Using the JNI with QT - Lib/DLLs are giving a file format error

RBrNx277

Hi there, I am currently trying to use the JNI with QT, to allow me to run some Java function inside my C++ application. I have added the correct INCLUDEPATH to the JNI.h and JNI_md.h files which is:

INCLUDEPATH += "C:/Program Files/Java/jdk1.8.0_65/include/"
INCLUDEPATH += "C:/Program Files/Java/jdk1.8.0_65/include/win32/"

I have also adds a LIBS flag to the .pro file to tell QT where the JNI dll is located which looks like the following:

LIBS += "C:/Program Files/Java/jre1.8.0_65/bin/server/jvm.dll"

However, once I attempt to build my program I get an error saying "file not recognized: File format not recognized" on the .DLL file. Has anyone else got the JNI to work with QT? Am I doing something wrong?

Thanks, Conor

hskoglund

Hi, are you building a 64-bit Qt program? Otherwise, the error could be because mixup of 32-bit and the 64-bit jvm.dll.

RBrNx277

I'm not sure, is there a way to check?

hskoglund

Sure (assuming you are using shadow builds) you can look at the directory name for your program's .exe file, something like "build-myprogram-Desktop_Qt_5_5_1..." it should say either 32bit or 64bit.

RBrNx277

I had a look and I'm building a 32 bit application. I would imagine that the DLL is 32bit as well. Unless you know of a way to check?

hskoglund

Easiest is to check if you have a 32bit or 64bit installation of Java, open a CMD window and type:
"C:\Program Files\Java\jdk1.8.0_65\bin\java.exe" -version

You should see 3 lines, the last one tells if you have 32bit or 64bit Java.

RBrNx277

It gave me this response: http://i.imgur.com/WDCfFhx.png
So I guess I do have the 64bit JNI? I'll have a look to see if there is a 32bit version. Or would it be easier to build my application as 64bit?

Thanks for your help.

hskoglund

Well 64-bit is the future :-) So I think try building your app as 64-bit.
However on Windows that requires you to have Visual Studio 2013 installed (there's no 64-bit option for the MinGW compiler :-(

RBrNx277

Ahh OK that makes sense. I have Visual Studio 2013 installed, is there a step by step process to compile my QT application for 64bit?

hskoglund

It's pretty straightforward: first run Qt's MaintenanceTool.exe app to add 64-bit Visual Studio 2013.
Then to select 64-bit build for your project, easiest is to remove the file ending with "pro.user" (like myprogram.pro.user) and next time you start Qt Creator you should be able to select the 64-bits build.

RBrNx277

OK so I haven't tried to build the 64bit application yet, but I did try to download and use a 32bit Java version. I updated the .pro file to use the new Include Paths and LIBS flag:

INCLUDEPATH += "C:/Program Files (x86)/Java/jdk1.8.0_74/include/"
INCLUDEPATH += "C:/Program Files (x86)/Java/jdk1.8.0_74/include/win32/"
LIBS += "C:/Program Files (x86)/Java/jre1.8.0_74/bin/client/jvm.dll"

The "file format not recognized" error seems to have disappeared (yay?) but now I get the following error:

undefined reference to `_imp__JNI_CreateJavaVM@12'

I assume I'm still doing something incorrectly? My guess is using the 64bit application will also produce this error?

I seen a few bits and pieces online about .Lib files and .a files, are they something I need?

Thanks

hskoglund

Yeas you have now opened another nice can of worms. For starters you'll need a jvm.lib, here's an example Good luck!

RBrNx277

Thanks for the link, it doesn't do anything differently from the way I have everything coded at the moment though. I have all the same (or similar) code to set up the JVM and call Java functions, but my project wing build because of that undefined reference error. It doesn't seem to mention anything about .lib/.a files either, do I just need to add the location of the JVM.lib to the libs flag in the .pro file? Or is there more to it?

jsulm

Shouldn't this one

LIBS += "C:/Program Files (x86)/Java/jre1.8.0_74/bin/client/jvm.dll"

either be

LIBS += -L"C:/Program Files (x86)/Java/jre1.8.0_74/bin/client" -ljvm"

But it probably will not work in any case because you use MinGW and JRE most probably another (VS?) compiler. On Windows you cannot mix compilers.

RBrNx277

Aren't the -L flags only used in Linux systems? Or did I read that wrong?
OK so the problem might be the JVM.dll was compiled with a VS compiler and I'm trying to use it with the MinGW compiler? Is there a way to check what compiler the DLL was compiled with? Could I change the Compiler QT is using to the same one?

SGaist

Hi,

No they are not.

The first problem you have is that you are trying to link against a .dll which is wrong. You need the corresponding .lib file.

But first thing, do you need that for an Android application ?

RBrNx277

Ok, I know the location of the .lib file, what should I do with it?

No this is just a regular windows application. There's a piece of code I'd like to use which is written in Java, I will perhaps look to convert it to C++ in the future but for now I'd just like to use the exisiting Java code.

SGaist

Follow the example that @jsulm provided:

LIBS += -L"C:/Path/to/Java/libs/" -ljvm"

RBrNx277

Nice one, thank you! Finally managed to get it working by only using these three lines in my .pro file

INCLUDEPATH += "C:/Program Files (x86)/Java/jdk1.8.0_74/include/"
INCLUDEPATH += "C:/Program Files (x86)/Java/jdk1.8.0_74/include/win32/"
LIBS += -L"C:/Program Files (x86)/Java/jdk1.8.0_74/lib/" -ljvm

and also by placing the JVM.dll where the .exe file is built by QT.

Unfortunately my program now crashes when it attempts to create a VM for Java and says

Error occurred during initialization of VM
Unable to load native library: Can't find dependent libraries

But I guess that's a different problem!

jsulm

It looks like the jvm DLL depends on at least one more DLL. You can use DependencyWalker to check which dependencies you have.

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