use java function on Qt

Question

I want call java function from Qt application. I have succeed to create javaVM but I have problem to get my java class.

main.cpp

#include <jni.h>
#include <qDebug>
#include <string.h>
#include <windows.h>

#ifdef _WIN32
#define PATH_SEPARATOR ';'
#else
#define PATH_SEPARATOR ':'
#endif

int main()
{
    JavaVMOption options[1];
    JNIEnv *env;
    JavaVM *jvm;
    JavaVMInitArgs vm_args;
    long status;
    jclass cls;
    jmethodID mid;
    jint square;
    typedef jint(JNICALL *pCreateJavaVM)(JavaVM **, void**, void *);

    HINSTANCE hInstance = LoadLibrary(L"C:\\Program Files (x86)\\Java\\jdk1.8.0_101\\jre\\bin\\client\\jvm.dll");
    qDebug()<<"histance"<<hInstance;
    pCreateJavaVM CreateJavaVM = (pCreateJavaVM)GetProcAddress(hInstance, "JNI_CreateJavaVM");

    options[0].optionString = "-Djava.class.path=.";
    vm_args.version = JNI_VERSION_1_2;
    vm_args.nOptions = 1;
    vm_args.options = options;
    vm_args.ignoreUnrecognized = JNI_TRUE;
    status = CreateJavaVM(&jvm, (void**)&env, &vm_args);

    qDebug()<<"status"<<status;
    if (status != JNI_ERR)
    {
        //cls = (env)->FindClass("Test");
        cls = (env)->FindClass("java/Test");
        if(cls !=0)
        {   mid = (env)->GetStaticMethodID( cls, "intMethod", "(I)I");
            if(mid !=0)
            {
                square = (env)->CallStaticIntMethod(cls, mid, 5);
                qDebug()<<"square"<<"5²"<<square;
            }else qDebug()<<"function not found";

        } else qDebug()<<"class not found";

        (jvm)->DestroyJavaVM();
        return 0;
    }
    else
        qDebug()<<"jni error"<<status;
        return -1;
}

Test.java

public class Test
  {
    public static int intMethod(int n) {
        return n*n;
    }
 }

.pro

TEMPLATE = app

CONFIG += c++11

SOURCES += main.cpp

# Default rules for deployment.
include(deployment.pri)

INCLUDEPATH += "C:/Program Files (x86)/Java/jdk1.8.0_101/include"
DEPENDPATH += "C:/Program Files (x86)/Java/jdk1.8.0_101/include"

INCLUDEPATH += "C:/Program Files (x86)/Java/jdk1.8.0_101/include/win32"
DEPENDPATH += "C:/Program Files (x86)/Java/jdk1.8.0_101/include/win32"

LIBS += -L"C:/Program Files (x86)/Java/jdk1.8.0_101/lib/" -ljvm

HEADERS +=

DISTFILES += \
    java/Test.java

folder

MyProject/
    |__MyProject.pro
    |__MyProject.pro.user
    |__deployment.pri
    |__main.cpp
    |__java/
        |__Test.java

I think my file is on the wrong place but i don't know where I have to copy it

Edit : I have compiled my file .java to .class (with javac) and edit my file main.cpp :

#include <jni.h>
#include <qDebug>
#include <string.h>
#include <windows.h>

#ifdef _WIN32
#define PATH_SEPARATOR ';'
#else
#define PATH_SEPARATOR ':'
#endif

int main()
{
    JavaVMOption options[1];
    JNIEnv *env;
    JavaVM *jvm;
    JavaVMInitArgs vm_args;
    long status;
    jclass cls;
    jmethodID mid;
    jint square;
    typedef jint(JNICALL *pCreateJavaVM)(JavaVM **, void**, void *);

    HINSTANCE hInstance = LoadLibrary(L"C:\\Program Files (x86)\\Java\\jdk1.8.0_101\\jre\\bin\\client\\jvm.dll");
    qDebug()<<"histance"<<hInstance;
    pCreateJavaVM CreateJavaVM = (pCreateJavaVM)GetProcAddress(hInstance, "JNI_CreateJavaVM");

    options[0].optionString = "-Djava.class.path=classes:.";
    vm_args.version = JNI_VERSION_1_2;
    vm_args.nOptions = 1;
    vm_args.options = options;
    vm_args.ignoreUnrecognized = JNI_TRUE;
    status = CreateJavaVM(&jvm, (void**)&env, &vm_args);

    qDebug()<<"status"<<status;
    if (status != JNI_ERR)
    {
        cls = (env)->FindClass("Test");
        if(cls !=0)
        {   mid = (env)->GetStaticMethodID( cls, "intMethod", "(I)I");
            if(mid !=0)
            {
                square = (env)->CallStaticIntMethod(cls, mid, 5);
                qDebug()<<"square"<<"5²"<<square;
            }else qDebug()<<"function not found";

        } else qDebug()<<"class not found";

        (jvm)->DestroyJavaVM();
        return 0;
    }
    else
        qDebug()<<"jni error"<<status;
        return -1;
}

folder

MyProject/
    |__MyProject.pro
    |__MyProject.pro.user
    |__deployment.pri
    |__main.cpp
    |__classes/
        |__Test.class

My class isn't found

Jonah Benton · Accepted Answer · 2016-09-14 01:55:08Z

1

3 things:

The .java file has to be compiled with javac into a .class file. Put this Test.class file in a "classes" directory.
the .class file has to be reachable in the path declared by

-Djava.class.path=.

Say instead

-Djava.class.path=classes:.
FindClass just takes a class name, not a path. So it should be "Test".

answered Sep 14, 2016 at 1:55

Jonah Benton

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2 Comments

helene Over a year ago

Thank you for your help but I have the same result. Maybe i's because I have warning "deprecated conversion from string constant to 'char *'; I have edit my post

Jonah Benton Over a year ago

A warning shouldnt matter. If the app is reporting the class is not found- where is the c++ app being run from? Does the compiled binary get dropped in an "out" directory or something? Try putting the Test.class file in that directory.

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