Java SWT interop with COM - putting a float[] into a Variant?

Question

In my Java SWT application I'm hosting an 3rd party ActiveX control. I'm using OleClientSite to do this.

// Ah, this works. :-)
OleAutomation comObject = new OleAutomation(...);

There are 2 easy little functions I want to call from Java. Here are the COM function definitions:

[id(5)]
void easyFoo([in] int blah);

[id(20)]
void problemFoo([in] VARIANT floatArray);

Easy, right? Here's my pretend code:

// Ah, this works. :-)
OleAutomation comObject = new OleAutomation("Some3rdPartyControlHere");

// Call easyFoo(42). This works. :-)
int easyFooId = 5;
comObject.invoke(easyFooId, new Variant[] { new Variant(42) });

// Call problemFoo(new float[] { 4.2, 7.0 }). This doesn't work. :-(
int problemFooId = 20;
comObject.invoke(problemFooId, [ACK! What goes here?]);

The problem is on the last line: how do I pass a float array to the 3rd party COM object? HELP!

Answer 1

I suspect there is no constructor that takes a float[] because VARIANTs don't have a float array member.

I think what you need to do to make this work is pack up your floats into a SAFEARRAY (ick; and I have no idea how to create one in Java).

Alternatively, you may try serializing your array to raw bits and use the BYTE* member of the VARIANT struct, and pass an int that has the count of bytes so you can accurately de-serialize on the other side (and I assume this is all in the same process and thread, otherwise it gets harder).

[id(20)]
void problemFoo([in] VARIANT bytes /* VT_BYREF|VT_UI1 */, [in] VARIANT byteCount /* VT_UI4 */);

Answer 2

What's wrong with creating an array of Variant and filling it with your float array values?

Variant[] problemFooArgs = new Variant[myFloats.length]; 
for( int i=0; i<myFloats.length; i++)
{
        problemFooArgs[i] = new Variant(myFloats[i]);
}

If it really needs only one argument (an array of float), you could try one level of indirection:

Variant[] problemFooArgs = new Variant[1]; 
Variant[] myFooArgs = new Variant[1]; 
for( int i=0; i<myFloats.length; i++)
{
        myFooArgs [i] = new Variant(myFloats[i]);
}
problemFooArgs[0] = myFooArgs;

---

If the simple approach does not work and you do need a SAFEARRAY, you could try and create one after the example "Reading and writing to a SAFEARRAY", using the constants of org.eclipse.swt.internal.win32.OS. But it seems for char[] only.

Other source of inspiration for creating the relevant SAFEARRAY:

class SafeArray of project com4j (and its associated class, like Variant)

Answer 3

Hi Judah,

I’m encountering exactly the same problem as you were. I’ve spent few weeks searching for the solution, but couldn’t fine one. Please let me know if you were able to solve this problem. Thanks!

Answer 4

You need to pass a float array. In COM terms, that mean s a VARIANT with vt set to VT_R4|VT_ARRAY. An array of variants may not work as the document does not say it can accept an array of variants (VT_VARIANT |VT_ARRAY). In java you should be able to use float[] as the parameter type. If not you can always call the Windows API to construct a safe array of desired type.