How to invoke Java method from C++

Question

Hi
My requirement is In C++ we are having three different structures, like
struct one{
int accontno;
char * accountholdername;
long balance;
struct two{
int startdate;
int enddate;
struct three{
char* userid;
char* userpassword;
from c++ how to pass these structures, these structures I have to read in Java class, and in java how receive these structures.
And the sample methods will be like: getCarsData(Input, output, Error);
How do i call the method in JNI, if it is static int method we can call like
getStaticIntMethodID(), instead of int if i have to pass the above mentioned structures how do i do? Should i create an object array in JNI for accessing these three structures?
Suggestions/Samples welcome
Thanks,

Answer 1

Why did you start another thread for this?
I was answering you in the [url http://forum.java.sun.com/thread.jspa?threadID=712137&messageID=4119725#4119725]old thread and you didn't even bother to answer back my (basic) questions.
I told you to use the code tags when pasting code. Got ignored. I also gave you a starting example on how to achieve your goal. Flushed.
Oh well, I'm getting used to it.
Again.
Before starting, allways keep near you the [url http://java.sun.com/j2se/1.5.0/docs/guide/jni/spec/jniTOC.html]JNI reference. All the JNI functions are documented in it.
First you need to create custom Java classes that will match the C++ stucts (remember?).
There is no magic wand in JNI that will transform the C++ structs into the custom Java classes and vice versa.
You'll have to code it by hand, one field at a time. It's a pain.
So for the one, two, three C++ structs (why not posting some real names instead of this crap?),
there will be JOne, JTwo and JThree Java classes.
There is also that other Java class already existing with getCarsData() static method that will fetch the data.
And finally the DLL function wich will call this Java static method, doing the bridge between the C++ structs and the custom Java classes, finally return the data.
Due to the lack of explanation from your posts, I will say that the getCarsData() static method receives a parameter of type JOne
and returns an array of JTwo objects.
So the get_cars_data() DLL function receives a parameter which is a struct one and
returns a pointer to an array of struct two elements.
// C++ structs
struct one {
    int accontno;
    char * accountholdername;
    long balance;
struct two {
    int startdate;
    int enddate;
struct three {
    char * userid;
    char * userpassword;
// custom Java classes
class JOne {
    int accontno;
    String accountholdername;
    int balance;
class JTwo {
    int startdate;
    int enddate;
class JThree {
    String userid;
    String userpassword;
// Java class with static method that gets the data
class DataFetcher {
    static JTwo[] getCarsData(JOne one) {
// DLL function
// struct 'one' parameter is already filled with values.
// Returns an array of 'two' C++ struct obtained from a Java static method
two * get_cars_data(one o) {
    JNIEnv * env;
    /** not show here is the code to either
        create the JVM or
        attach it to the current thread **/
    // get the data from 'one' C++ struct and put it in a new instance of JOne Java class
    jclass oneCls = env->FindClass("JOne");
    jmethodID oneCtorID = env->GetMethodID(oneCls, "<init>", "()V");
    jfieldID accontnoID = env->GetFieldID(oneCls, "accontno", "I");
    jfieldID accountholdernameID = env->GetFieldID(oneCls, "accountholdername", "Ljava/lang/String;");
    jfieldID balanceID = env->GetFieldID(oneCls, "balance", "I");
    jobject oneObj = env->NewObject(oneCls, oneCtorID);
    env->SetIntField(oneObj, accontnoID, o.accontno);
    env->SetObjectField(oneObj, accountholdernameID, env->NewStringUTF((const char *)o.accountholdername));
    env->SetIntField(oneObj, balanceID, o.balanceo);
    // call the Java static method
    jclass dfCls = env->FindClass("DataFetcher");
    jmethodID getCarsDataID = env->GetStaticMethodID(dfCls, "getCarsData", "(LJOne;)[LJTwo;");
    jobjectarray twoArray = (jobjectarray)env->CallStaticObjectMethod(dfCls, getCarsDataID, oneObj);
    // create the 'two' C++ struct array from the Java array of JTwo objects
    jclass twoCls = env->FindClass("JTwo");
    jfieldID startdateID = env->GetFieldID(twoCls, "startdate", "I");
    jfieldID enddateID = env->GetFieldID(twoCls, "enddate", "I");
    jint len = env->GetArrayLength(twoArray);
    two * pt = new two[len];
    for (int i = 0; i < len; i++) {
        jobject twoObj = env->GetObjectArrayElement(twoArray, i);
        pt.startdate = env->GetIntField(twoObj, startdateID);
pt[i].enddate = env->GetIntField(twoObj, enddateID);
// return the resulting 'two' C++ struct array
return pt;
Regards