header file to include java classes?

Discussion in 'C++' started by Swapnil Kale, Dec 19, 2007.

  1. Swapnil Kale

    Swapnil Kale Guest

    Hi,
    I'm working on a Migration project (Forte to JAVA).

    The forte client had a C++ dll which used to call one more FORTE dll
    for a complex database calculations.

    Now all the forte code has been migrated to JAVA except this piece of
    code where C++ dll calls Forte DLL.


    I'm using JNI to call the Java classes from C++. Some of the methods
    in C++ has the header files where the signature has reference to the
    Forte Classes (Which are now Java Classes).

    How do I change the header files to include Java classes in the
    signature?

    (I couldnt find a strong reference for this, but i assume that for all
    the classes in Java I'll need to create the .h file using javah
    utility.


    Here is the Sample in one of the C++ header files:

    qqhConnectionHandleClass * m_ForteConnHandle; //
    qqhConnectionHandleClass was Forte class which is now a
    ConnectionHandleClass in Java.


    HWND m_hwndParent;
    int m_autocommit;
    StatementHandle * m_StmtHandles;
    .....

    Almost all the signatures in the header files of C++, have references
    to Forte / Java classes.

    Do I need to use javah and create a header file for the classes and
    then include in the C++ header file?


    In the implementation class (C++) just including jni.h solves my
    problem as it is enough to create the jvm and loading classes/
    executing methods..


    Any help is appreicated.

    Newbie in C++ :)

    Thanks,
    Swapnil.
    Swapnil Kale, Dec 19, 2007
    #1
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  2. On Dec 19, 2:33 pm, Swapnil Kale <> wrote:
    [...]
    > I'm using JNI to call the Java classes from C++. Some of the methods
    > in C++ has the header files where the signature has reference to the
    > Forte Classes (Which are now Java Classes).

    [...]

    This is offtopic here.

    I do have a bit of experience in this myself, but I am not sure what
    your problem is. You mention in the last paragraph of yours that you
    made the link between java and c++. Do you have any particular problem
    or are you just unsure?

    I do recognize, though, that there is not too much documentation on
    the net, and a large part of it is very outdated.

    Well, in short, you design your Java classes and use javah to create
    the headers. In C++, you can #include this header alongside jni.h and
    make a library. In it, you can communicate with Java through
    GetObjectClass, GetFieldID etc. When you are done, in Java, you
    declare those functions (take note of the native keyword) preferably
    in some wrapper class, which also has something like:

    public class BlahBlah{
    static {
    System.load("/home/trail/libSomething.so");
    }

    to load the library.

    Note that there is a better way to load it that does not need an
    absolute path, but I don't remember the methods atm.

    The C++ function would like like this:

    JNIEXPORT void JNICALL Java_ClassName_MethodName(
    JNIEnv * JAVA, jclass Ccls,
    /*in*/ jstring j_something, jint j_somethingelse,
    /*out*/ jobject ret_obj)
    {
    /* get info from Java */
    int shalala = j_somethingelse;
    const char* c_something = JAVA->GetStringUTFChars(j_something,
    NULL);
    ...
    JAVA->ReleaseStringUTFChars(j_something, c_something);
    ...
    /* change Java class members' data */
    jclass cls = JAVA->GetObjectClass(ret_obj);
    jfieldID fid_res = JAVA->GetFieldID(cls, "resultCode", "I");
    JAVA->SetIntField(ret_obj, fid_res, RES_SUCCESS_OR_WHATEVER);
    }

    > How do I change the header files to include Java classes in the
    > signature?


    I am unsure of what you mean by this.
    Ioannis Gyftos, Dec 19, 2007
    #2
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  3. Swapnil Kale

    Swapnil Kale Guest

    Hi,
    Thanks for the update.

    Let me be more specific on this.

    I've a C++ dll which is calling FORTE (the language developed by SUN
    10/15 yrs back. Now it is outdated). Now after I migrate the project
    to JAVA (Forte was pretty similar to JAVA), My task would be to call
    the Java programs into the existing C++ dlls.

    I could use the JVM and the getobject methods etc...using JVM
    creattion in C++.

    My problem: The existing header files of the dlls / C++ contains the
    type of the classes .

    /*
    // EnvironmentHandle.h: interface for the EnvironmentHandle class.
    */
    e.g. qqhEnvironmentHandleClass * GetForteHandle(void) { return
    m_ForteEnvHandle;}

    qqhEnvironmentHandleClass was a Forte class which is a
    EnvironmentHandleClass in Java now. (not reflected in the code),

    The new code could be EnvironmentHandleClass * GetForteHandle(void)
    { return m_ForteEnvHandle;}

    //just an assumption, didnt find the right way yet.

    Now How do I refer this converted Java Class in the C++ header.

    How do I tell the C++ header where to look for the Java classs?

    Regards,
    Swapnil
    --------------------------------------------------------------------



    On Dec 19, 7:20 am, Ioannis Gyftos <> wrote:
    > On Dec 19, 2:33 pm, Swapnil Kale <> wrote:
    > [...]> I'm using JNI to call the Java classes from C++. Some of the methods
    > > in C++ has the header files where the signature has reference to the
    > > Forte Classes (Which are now Java Classes).

    >
    > [...]
    >
    > This is offtopic here.
    >
    > I do have a bit of experience in this myself, but I am not sure what
    > your problem is. You mention in the last paragraph of yours that you
    > made the link between java and c++. Do you have any particular problem
    > or are you just unsure?
    >
    > I do recognize, though, that there is not too much documentation on
    > the net, and a large part of it is very outdated.
    >
    > Well, in short, you design your Java classes and use javah to create
    > the headers. In C++, you can #include this header alongside jni.h and
    > make a library. In it, you can communicate with Java through
    > GetObjectClass, GetFieldID etc. When you are done, in Java, you
    > declare those functions (take note of the native keyword) preferably
    > in some wrapper class, which also has something like:
    >
    > public class BlahBlah{
    > static {
    > System.load("/home/trail/libSomething.so");
    > }
    >
    > to load the library.
    >
    > Note that there is a better way to load it that does not need an
    > absolute path, but I don't remember the methods atm.
    >
    > The C++ function would like like this:
    >
    > JNIEXPORT void JNICALL Java_ClassName_MethodName(
    > JNIEnv * JAVA, jclass Ccls,
    > /*in*/ jstring j_something, jint j_somethingelse,
    > /*out*/ jobject ret_obj)
    > {
    > /* get info from Java */
    > int shalala = j_somethingelse;
    > const char* c_something = JAVA->GetStringUTFChars(j_something,
    > NULL);
    > ...
    > JAVA->ReleaseStringUTFChars(j_something, c_something);
    > ...
    > /* change Java class members' data */
    > jclass cls = JAVA->GetObjectClass(ret_obj);
    > jfieldID fid_res = JAVA->GetFieldID(cls, "resultCode", "I");
    > JAVA->SetIntField(ret_obj, fid_res, RES_SUCCESS_OR_WHATEVER);
    >
    > }
    > > How do I change the header files to include Java classes in the
    > > signature?

    >
    > I am unsure of what you mean by this.
    Swapnil Kale, Dec 19, 2007
    #3
  4. On Dec 19, 4:22 pm, Swapnil Kale <> wrote:
    > Now How do I refer this converted Java Class in the C++ header.
    >
    > How do I tell the C++ header where to look for the Java classs?


    I believe you can't use the whole Java class in C++ through JNI. JNI
    is used to access and call member functions. The 'native' Java keyword
    refers to functions that run outside the JVM, but not to classes

    If you use javah on a Java member function (well, on a .java file but
    you get what i mean), you get a C++ header with also a parameter that
    is a handle to that Java object. You can use that handle to get access
    to other variables of that class and stuff like that through the JNI
    API. You also have similar access to objects that are parameters to
    the function call. But you don't get a full class in the C++ header.

    Why do you need the full Java Class in C++? If, for example, you are
    trying to create an object through its Java class in C++, i doubt you
    would be able to do it directly. JNI might provide functionality for
    something like that, though, but i didn't need it so I didn't
    research :) However I don't recall seeing anything like that, either.
    However, you could create that Java object through a Java factory,
    that returns an instance of that Java object, that C++ can have access
    to.

    Are you trying to have a C++ executable that creates the JVM? In that
    case, from what I understand, it might be better to have a Java jar
    that loads a C++ library that only deals with the dll.

    Think of JNI as a bridge between functions, not classes. Besides,
    classes are a compile-time construct and objects are a run-time one.
    This communication is done dynamically, so you have to deal with
    objects.

    Forgive my horrible terminology, I was in a hurry :)
    Ioannis Gyftos, Dec 19, 2007
    #4
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