I just saw the open source project on this in github (googlevr) and my question is how is it possible for C++ work with Java? I can understand that Java is for android stuff and C++ is for graphic, memory and tracking but how does two different compiled language work together?
In C and C++ you can create shared libraries. They are handled a bit differently for each platform, but do roughly the same thing.
Windows creates a .dll
Mac creates a .dylib
Linux creates a .so
These represent executable code that can be called by any process. This means that java code, matlab code, python code, etc can call code written in C/C++. Java uses a feature called JNI (Java Native Interface) to do this. JNI is notoriously tricky to setup and manage, so a lot of people use a library like Swig which essentially manages everything you need related to JNI in order to make calling precompiled C++ code from Java easier.
The key here is "precompiled". Someone, at some point, maybe even you, had to take the source code and compile it into a dll, dylib, or so and you have to have that shared library set up where the code that needs to use it (in this case your java app) can see it so that when the java app starts it can load the shared library and make calls into it.
For java one consideration is that java code is inherently cross-platform. C++ code needs to be compiled against each platform. So when you distribute your java app, you need to make sure you have a shared library available that is accessible for whichever platform it is being run on.
Related
I have some Java-app and a customer with some UWP-app implemented in C#, distributed through the Windows Store etc., who wants to use some pieces of my app. Those pieces are pretty OS-independent, only parsing of some special binary file formats, applying some business logic configured using YAML files and stuff. No network, GUI, only some accesses to files etc.
We currently use IKVM to make the code of interest available to C# but ran into different problems already. Some were supporting .NET Core, some had to do with the native toolchain in Release etc. While right now things seem to work after applying some workarounds, I'm looking for alternatives to IKVM already a bit.
The only thing I currently use of IKVM is simply creating a DLL of my code using ikvmc, which can then be referenced in the UWP-project. The compiler is summarized like the following:
The ikvmc tool converts Java bytecode to .NET dll's and exe's.
That's where the support to create native Windows images of GraalVM came into my mind. Others seem to already build native binaries for Windows and according to the docs, GraalVM is able to create shared libs using "--shared". From my understanding, IKVM implements a JVM in .NET and maps things as needed and possible. That sounds pretty much like what "Substrate VM" does in case of a native image, doesn't it?
This executable includes the application, the libraries, the JDK and
does not run on the Java VM, but includes necessary components like
memory management and thread scheduling from a different virtual
machine, called “Substrate VM”. Substrate VM is the name for the
runtime components (like the deoptimizer, garbage collector, thread
scheduling etc.).
https://www.graalvm.org/docs/reference-manual/native-image/
So, is there any chance that a native image in form of a DLL can replace the DLL created by ikvmc currently? Did anyone try that already and has any experiences? Did anyone try already to create a native DLL and consume that in some other native Windows app? From my understanding UWP "only" applies additional restrictions which one might be able to work around again. Or is this approach totally impossible for some reasons?
Thanks all for your input!
I'm not very familiar with the IKVM project, so this answer is mostly about the generic question:
Can you create a native DLL/shared library and consume that in some other native Windows app?
It should be possible. You can compile Java code into a shared library. The entry points are marked with the #CEntrypoint annotation.
You can then use the generated shared library and the header files to consume your library from a native application.
This way for example GraalVM distributions use the GraalVM JIT compiler by default:
The GraalVM JIT is written in Java
Compiled as a shared library with the native-image
Used in Hotspot.
Here's a page describing how to consume those from Java through the JNI: https://www.graalvm.org/reference-manual/native-image/ImplementingNativeMethodsInJavaWithSVM/
which could be very similar to how would you use a shared library from a C# application.
GraalVM native images are not very flexible, unlike IKVM.NET images. Unless you like writing wrappers and playing with P/Invoke, you should stick to IKVM.NET.
NOTE: I am behind an IKVM.NET fork
My problem is as follows.
I need to create a library in Java. So far so good.
Like any decent Java Libraries, it must be usable on any JVM.
But the difficulty here is that this library will contain a lot of native code.
I have some knowledge in JNI.
I've tried creating a few native methods in Java in an Android application with Android Studio and with the Android NDK.
That's great I can do a lot of HelloWorld examples. But how can I export this as a library ?
My project needs external C++ Libraries.
I would like to wrap these libraries and use them in a JNI wrapper wrapped inside a Java Library.
See the following schema :
To make things even simpler, take a simple HelloWorld JNI API for example.
Create a Java Desktop App (or anything else). Import the Helloworld JNI API (as jar), call String HelloWorldJNI_API.sayHello(String yourName) and print the result.
What is required here :
The JNI API will obviously declare the sayHello() method as a
native method ;
The argument (String yourName) is sent to JNI ;
The JNI code calls an internal awesome C++ so library that crunches the data and returns a "hello" + "yourName" (awesome, right ?)
The JNI code returns the result as a jstring
Finally, the Java API returns the result as a String
and voila !
This simple example should show you what I am trying to do.
Well, your Library will contain both the .jar file with the java wrapper code as well as the native files (.so if you're on linux based, or .dll if you're on windows).
Here's where the fun begins :
Because native is compiled in processor assembly language you will have to compile the .so for all your supported target (eg for all android with native support since like forever):
armv5, armv7, armv7s , arm64
Now, you will have to provide an archive with all the above.
This is the case where you want a stand alone library, without providing the code to the developer.
If you can provide the code,then you don't need to worry about different architectures.
I have an algorithm and some other code which is in MATLAB and I want to use it in my Android application.
How can I do this?
Can I make a jar file from MATLAB for use with Android?
I have to do something else?
If you have an additional product, MATLAB Builder JA for Java, you can produce a .jar file from your MATLAB code.
However, the .jar file requires the MATLAB Compiler Runtime (a freely redistributable component that you get with MATLAB Compiler and MATLAB Builder products) to be present. The MCR has a much larger footprint than is suitable for the typical Android device (it's like a copy of MATLAB itself, without the user interface).
You could think about either
Running your MATLAB .jar file remotely on a server, and having your Android application connect to it, or
Instead of using MATLAB Compiler and Builder products, use MATLAB Coder, which will convert a subset of the MATLAB language directly into C code. This C code doesn't require the MCR, and could be compiled to run directly on Android. Make sure your MATLAB algorithm falls within, or can be expressed in, the appropriate subset of the MATLAB language.
Edit: As of R2015a, functionality from MATLAB Builder JA for Java has been replaced by a new product, MATLAB Compiler SDK.
I am the developer of Addi. http://addi.googlecode.com Addi is quickly becoming a full port of Octave (which is an open source tool that uses Matlab syntax). Addi is going to have intents for other applications to use it as their math engines or plotting engines. So, if you can run your code on Octave, then very soon you will be able to run it on Android.
Our only option is to get C++ code from M code using MATLAB Coder toolbox, that generates standalone C and C++ code from MATLAB® code. It supports only some subset of all Matlab functions, therefore might be not suitable for your needs.
Having C code you can compile it using NDK. MATLAB Compiler is not an option here.
A new feature in Matlab 2014a:
http://www.mathworks.com/help/simulink/samsung-galaxy-android-devices.html
You can now directly install (limited set of) models to Samsung Android devices, and this should work actually on any Android device.
You can convert matlab code into python and then use the python code in the android .There are many tools to do this conversion. Python goes well with android than matlab.
You have 2 options,
Create a JAR and include in your Java Application and start using it. (I have not tested this by creating a JAR outside Eclipse)
You can code the same thing in C and use Android NDK to process it. (This will be faster and safer way)
I need to access a list of Wifi devices on Mac OSX from Java code and after researching it, I've found that I need to resort to "native" code - namely the CoreWLAN framework and the CWInterface.h class (https://developer.apple.com/library/mac/#documentation/CoreWLAN/Reference/CWInterface_reference/translated_content/CWInterface.html)
I initially thought I would be able to call this code using JNA but realized that I needed something since CoreWLAN is Objective-C (not C/C++).
I've tried this Objective-C Java bridge (https://github.com/shannah/Java-Objective-C-Bridge) but I can't work out how to make it find the CoreWLAN framework.
So I've also tried using JNAerator (https://code.google.com/p/jnaerator/wiki/ObjectiveC) so I can I use BridJ (https://code.google.com/p/bridj/), but I can't make it generate the right Java code.
Just using this style java -Xmx1000m -jar jnaerator.jar -framework CoreWLAN -jar CoreWlan.jar runs quickly but results in a jar that only contains mappings for CoreWlan.h
If I run jnaerator against CWInterface.h then jnaerator fails.
(I'm using the latest snapshot version of jnaerator)
What is the best way to call methods on CWInterface.h from Java?
What I would do is create a C++ class that communicates with the Java code through JNI. You can use both C++ and Obj-C in your xCode project. I haven't tried myself to use a Obj-C++ class with JNI, but I would just create a C++ class responsible for all communication between Java and Obj-C, this class can just be included in the Obj-C class where you need it (change the extension of this Obj-C file to .mm since it'll include C++ code).
I found this a helpful article on JNI: http://www.ibm.com/developerworks/java/tutorials/j-jni/
Personally I would do this via two totally separate programs that communicate via a shared memory mapped file. One program written in straight Objective-C, and the other in straight Java.
Memory mapping would mean that both programs would have access to the same region of memory, without having to resort to JNI at all.
Notes on memory mapping from Java:
http://javarevisited.blogspot.co.uk/2012/01/memorymapped-file-and-io-in-java.html
Notes on memory mapping from Objective C:
https://gist.github.com/jverkoey/2985830
That said, if you do want to do go the JNI route, then Apple has the following advice:
It is recommended that you use the Java JNI template in Xcode as a starting point for your JNI development.
To interoperate with the Objective-C runtime from JNI, link against JavaNativeFoundation.framework, which
is a sub-framework of JavaVM.framework. It contains Objective-C classes and macros to automatically set up
and tear down autorelease pools, catch and re-throw Java and Cocoa exceptions, hold JNI global references
in Foundation container classes, and convert object graphs of strings, numbers, lists, maps, and sets.
The above quotes were taken from here:
https://developer.apple.com/library/mac/documentation/java/conceptual/java14development/Java14Development.pdf
You can use the java objective-c bridge for this. You just need to load the framework you want to use with jna's Native.loadLibrary() method.
This example uses the WebKit framework. https://github.com/shannah/Java-Objective-C-Bridge/blob/master/java/test/ca/weblite/objc/TestWebView.java
Is there a Java library to access the native Windows API? Either with COM or JNI.
You could try these two, I have seen success with both.
http://jawinproject.sourceforge.net
The Java/Win32 integration project
(Jawin) is a free, open source
architecture for interoperation
between Java and components exposed
through Microsoft's Component Object
Model (COM) or through Win32 Dynamic
Link Libraries (DLLs).
https://github.com/twall/jna/
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.
JNA allows you to call directly into
native functions using natural Java
method invocation. The Java call looks
just like it does in native code. Most
calls require no special handling or
configuration; no boilerplate or
generated code is required.
Also read up here:
http://en.wikipedia.org/wiki/Java_Native_Interface
The Java Native Interface (JNI) is a
programming framework that allows Java
code running in a Java Virtual Machine
(JVM) to call and to be called1 by
native applications (programs specific
to a hardware and operating system
platform) and libraries written in
other languages, such as C, C++ and
assembly.
http://en.wikipedia.org/wiki/Java_Native_Access
Java Native Access provides Java
programs easy access to native shared
libraries without using the Java
Native Interface. JNA's design aims to
provide native access in a natural way
with a minimum of effort. No
boilerplate or generated glue code is
required.
JNA is pretty nice. I'm just a beginner and I found it very easy. Works not only for the Win32 API but for almost any other DLL.
Jacob is quite good on the COM side (but it's real COM - like you write in C++ - if you are familiar with true COM programming, then Jacob is a snap to use)
JNIWrapper or ConfyJ from from TeamDev.
One more option is WinRun4J. It has a native binding layer that aims to be compatible with pinvoke.net (the native binding format used in dot net). See examples for more information. Its a little early days so YMMV.
(full disclosure: i work on the project).
Yet another option is JFFI - this is used in jruby and jython to interact with native libraries.
Check out Waffle