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Java Forum / General / March 2006JNI: instace object C++ from Java
I'm a newby about JNI. I have an application write in C++ and I have to migrate it to Java. I have seen that JNI permit to call c++ methods that return only primitive type as string or int. My question is simple: if I have a C++ class that we call "ClassA", can I instance an object "ClassA" in Java that run the constructor and where can I call the methods of "ClassA" as I was in C++? I want something like this in Java: jClassA a; jClassB b b = h.methodClassA(int d); Can I do that? Thank you! > I have an application write in C++ and I have to migrate it to Java. > I have seen that JNI permit to call c++ methods that return only > primitive type as string or int. Not just primitives: you can pass *any* Java types (primitives or Objects) between Java and native code. Please note that String is not a primitive. > My question is simple: if I have a C++ class that we call "ClassA", > can I instance an object "ClassA" in Java that run the constructor > and where can I call the methods of "ClassA" as I was in C++? No. JNI doesn't let you invoke arbitrary methods written in C or C++, or create C++ objects. JNI is a mechanism that lets you implement some (or all) of a Java class's methods in C or C++. Furthermore those methods must have been written with the intent of being called through JNI, i.e. they must use signatures and data types as defined by JNI, and follow a specific calling convention. However from those native methods you are free to create C++ objects and invoke their methods however you like. JNI also lets your native code invoke any Java methods or constructors, or access fields in Java objects. /gordon  Signature[ do not email me copies of your followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e Are you sure that there isn't some trick to use c++ methods? The doubt is coming when I have read this post: http://groups.google.it/group/comp.lang.java.programmer/browse_thread/thread/b4b fa7d78f205066/3ac2b31639123573?lnk=st&q=instance+c%2B%2B+object+in+java+jni&rnum =19&hl=it#3ac2b31639123573 What do you think about that? > Are you sure that there isn't some trick to use c++ methods? Did you actually read the thread you referred to? The example shows that it is possible to call a native method, and _from_there_ create the C++ object. If you read my earlier answer, you'll see that this is exactly what I said you must do. The only "trick" here is that the C++ pointer is passed back to Java as an int (although I would recommend a long). You still can't use the object from Java. To do anything meaningful with it you must first pass it back to a native method, then cast it to whatever object it once was. /gordon Signature[ do not email me copies of your followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e >No. JNI doesn't let you invoke arbitrary methods written in C or C++, >or create C++ objects. More precisely, from Java you cannot create arbitrary C or C++ objects, but of course you can create them on the C++ side. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. > More precisely, from Java you cannot create arbitrary C or C++ > objects, but of course you can create them on the C++ side. Which is exactly what I wrote a couple of sentences later. /gordon Signature[ do not email me copies of your followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e >Which is exactly what I wrote a couple of sentences later. I thought your explanation was going over OP's head. So I decided to restate it in a more elementary way. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. First, thanks to all for response. I haven't understand how I can manage the object pointer. Can someone do an explicit example? We can suppose that we have in java this code: public class JObject { private final long objectPtr = 0; private native void objectCreate(); public static void main(String[] args){ JObject my = new JObject(); my.objectCreate(); } } In native code what will we write? JNIEXPORT void JNICALL Java_objectManager_objectCreate(JNIEnv *env, jobject obj) { CppObject *ptr = 0; CppObject myCppObject; ptr = &myCppObject; ?????????????????? :-( } Can someone rewrite correctly the above code with the target to instance a C++ class (CppObject) and return to Java its pointer? Thank you!!!!! > Can someone rewrite correctly the above code with the target to > instance a C++ class (CppObject) and return to Java its pointer? It's trivial if you declare the native method so that it *returns* the long: private native long objectCreate(); Then: return (jlong)&myCppObject; /gordon Signature[ do not email me copies of your followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e > private native long objectCreate(); > > Then: > > return (jlong)&myCppObject; Ok, this cast is simple. But suppose that I must call a method of my object. I have to call in java a function that give as member the long variable pointer: private native void objectMethodA(objectPtr); public class JObject { private final long objectPtr = 0; private native long objectCreate(); private native void objectMethodA(long objectPtr); public static void main(String[] args){ JObject my = new JObject(); my.objectCreate(); my.objectMethodA(objectPtr); } } Then, how can I call "methodA" (myCppObject.methodA) in native code? JNIEXPORT jlong JNICALL Java_objectManager_objectMethodA(JNIEnv *env, jobject obj, jlong ptr) { ???????????????????? } JNIEXPORT jlong JNICALL Java_objectManager_objectCreate(JNIEnv *env, jobject obj) { CppObject *ptr = 0; CppObject myCppObject; ptr = &myCppObject; return (jlong)&ptr; } Thanks! > Then, how can I call "methodA" (myCppObject.methodA) in native code? Obviously you have to cast the object back: myCppObject = (CppObject*)ptr; /gordon Signature[ do not email me copies of your followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e Java console give me this error: unsatisfiedLinkError: JAxObjectManager.axObjectManagerConstructor() ect..... I don't understand where is the error.. can someone help me? I use: - jvm--> j9 ibm - pocket pc 2003 - evc4 I use this link: 255#"\Programmi\J9\MIDP20\bin\j9.exe" "-jcl:midp20:loadLibrary=AxomStudentVCEmbeddedDLL" "-Xbootclasspath:\Programmi\J9\MIDP20\lib\jclMidp20\jclMidp20.jxe" "-cp" "\Programmi\J9\MIDP20\examples\Axom2\axom.jar" "JAxObjectManager" -------------------------------- -------------------------------- class JAxObjectManager { // salvo il puntatore all'oggetto allocato nel codice nativo private static long axObjectManagerPtr = 0; private native long axObjectManagerConstructor(); private native void jgetRootIndex(long axObjectManagerPtr); public static void main(String[] args){ JAxObjectManager my = new JAxObjectManager(); axObjectManagerPtr = my.axObjectManagerConstructor(); my.jgetRootIndex(axObjectManagerPtr); //System.out.println("Dopo: "+axObjectManagerPtr); } } -------------------- -------------------- /* DO NOT EDIT THIS FILE - it is machine generated */ #include <jni.h> /* Header for class JAxObjectManager */ #ifndef _Included_JAxObjectManager #define _Included_JAxObjectManager #ifdef __cplusplus extern "C" { #endif /* * Class: JAxObjectManager * Method: axObjectManagerConstructor * Signature: ()J */ JNIEXPORT jlong JNICALL Java_JAxObjectManager_axObjectManagerConstructor (JNIEnv *, jobject); /* * Class: JAxObjectManager * Method: jgetRootIndex * Signature: (J)V */ JNIEXPORT void JNICALL Java_JAxObjectManager_jgetRootIndex (JNIEnv *, jobject, jlong); #ifdef __cplusplus } #endif #endif -------------------- -------------------- #define WIN32 #include <jni.h> #include "windows.h" #include <iostream> #include <sstream> #include <fstream> #include <stdexcept> #include <string> #include <axom/axobjectmanager.h> ect..... other include... #include "JAxObjectManager.h" JNIEXPORT jlong JNICALL Java_JAxObjectManager_axObjectManagerConstructor(JNIEnv *env, jobject obj) { AxObjectManager *axObjectManagerPtr = 0; AxObjectManager myAxom; axObjectManagerPtr = &myAxom; return (jlong)axObjectManagerPtr; } JNIEXPORT void JNICALL Java_JAxObjectManager_jgetRootIndex(JNIEnv *env, jobject obj, jlong axObjectManagerPtrLong) { AxObjectManager* myAxom; myAxom = (AxObjectManager*) axObjectManagerPtrLong; //(myAxom).getRootIndex(); AxCommandExpand *beforeExpand = new AxCommandExpand((*myAxom).getRootIndex()); (*myAxom).executeCommand(*beforeExpand); TEST("expand (empty)", beforeExpand->getChildrenIndexes().size()==0); } > Java console give me this error: unsatisfiedLinkError: > JAxObjectManager.axObjectManagerConstructor() ect..... > > I don't understand where is the error.. I don't know j9, but AFAICT you failed to load the DLL in your code (i.e. System.loadLibrary() or equivalent). /gordon Signature[ do not email me copies of your followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e > I don't know j9, but AFAICT you failed to load the DLL in your code > (i.e. System.loadLibrary() or equivalent). i run dll "AxomStudentVCEmbeddedDLL" from link path, there isn't a System.loadLibrary() in the code, it's not supported: 255#"\Programmi\J9\MIDP20\bin\j9.exe" "-jcl:midp20:loadLibrary=AxomStudentVCEmbeddedDLL" "-Xbootclasspath:\Programmi\J9\MIDP20\lib\jclMidp20\jclMidp20.jxe" "-cp" "\Programmi\J9\MIDP20\examples\Axom2\axom.jar" "JAxObjectManager" > i run dll "AxomStudentVCEmbeddedDLL" from link path, there isn't a > System.loadLibrary() in the code, it's not supported: This article on using JNI with j9 seems to disagree with you: http://www.cs.hku.hk/~fyp05016/kyng/ibm_j9.htm Other posts I've seen claim that JNI itself isn't supported by early versions of j9. Perhaps you need a later version that supports JNI and thus has System.loadLibrary(). Again, I don't know j9 or midp... Other things to check: Is there (or has there been) a package declaration in your java source? If so, there is a chance that the generated native symbol names are wrong as a result. Have you confirmed with a binary inspection tool that the native symbols in the DLL *exactly* match those in the generated header file? Failing that, post the complete and exact error message, it may contain some more clues. /gordon Signature[ do not email me copies of your followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e >jClassA a; >jClassB b >b = h.methodClassA(int d); Normally, Java calls C/C++, but you can do the reverse. Normally you just pass primitives back and forth. On the C++ side, you can create a Java object with the JNI API and populate its fields and return it, most commonly a String. You can also create C++ objects and use them on the C++ side. They mean nothing on the Java side so you can't bring them over into Java without converting them to Java objects first. see http://mindprod.com/jgloss/jni.html SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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