Looking for a convenient way to call Java from C++

Question

It seems most documentation or helper libraries relating to JNI (Java Native Interface) are concerned with calling native code from Java. This seems to be the main use of it, ev

Answer 1

Maybe a bit of a too large hammer for this nail, but isn't that what CORBA was built for?

Answer 2

Answering my own question:

http://java4cpp.kapott.org/

Doesn't appear to be an active project. The author recommends it not be used with JDK 1.5 or later.

It appears to have a serious problem: it passes around naked pointers to its wrapper objects:

java::lang::Integer* i = new java::lang::Integer("10");

delete i; // don't forget to do this!

It also causes a more subtle problem that in order to represent assignment compatibility (e.g. a class as a subtype of the interface it implements) the wrappers have to inherit from each other.

Answer 3

I also had many difficulties getting JNI to work on different operating systems, coping with 32/64-bit architectures and making sure the correct shared libraries were found and loaded. I found CORBA (MICO and JacORB) difficult to use too.

I found no effective way to call from C/C++ into Java and my preferred solutions in this situation are to run my Java code as either:

1. a stand-alone program that I can easily run from C/C++ programs with java -cp myjar.jar org.foo.MyClass. I guess this is too simplistic for your situation.

2. As a mini-server, accepting requests from C/C++ programs on a TCP/IP socket and returning results through this socket too. This requires writing networking and serializing functions but decouples the C/C++ and Java processes and you can clearly identify any problems as being in the C++ side or Java side.

3. As a Servlet in Tomcat and make HTTP requests from my C/C++ program (other servlet containers would also work too). This also requires writing networking and serializing functions. This is more like SOA.

Answer 4

Yes, there are existing tools that do exactly this -- generate C++ wrappers for Java classes. This makes use of Java APIs in C++ more transparent and enjoyable, with lower cost and risk.

The one that I've used the most is JunC++ion. It's mature, powerful and stable. The primary author is very nice, and very responsive. Unfortunately, it's a commercial product, and pricey.

Jace is a free, open-source tool with a BSD license. It's been years since I last played with jace. Looks like there's still some active development. (I still remember the USENET post by the original author, over a decade ago, asking basically the same question you're asking.)

If you need to support callbacks from Java to C++, it's helpful to define C++ classes that implement Java interfaces. At least JunC++ion allows you to pass such C++ classes to Java methods that take callbacks. The last time I tried jace, it did not support this -- but that was seven years ago.

Answer 5

I'm one of the prinicpal architects for Codemesh's language integration products, including JunC++ion. We have been doing this kind of integration since 1999 and it works really well. The biggest problem is not the JNI part. JNI is tedious and hard to debug, but once you get it right, it mostly just keeps working. Every now and then, you get broken by a JVM or an OS update, and then you have to fine-tune your product, but in general it's stable.

The biggest problem is the type system mapping and the trade-offs between general usability and targeted solution. You state for example that you don't like the fact that JACE treats all object references as globals. We do the same thing (with some escape hatches) because it turns out that this is the behavior that works best for 95% of customers, even if it hurts performance. If you're going to publish an API or a product, you have to pick the defaults that make things work for most people. Picking local references as the default option would be wrong because more and more people are writing multithreaded applications, and a lot of Java APIs that people want to use from other languages are intrinsically multithreaded with asynchronous callbacks and the like.

We also found out that you really want to give people a GUI-based code generator to create the integration specification. Once they've specified it, you use the CLI version to integrate it into the nightly build.

Good luck with your project. It's a lot of work to get right. We spent several years on it and we're still making it better regularly.

Answer 6

Re calling Java from C++.

You can do what you wish but you must let Java be in control. What I mean by this is that you create Java threads that call into Native code and from there they block, kind of waiting for your native code to give it something to do. You create as many Java threads as you need to get enough work / throuhput done.

So your C++ application starts up, it creates a JVM/JavaVM (as per the documented way, example exists in qtjambi codebase see below), this in turn perform the usual JNI initialization and System.loadLibrary() and provides JARs with "native" linkage. You then initialize a bunch of threads and call some JNI code (that you created) where they can block in wait for your C++ code to give them some work to do.

Your C++ code (presumabily from another thread) then sets up and passes all the information needed to one of the blocked and waiting Java Thread workers, it is then given the order to run, it may then go back into pure-Java code to do work and return with a result.

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It is possible to setup and create and contain a JavaVM instance from C++ code. This can be force fed your own CLASSPATH/JARs to setup the contained environment you need encapsulated inside your C++ program.

Outline of that as I'm sure you have found already at http://download.oracle.com/javase/1.5.0/docs/guide/jni/spec/invocation.html

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There is a kind of C++ => Java JNI generator in the QtJambi project (that I work on and help maintain). This is quite bespoke for the Qt toolkit but essentially it translates a bunch of C++ header files into a collection of C++ .cpp/.h files and *.java file to provide linkage and shell containment of the object so that the competing memory allocation schemes play well together. Maybe there is something to be taken from this.

This is certainly a proof in cencept for what you are asking the generator just happens to be contained in the qtjambi project (but could be made independant with some work) and this is LGPL licensed (open-source). The Qt toolkit is not a small API yet it can generated 100s of classes to cover high % of API (>85% and almost 100% of Core/GUI parts).

HTH