I am working on a project where I have some sensors and I want them to connect to a PC via USB. now they write there is a windows lib written in c and all functions are using the cdecl call convention of c and I have to be sure that if I programm in an other language (java!) that the call convention is given.
So I researched a bit and found a few tutorials to bind dlls in java and or use JNA. but this are all new things for me and I want to be sure, that it is possible (and maybe an example) - the device isn't that cheap and I want to know that before I buy it.
You can call C code from Java - there are no issues here.
Then, if you have something very specific in your C routines, you can always go via wrapper.
Take a look here.
http://jnicookbook.owsiak.org/recipe-No-018/
In this sample, you are calling C code that - in turn - calls another library.
You can also do it by linking your JNI code with some other library. Like here:
https://github.com/mkowsiak/jnicookbook/tree/master/recipes/recipeNo023
There are plenty of options. When it comes to JNI vs. JNA. Make sure that JNA will be efficient enough. In general, JNA tends to be slower. Take a look here:
https://github.com/mkopsnc/keplerhacks/tree/master/jnijna
Hope this helps.
I don't post codes here - all the samples in JNI Cookbook are available from github.
Related
I have existing cpp files from a project which I would like to use it for my application which is in Java. I cannot change the cpp files. How can I call the functions from Java?
I'm working in Windows 10 using JavaFX for my application. I've seen some articles about JNI but none seem to solve my issue.
If JNI or swig is not desired or seems too low level,
A really blunt approach is to wrap the .cpp in c/c++ program and built an .exe that dumps to stdout/file. Then execute that in java via an external shell command.
Another good alternative is
Apache thrift
This basicly handles everything and goes everywhere so to speak (works by auto-generating code to target languages) and it is one I usually recommend in RPC situations. However there could be more setup cost involved (in the end, depends on your actual needs) - also since you need to host the .cpp in a service, in your case, locally.
If you package your library inside a shared object or a dll, you can also use JNA: https://github.com/java-native-access/jna or https://github.com/java-native-access/jna/blob/master/www/GettingStarted.md
For example, you already have mapping to Windows API.
Another example is a mapping of mediainfo in Java: https://github.com/andersonkyle/mediainfo-java-api/blob/master/src/main/java/org/apothem/mediainfo/api/MediaInfo.java
Note that, as far as I understand it, this is based on JNI: it simplify the process since you mostly have to only declare interface on Java side and call appropriate method.
This may be a bit of an odd question, but I would like to know if you can use Java inside c++14. I don't really care for GTK (I find it confusing and over complicated). Swing, however, is very easy to use, and you can get a working project very quickly. So I would like to know if you can use Java Swing inside c++, so I can use Swing as the foreground, as in what you see like graphics, and use c++ for the background, as in stuff you cant see, like calculations and objects and stuff. So if I can have c++ code tell Swing what to look like, or when to update, that would be very useful for the project I have in mind. Thanks in advance for any advice I may receive.
EDIT: Being able to use c++14 inside Java would be acceptable as well. Also, if anyone could get me example code also, this would be very helpful. Thanks!
You shouldn't run Java from C++, but rather C++ from Java. Oracle gives you a way to load native shared libraries, using JNI.
So you would create your view in Java, using Swing, then you would update your view by calling C++ functions that were pre-compiled and exported in a shared library.
That said, using JNI is quite tricky and the speed improvements of C++ might not be worth it; so you should consider using only Java (or only C++ and a library to create your GUI, such as Qt)
In which direction you go is a matter of taste (loading the JVM from a C++ program or loading DLLs from the JVM side).
Usually you go the way which is more logical, e.g. if you already have a C++ program you likely want to load the JVM from the C++ side. That would be your case. Especially if the Java you want to add is essentially "scripting" the C++ application.
If you already have a Java program and want to access a C++ DLL, you load the DLL from Java and write a simple JNI / native Java class.
In our times you would use tools like JNA for that (instead of JNI): https://github.com/java-native-access/jna
Or you can use SWIG to generate wrappers for your C++ classes: http://www.swig.org/
I need to find out which default windows program is executed when opening a text (.txt) file. For this purpose I want to call the FindExecutable function from windows.
How can this be achieved from Java?
I hope I do not have to make a JNI-implementation to call Windows via C for this purpose.
You will have to use JNI. If you don't to code it yourself, you can use JNA - I believe that JNA has moved to GitHub, but the documentation at the previous link is the best I could find. Note that you will be using JNI no matter what (and deploying a native dll), but JNA does make it a lot easier.
I've been playing around with the NDK recently, finding that many of the tutorials available online really don't help. I've been using this tutorial and I've got it running great.
However. Is this the correct use of the NDK? I mean if I have a game say with many classes all in C++ that I wish to port over to the android. Do I have to really manually change all my methods to the likes of:
JNIEXPORT jstring JNICALL Java_com_domain_ndk_NativeLib_hello
(JNIEnv * env, jobject obj) {
return (*env)->NewStringUTF(env, "Hello World!");
}
I can't see this being a very efficient way of porting my code over and I get the feeling I'm using the NDK wrong. I also have no idea how the NDK samples are supposed to work. Could anyone point me in the right direction?
Thanks
You do not need to change ANY of your existing methods to use the "auto-bound" JNI format. There aren't good examples in the Google NDK samples, but the Google SDK uses the other JNI binding method internally: Search for RegisterNatives() in the JNI docs. But even then, you really only need to create a wrapper for the few places that the Android SDK needs to talk to your library: user input/sensors in general, a draw callback, and possibly an "update" callback. Aside from that the rest of your app should remain untouched, except to get it to build.
To simplify things, I'd recommend you not use the native widgets; if you keep your game entirely in OpenGL, you can avoid going through Java to, e.g., draw textures to the screen, at least if you target 1.6 devices and newer, where the NDK has native bindings for OpenGL.
Contrary to some opinions (including those of Google), the NDK can and frequently is used for porting games to Android. I'm using it that way right now, in fact. You need to pass user events from Java to C++, and you need to have Java set up the basic GL context (there are no EGL bindings in the NDK), but you can do pretty much everything else in C++ with OpenGL and, once your basic Java messages are being passed to C++, you can (mostly) ignore Java. Thank goodness. :)
You'll still need to talk to Java for sound, and to get access to your files: Both still need to go through Java, and so you'll probably want to use "reverse JNI," where you call a Java function from C++. Also not hard, and there are examples all over the place.
I talk more about how to access your assets here: File Operations in Android NDK
Good luck.
I'd say your way isn't wrong, but be aware that passing data between Java and C/C++ code is a time consuming thing. Because of that I would suggest you write the most of the code actually in C/C++ and just call C/C++ functions from Java if it can't be avoided. For example you will need to pass data back for the GUI. Especially when you are programming a real time game I would avoid passing data between Java and C/C++ code too much. So just changing all the methods and then calling them in your Java code wouldn't be such a great idea.
Btw if you have to pass something back from C/C++ to Java, I would not pass it as a return value, but rather give the destination as a parameter in the function call.
The NDK is just a native C and C++ development environment for Android. Bionic (the Android version of libc) is more slightly more limited not by much. The only real complication is integrating you new component into the Android build tool chain. Android has a highly customized Makefile system. Downloading the source and looking at simple native components should give you an idea how to use it though.
Now if you want to integrate Java with C or C++ you will need to understand the Java Native Interface (JNI). You should be able to find good documentation of the JNI online.
The new release of the ndk (r5) now includes the capability to write apps entirely in native code.
I'm trying to make a program like AutoHotKey, but with a graphical interface.
I'm using java.awt.Robot
Now I want to make the code for checking the state from a key (In AHK: getKeyState)
Of course somthing like a KeyListener without having focus.
I read already something with JNI and C++, but....
I can't find some information.
Can somebody help me??
There are lot of good JNI resources for starting out with JNI Programming like the Sun JNI Tutorial. Almost all Tutorials assume a good knowledge of C/C++ because the Java Native Interface (JNI) is the bridge between native C/C++ code, the Java Virtual Machine and everything running in there (meaning your Java Bytecode).
What you may want to do first is to find a key capturing library for your operating system of choice (you didn't mention anything specific here) in C++ and try that out as well as checking if there are already some Java bindings (libraries that use JNI and offer Java classes) to interact with. I didn't find any promising on a quick search unfortunately.
works perfect for windows 32/64 Bit. It's not necessary to integrate the dll files into the (eclipse)workspace / deployment process. Amazing Lib:
Hot stuff!
http://ksquared.de/blog/2011/07/java-global-system-hook/