I'm a C programmer with limited Java experience, and need to use JNI to implement a library in Java on top of a C library that consists of a bunch of functions, each receiving some parameters from the operating system. For each C function, the corresponding Java method will be called, and some values would be returned back to the C function. i.e. C -> Java -> C.
I know how to deal with simple primitive types, but I need to pass and return multiple fields which could be primitive types or C structs. I understand that to pass C structs I would need to define a corresponding Java class and copy over the struct fields.
The question, though, is how to deal with multiple parameters/return values of different types. Here is a sample C function specification:
int getattr (const char *, struct stat *)
I have approximately 35 such functions with differing number and types of parameters. The corresponding Java method needs to be passed both these parameters, and it also needs to return both of them! Since I'm new to JNI the only way I can think of is to define 35 Java classes, each corresponding to a particular set of parameters. Hopefully there is a better way to do it!
adityar7 wrote: > I'm a C programmer with limited Java experience, and need to use JNI > to implement a library in Java on top of a C library that consists of > a bunch of functions, each receiving some parameters from the > operating system. For each C function, the corresponding Java method > will be called, and some values would be returned back to the C > function. i.e. C -> Java -> C.
> I know how to deal with simple primitive types, but I need to pass and > return multiple fields which could be primitive types or C structs. I > understand that to pass C structs I would need to define a > corresponding Java class and copy over the struct fields.
> The question, though, is how to deal with multiple parameters/return > values of different types. Here is a sample C function specification:
> int getattr (const char *, struct stat *)
> I have approximately 35 such functions with differing number and types > of parameters. The corresponding Java method needs to be passed both > these parameters, and it also needs to return both of them! Since I'm > new to JNI the only way I can think of is to define 35 Java classes, > each corresponding to a particular set of parameters. Hopefully there > is a better way to do it!
You can put all the native methods in one class.
But if you need 35 different classes with data, then I can not see any way to avoid writing them.
On 4 Jul, 03:34, adityar7 <adity...@gmail.com> wrote:
> I'm a C programmer with limited Java experience, and need to use JNI > to implement a library in Java on top of a C library > The question, though, is how to deal with multiple parameters/return > values of different types. Here is a sample C function specification:
> int getattr (const char *, struct stat *)
I recommend SWIG. See www.swig.org. This is what I used to interface to a proprietary C library for a server that we wanted to be in java. SWIG would take the prototype above and create corresponding java classes for where there is no natural mapping, e.g it would create a class for stat.
Output from SWIG is in the form of two files, one is C glue to go into a dynamic library, the other is the java class that contains all the methods for the functions you want to call. Other java files are created for the new classes it creates, like stat. Your java program has to load the dynamic library first.
On Thu, 3 Jul 2008 19:34:30 -0700 (PDT), adityar7 <adity...@gmail.com> wrote, quoted or indirectly quoted someone who said :
>I have approximately 35 such functions with differing number and types >of parameters. The corresponding Java method needs to be passed both >these parameters, and it also needs to return both of them! Since I'm >new to JNI the only way I can think of is to define 35 Java classes, >each corresponding to a particular set of parameters. Hopefully there >is a better way to do it!
This is one of the uglier parts of Java. It supports multiple inputs to a method but only one output.
How to kludge?
one way is to return an array of double or array of objects. You to lose typing and keyword identification of results.
Another approach is to create a class with a number of internal variables. To use it you call setXXX to set each of the inputs, then call calcXXX to calculate. Then call getXXX to get each of the results. This is the most Javaesque solution.
It is considered bad form to create result classes of scattered crap that is not logically related.
Have your method return a result, and if there is a problem throw an exception, rather than returning a result and status as you would in C.
Rethink your methods so that there are a small number of possible result classes that have physical meaning.
They problem you are having is made worse if you try to write C in Java, rather than thinking OO. --
On 4 Jul, 13:54, Roedy Green <see_webs...@mindprod.com.invalid> wrote:
> This is one of the uglier parts of Java. It supports > multiple inputs to a method but only one output.
> How to kludge?
> Another approach is to create a class with a number of internal > variables.
This is what SWIG does. The classes it creates has setters and getters. You have to write a very thin C wrapper which calls the C code you first thought of. The wrapper also manages this way of interfacing. The C code will have to populate the structure ready for return, your java code just puts the result into an object of the type generated by SWIG. You use the getters to obtain the member values.
I reckon this is MUCH easier than doing raw JNI code.
marlow.and...@googlemail.com wrote: > On 4 Jul, 03:34, adityar7 <adity...@gmail.com> wrote: >> I'm a C programmer with limited Java experience, and need to use JNI >> to implement a library in Java on top of a C library
>> The question, though, is how to deal with multiple parameters/return >> values of different types. Here is a sample C function specification:
>> int getattr (const char *, struct stat *)
> I recommend SWIG. See www.swig.org. This is what I used to interface > to a proprietary C library for a server that we wanted to be in java. > SWIG would take the prototype above and create corresponding java > classes for where there is no natural mapping, e.g it would create a > class for stat.
> Output from SWIG is in the form of two files, one is C glue to go into > a dynamic library, the other is the java class that contains all the > methods for the functions you want to call. Other java files are > created for the new classes it creates, like stat. Your java program > has to load the dynamic library first.
SWIG sounds like a real find. Thanks for bringing it to our attention.
Regarding the example with the 'stat' struct, does SWIG let you turn that into a convention-compliant type name, 'Stat'?
> marlow.and...@googlemail.com wrote: > > On 4 Jul, 03:34, adityar7 <adity...@gmail.com> wrote: > >> I'm a C programmer with limited Java experience, and need to use JNI > >> to implement a library in Java on top of a C library
> >> The question, though, is how to deal with multiple parameters/return > >> values of different types. Here is a sample C function specification:
> >> int getattr (const char *, struct stat *)
> > I recommend SWIG. Seewww.swig.org. This is what I used to interface > > to a proprietary C library for a server that we wanted to be in java. > > SWIG would take the prototype above and create corresponding java > > classes for where there is no natural mapping, e.g it would create a > > class for stat.
> > Output from SWIG is in the form of two files, one is C glue to go into > > a dynamic library, the other is the java class that contains all the > > methods for the functions you want to call. Other java files are > > created for the new classes it creates, like stat. Your java program > > has to load the dynamic library first.
> SWIG sounds like a real find. Thanks for bringing it to our attention.
> Regarding the example with the 'stat' struct, does SWIG let you turn that into > a convention-compliant type name, 'Stat'?
> -- > Lew
Yes it does. I must admit, I am suprised when java people tell me they don't know about SWIG. I am relatively new to java and I was lukcily to come across it quite early. I saw it used about a year ago on a C++ project where suddenly a java interface was required by certain customers. It was quickly knocked up using SWIG. The C++ was concerned mainly with returning structures that represented trades for particular financial instruments. Each instrument had its own type and several were expressed in terms of other types.
marlow.and...@googlemail.com wrote: > project where suddenly a java interface was required by certain > customers. It was quickly knocked up using SWIG.
Aside: I enjoyed the momentary mental hiccough - "knocked up" as you meant it is a regional expression meaning "built" or "prototyped". Around these parts it means "impregnated", usually with an implication of out-of-wedlock relations. Often the latter sense is quickly achieved after a few swigs of something intoxicating.
On Thu, 3 Jul 2008, adityar7 wrote: > I'm a C programmer with limited Java experience, and need to use JNI > to implement a library in Java on top of a C library that consists of > a bunch of functions, each receiving some parameters from the > operating system. For each C function, the corresponding Java method > will be called, and some values would be returned back to the C > function. i.e. C -> Java -> C.
> I know how to deal with simple primitive types, but I need to pass and > return multiple fields which could be primitive types or C structs. I > understand that to pass C structs I would need to define a > corresponding Java class and copy over the struct fields.
> The question, though, is how to deal with multiple parameters/return > values of different types. Here is a sample C function specification:
> int getattr (const char *, struct stat *)
> I have approximately 35 such functions with differing number and types > of parameters. The corresponding Java method needs to be passed both > these parameters, and it also needs to return both of them! Since I'm > new to JNI the only way I can think of is to define 35 Java classes, > each corresponding to a particular set of parameters. Hopefully there > is a better way to do it!
> Any help would be greatly appreciated :D
Well, firstly, are you aware that there is already a java binding of FUSE?
If you don't want to use that, or if this isn't what you're working on, to answer your question, i'd do a straightforward conversion of the C interface, and pass a writeable object from C to java, get java to write into it, then copy back from the java object to the C one. Like so:
// java:
public class Stat { public int ino ; // etc
}
public interface Filesystem { public int getattr(String path, Stat sb) ;
}
public class FUSEManager { static { System.loadLibrary("fuse") ; } public static native void init(Filesystem fs) ;
}
// C:
/* error checking omitted for clarity! */
#define OK 0
JavaVM *jvm ; // JVM ref, get in a setup method
jobject fs ; // ref to a Filesystem object, get in a setup method jclass fsClass ; jmethodID getattrID ;
jclass statClass ; jmethodID statCtorID ; jfieldID statInoID ; // etc for other fields
void copyStatIn(JNIEnv *env, jobject statObj, struct stat *stat) { stat->st_ino = (*env)->GetIntField(env, statObj, statInoID) ; // etc for other fields
}
// would define a copyStatOut for setattr
int getattr(const char *path, struct stat *stat) { JNIEnv *env = NULL ; (*jvm)->AttachCurrentThread(jvm, &env, NULL) ; jstring pathStr = (*env)->NewStringUTF(env, path) ; jobject statObj = (*env)->NewObject(env, statClass, ctorID) ; /* i don't copy the struct into the object here - hope this is right */ jint ret = (*env)->CallIntMethod(env, fs, getattrID, pathStr, statObj) ; /* don't copy back if call failed */ if (ret == OK) copyStatIn(statObj, stat) ; /* don't think we need to release local refs explicitly before detach */ (*jvm)->DetachCurrentThread(jvm) ; return ret ;
}
Does that make sense? This means defining one class per struct, plus one copy-in method and one copy-out method, as needed. No need to write 35 different classes!
I've assumed that, despite what you said, the const char* parameter isn't 'returned' in any sense.
tom
-- Know who said that? Fucking Terrorvision, that's who. -- D
> On 4 Jul, 03:34, adityar7 <adity...@gmail.com> wrote:
> > I'm a C programmer with limited Java experience, and need to use JNI > > to implement a library in Java on top of a C library > > The question, though, is how to deal with multiple parameters/return > > values of different types. Here is a sample C function specification:
> > int getattr (const char *, struct stat *)
> I recommend SWIG. Seewww.swig.org. This is what I used to interface > to a proprietary C library for a server that we wanted to be in java. > SWIG would take the prototype above and create corresponding java > classes for where there is no natural mapping, e.g it would create a > class for stat.
> Output from SWIG is in the form of two files, one is C glue to go into > a dynamic library, the other is the java class that contains all the > methods for the functions you want to call. Other java files are > created for the new classes it creates, like stat. Your java program > has to load the dynamic library first.
> Regards,
> Andrew Marlow
Sounds excellent. I have seen that term (SWIG) often but not being a regular Java programmer, didn't know what that means. I'll check it out and see how it goes. Thanks for pointing it out!
