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Most of the examples I've seen for JNI seem to allow access to 'top
level' instance variables like this:

public class HelloNative{

private String str;
private native void setObjLong();

public static void main(String[] args) {
HelloNative test = new HelloNative();
test.setObjLong();
System.out.println("The java s is: "+ test.str);
}
}

The string is created in the c code:
JNIEXPORT void JNICALL
Java_HelloNative_setObjLong (JNIEnv* env, jobject obj) {

/* get the class object */
jclass cls = (*env)->GetObjectClass(env, obj);
/* get the field id */
jfieldID fid = (*env)->GetFieldID(env, cls, "str",
"Ljava/lang/String;");
/* make new string */
jstring myStr = (*env)->NewStringUTF(env,"1234");
/* set the value */
(*env)->SetObjectField(env, obj, fid, myStr);
return 0;
}

This all works fine, the question is how do you get to user-defined
object's instanace variables?
So if I change the java bit thus:
public class HelloNative {

private MyClass myObj = new MyClass();
private native void setObjLong();

class MyClass{
private long address;
// getters and setters omitted
}

public static void main(String[] args) {
HelloNative test = new HelloNative();
test.setObjLong();
System.out.println("The java long is: "+
test.myObj.getAddress());
}
}

How can I set the address field of myObj?

If I try:
jfieldID fid = (*env)->GetFieldID(env, cls, "myObj",
"Ljava/lang/Object;");

I get:
java.lang.NoSuchFieldError: HelloNative.myObj

....and anyway this would only get me to the object and not to the
address instance which I wish to set.

Thanks,
Andy

On 2 Feb 2006 08:23:42 -0800, stixwix wrote:
> the question is how do you get to user-defined
> object's instanace variables?
>
> So if I change the java bit thus:
> public class HelloNative {
>
> private MyClass myObj = new MyClass();
> private native void setObjLong();
>
> class MyClass{
> private long address;
> // getters and setters omitted
> }
>
> public static void main(String[] args) {
> HelloNative test = new HelloNative();
> test.setObjLong();
> System.out.println("The java long is: "+
> test.myObj.getAddress());
> }
> }
>
> How can I set the address field of myObj?
>
> If I try:
> jfieldID fid = (*env)->GetFieldID(env, cls, "myObj",
> "Ljava/lang/Object;");
>
> I get:
> java.lang.NoSuchFieldError: HelloNative.myObj

Your call to GetFieldId() says "look in HelloNative for a field called
myObj whose type is java.lang.Object". But myObj in HelloNative is
clearly of type HelloNative$MyClass (because MyClass is an inner class
to HelloNative), not java.lang.Object.

"javap -s HelloNative" will show you the correct signatures to use for
all of the fields and methods in that class.

> ...and anyway this would only get me to the object and not to the
> address instance which I wish to set.

To create an instance of the inner class, look up that class, then
look up one of its constructors (realizing that there is a hidden
first argument, an object of the enclosing class) and use that to
invoke NewObject().

/gordon

--
[ do not email me copies of your followups ]
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Gordon Beaton wrote:
> To create an instance of the inner class, look up that class, then
> look up one of its constructors (realizing that there is a hidden
> first argument, an object of the enclosing class) and use that to
> invoke NewObject().
>
Thanks for that. I now get a little further. With regard to the clip
above, do i need to do this if I am passing an (instantiated) object
into the c code from the java code?

To get to the address field (String) in the MyClass, I was thinking the
following should work but it crashes nastily. (Note: I have now changed
this field in MyClass from long to String).

/* get the class object */
jclass cls = (*env)->GetObjectClass(env, obj);

/* get the myObj instance from HelloNative */
jfieldID fid = (*env)->GetFieldID(env, cls, "myObj",
"LHelloNative$MyClass;");

/* get the address field from MyClass */
jfieldID fidStr = (*env)->GetFieldID(env, fid, "address",
"Ljava/lang/String;");

It crashes when I introduce the last line.
There is a compiler warning:
"passing arg 2 of pointer to function from incompatible pointer type"

but I've no idea what I can pass to that function instead.

Thanks,
Andy

On 2 Feb 2006 10:32:31 -0800, stixwix wrote:
> With regard to the clip above, do i need to do this if I am passing
> an (instantiated) object into the c code from the java code?

Yes, it's perfectly reasonable to use the enclosing object ("this")
when you create instances of its inner class.

> /* get the address field from MyClass */
> jfieldID fidStr = (*env)->GetFieldID(env, fid, "address",
> "Ljava/lang/String;");
>
> It crashes when I introduce the last line.

You're attempting to find the fieldID by looking it up in another
fieldID, but you should be looking in a class instead.

Start with the class HelloNative$MyClass, and from there you can look
up its fields:

jclass my_cls = (*env)->FindClass(env,"HelloNative$MyClass");
jfieldID addr_fid = (*env)->GetFieldID(env,my_cls,"address",
"Ljava/lang/String;");

Now to actually get or change the *value* associated with this field,
you need an *object* of that type (i.e. one that holds the field you
just looked up), and you choose one of the Get/Set<type>Field()
functions based on the type of the field itself (i.e. one of the
primitive types, or Object for all others).

/gordon

--
[ 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

Gordon Beaton wrote:

> Start with the class HelloNative$MyClass, and from there you can look
> up its fields:
>
> jclass my_cls = (*env)->FindClass(env,"HelloNative$MyClass");
> jfieldID addr_fid = (*env)->GetFieldID(env,my_cls,"address",
> "Ljava/lang/String;");
>
> Now to actually get or change the *value* associated with this field,
> you need an *object* of that type (i.e. one that holds the field you
> just looked up), and you choose one of the Get/Set<type>Field()
> functions based on the type of the field itself (i.e. one of the
> primitive types, or Object for all others).

When I try the above with the following additional code - jstr is
NULL.:
jstring jstr = (*env)->GetObjectField(env, obj, addr_fid);
const char *str = (*env)->GetStringUTFChars(env, jstr, NULL);

All the examples I see use GetObjectClass instead of FindClass. I
can't find/understand any docs relating to FindClass, but it doesn't
imply object instance scope.

On 6 Feb 2006 02:57:01 -0800, stixwix wrote:
> When I try the above with the following additional code - jstr is
> NULL.:
> jstring jstr = (*env)->GetObjectField(env, obj, addr_fid);
> const char *str = (*env)->GetStringUTFChars(env, jstr, NULL);

It's hard to tell from these code fragments, but is "obj" really an
instance of the class containing the field you are looking up?

I.e. you looked up up a field ID in HelloNative$MyClass, so "obj" must
be a HelloNative$MyClass object (or it won't have that field).

/gordon

--
[ 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

[Varun Wadhwa]

Hi,
I need to access instance fields of a java class which is not the class from which i am calling the native function.So i cannot use the obj that is passed by default as second argument to the native function and use getfieldid(). This is the code structure:

class data
{
int a;
}
class ReadFile
{
private native void loadFile(data d);
static
{
System.loadLibrary("ReadFile");
}
public static void main(String args[])
{
data d= new data();
ReadFile mappedFile=new ReadFile();
mappedFile.loadFile(d);
}
}

I now want to access 'a' from native code.How can i do that. I have used findclass but it does not work, the field value shows 0 evenafter setting it to some value in native code.