I am able to call C++ code from Java using SWIG but I can't find any documentation on how to do the reverse (call Java from C++).
The official SWIG documentation says (http://www.swig.org/Doc1.3/Java.html#java_overview):
"SWIG enables a Java program to easily call into C/C++ code from Java. Historically, SWIG was not able to generate any code to call into Java code from C++. However, SWIG now supports full cross language polymorphism and code is generated to call up from C++ to Java when wrapping C++ virtual methods."
But I can't find where it says how to do it! Any help is greatly appreciated. Thank you.
Actually this is possible and I have implemented it based on inheriting a virtual class from C++ to Java.
You can find solution in chapter "24.5 Cross language polymorphism using directors" of Java part of SWIG document.
Based on the rather strange wording of the statement I'd say what you want isn't generally possible. "...call up from C++ to Java when wrapping C++ virtual methods." The "...call up..." leads me to believe you're calling protected or public members of an inherited interface and, "...when wrapping C++ virtual methods," leads me to conclude you can only do so when you're overriding an inherited interface. So it sounds like the actual use case is very narrow.
But then, I'm just basing this on the wording of the text you've pasted. It may or may not help you.
There is a tiny official example on calling Java callbacks from C++. Works well.
Your C++ callback class better be non-abstract, you gonna loose default constructor in Java otherwise.
Related
Is it possible to call matlab function in java, if so, How can I use it.
For example there is a function in matlab called scatter3 and plotter3
How can I use them in Java.
Thank you
There are undocumented ways to call MATLAB from Java. You can learn a lot more about them from #YairAltman 's blog Undocumented MATLAB. Some of these methods are captured in the matlabcontrol package.
Since the functionality is undocumented and subject to change (and since matlabcontrol is not updated frequently), you should not rely on this ability for anything critical.
I have a .dll that was provided, and I need to interface with it using java.
Do I have to rewrite the C to use the JNI types provided in jni.h? as seen in java sun's jni example.
Otherwise, How do i declare the native function my java function to pass and receive pointers?
thanks
You may find JNA useful
"JNA provides Java programs easy access to native shared libraries (DLLs on Windows) without writing anything but Java code—no JNI or native code is required. This functionality is comparable to Windows' Platform/Invoke and Python's ctypes. Access is dynamic at runtime without code generation."
In a way, yes.
Most of the time you would just write a small "wrapper" dll with functions that would do the type conversions and delegate to the corresponding functions in the "real" DLL.
You can rewrite the C code of course, but it's not uncommon to write a wrapper (technically, using the Facade or Bridge pattern) for the C code. You write code that matches your expectations in the Java, and have that code call your existing C code.
Is there a way to call STL libraries from JNI, I believe JNI provides a C like interface for native calls, how do we achieve this for the C++ template libraries?
I agree that if you're looking for just the plain STL, you could probably use a Java library instead. However, if you insist on wrapping STL, SWIG provides some STL wrapping in JNI out of the box (see this for the basic mechanism), which should produce relatively stable, tested code.
I know this is possible, although I haven't done it myself. But JNI code can crash the JVM if you are not VERY CAREFUL. Also, JNI code is much more difficult to maintain than Java code. I was on one project where the Java -> JNI -> STL and COM code was thrown away and we replaced it with C# accessing the same STL and COM and a socket. We never looked back.
If you are doing only a small amount of JNI, it may be worth it. If you are creating a large interface via JNI, I strongly recommend instead writing the component that interacts with STL in C# instead of Java, and using a socket to communicate between the C# and Java components. It will be far easier to write, test, and maintain.
I have to ask why you find it necessary to use the STL libraries from Java. Is there something in them that's not provided by Java's own massive set of libraries?
I understand why you'd have to call your own code (and I'm assuming you've profiled the Java code and found it wanting, otherwise your effort is wasted) but you seem to be asking how to call STL stuff directly.
We've often had trouble with JNI and it makes our code less portable, so you have to have a very good reason for wanting to use it in our shop, and you have to prove with hard data, why the Java-supplied stuff is not adequate.
Yes. You required to use the swig typemap to convert C++ STL classes to Java objects... and vice versa.
For more information, please visit,
http://www.swig.org/Doc2.0/Android.html
Does anyone know of a way to issue commands to a hard drive within Java? Does Java even support this kind of hardware interaction?
For example, if I have a SCSI hard drive that I would like to inquiry, is there a pre-existing Java method to do this, or would I have to write my own?
http://en.wikipedia.org/wiki/SCSI has some general information on SCSI commands in case you aren't familiar.
Java doesn't support talking directly to hardware like that. However, you can use JNI to call a C/C++ function from Java that can.
Three words "JNI or JNA". I strongly recommend taking a look at the latter to see if it suits your situation, instead of just opting for JNI.
No, since Java runs in a "virtual" machine rather than a real one. But it could be used as a bridge as dj mentioned earlier using JNI.
According to Wikipedia JNI can also call assembly directly. JNI could be used to call complete programs written in C or C++
you need to write the HDD interface code in C/C++ and then call that from Java using JNI
Has anyone successfully "wrapped up" a C++ API in Java or .NET? I have an application that provides a C++ API for writing plug-ins. What I'd like to do is access that API from .NET or Java.
Would I need to use COM, or are there simpler/better alternatives?
If you have a very straight forward method signatures (i.e. methods that takes and returns primitive types such as int, char[], void* ... etc), it is reasonably easy to do so in .NET and still possible but a bit harder in Java with JNI.
However, if your class methods uses modern C++ programming techniques such as boost shared pointers and STL containers then it is a very different story. You will need to be really careful with memory management in this case.
EDIT:
It is gonna be even more interesting if the method has C++ template arguments because C++ template system is very different from C# or Java generics in that it is only a compile time mechanism. Basically this means the signature of the method or class is different every time you pass a different data type to the template argument. This made the method impossible to wrap in C# or Java.
I think that you may be asking to something similar to the Java Native Inferface It allows you to call code written in languages such as C++. I've never worked with it though. But you may want to check it out. Hope it helps.
On the Java side there are many options here .. this question actually is fairly close to what you are looking for provided your API can be boxed in a DLL, com or activex objects.
I personnaly used JIntegra to wrap API calls to office (Word) and used it directly witin Java. It did take some hacking to get the desired functionnality but we ended up making it work. The fidling was actually on the Word side, the actual integration was relatively easy.
If you are going to use .NET, I would recommend creating a C++/CLI wrapper. C++/CLI lets you mix the managed .NET code with native C++ code. There are some tricks to this approach, but it works very well in majority of cases.
Wikipedia entry has also some good links.
Using SWIG to handle the JNI stuff makes wrapping a cpp API and using it from Java quite painless.
Here's a SWIG tutorial
I'm the lead author on SlimDX. It may not be the largest open source interoperability project out there, but at 150K+ lines across the project, it's fairly substantial. It's written using C++/CLI, which is a Microsoft developed language, largely compatible with C++, that is designed to allow you to build wrappers. It works very well, with the caveat that it is Windows only. Mono will not take it. I don't know how much of a concern that is for you, but it's probably a better alternative than COM. Unfortunately there's a lot of tips and tricks that you have to basically come up with yourself. I've meant to blog about a lot of the ones we use on SlimDX, but somehow I never quite get around to it.
On the Java side, I believe you have to use JNI. Good luck with that. I've never talked to anyone who's worked with JNI without both of us laughing at it, in a bad "ouch that hurt" sort of way.
I maintain software that is implemented in C++ but must have interfaces in multiple languages including Java and .NET, but also including delphi and VB6. Plus, it has to work on multiple platforms (so Java needs to work on unix).
The way it was done was to use a single DLL exporting plain C functions using primitive types. For example, given an class Foo:
long MY_EXPORT_FLAG FooCreate()
{
return (long)new Foo();
}
void MY_EXPORT_FLAG FooDestroy(long Handle)
{
delete (Foo*)Handle;
}
void MY_EXPORT_FLAG BarMethod(long Handle, const char* pStr1, long* pReturnValue)
{
*pReturnValue = ((Foo*)Handle)->BarMethod( pStr1 );
}
Then your JNI/.NET/VB6/Delphi code implements language specific class wrappers, but call these C dll functions. Each class wrapper would contain a Handle, and pass it into the C functions.
This works quite well because most languages tend to use C as the lowest common denominator, so as long as you can export your interfaces through a thin C api you can build interfaces for other languages.
We export a C (and not C++) api in the DLL because function signatures are much more standardized across platforms.
Don't forget, that on C# and Java, you'll have to deal with multibyte string encoding, so you should figure out how to transcode your strings to/from your C++ code. This typically involves knowing about locales, or you can cheap out and just support UTF-8.