course!" moment).
Again the point is that you must know C/C++ first.
From the "The Java Native Interface" book from Sun first page of the introduction
"The book assumes that you have basic knowledge of the Java, C and C++ programming languages..."
My point is: since integrating existing code of both languages should be the most common use case for JNI, why not provide an explanation about this scenario up front in the documentation? Something like: "if you have an existing DLL, you will need to write some C++ code that calls that DLL. In the following, we will explain how you can make the C++ code you write yourself callable from Java with JNI. Chapter 9 goes into more detail about how to write wrapper code for existing DLLs." Just so people that are not Einstein and are in a hurry can figure out where to go quickly.
And my point is that what you are stating was immediately obvious the first time I saw the JNI specification.
Anyone that tries to "go quickly" with C/C++ code is going to end up crashing the application (whether it is C/C++ code or JNI code.)
My second point is: the [Guide to Features/JNI|http://java.sun.com/j2se/1.5.0/docs/guide/jni/index.html] that ejp urged the OP to Google for is rubbish. Most people don't want a JNI specification, they want a tutorial. The [Java Native Interface tips|http://java.sun.com/products/jdk/faq/jnifaq.html] page is slightly less rubbish, but it still stinks. It doesn't explain anything. Try following the "resources on the Net" links. 3 out of 5 are broken or it is explicitly stated that they are outdated. The other 2 are decent, but again don't provide a straight statement on the most common scenario. Why doesn't either the guide to features or the tips page provide a link to the [JNI Tutorial and Programmer's Guide|http://java.sun.com/docs/books/jni/html/jniTOC.html]?
You would need to provide some actual numbers to support your contention as to what "most people" want.
Most developers never use JNI at all.
As for the spec being rubbish if a developer claimed experience in C and C++ and yet claimed they didn't understand the spec then myself I would question their contention that they actually knew C/C++.
For someone who doesn't know C/C++ then the first step is to learn those.
It would also be handy to have a basic idea of how shared libraries (on any OS) work and how to dynamically use such libraries (versus statically.)
My third point is: why be so rude to the OP almost from the start?Obviously the first question is why you think it is necessary that you need to comment on something that didn't involve you in the first place.
But other than that the posters on these forums answer a lot of questions and the forum is NOT for any of the following
1. Teaching someone how to program
2. Teaching someone how to ask questions
3. Teaching someone how to think logically.
4. Teaching someone how to design.
5. Teaching anyone about etiquette.
6. Anything at all except java.
So for example we do not nor should anyone expect us to teach a beginning programmer how to program in C/C++. And we need not be polite when we tell someone that the forum is not suited for those purposes.