Dispatch not hooked to windows memory

Question

Sometimes I get an error (exception):

java.lang.IllegalStateException: Dispatch not hooked to windows memory

What does it mean? How to prevent it?

This is a sample code that results in this error:

import com.jacob.activeX.*;
import com.jacob.com.*;

public class Hooked {

  public static void main(String[] args) {
    ActiveXComponent e = new ActiveXComponent("Excel.Application");
    ActiveXComponent sh = new ActiveXComponent(
      e.getProperty("ActiveSheet").toDispatch());
    System.out.println(sh.getPropertyAsString("Name"));
  }

}

Answer 1

That means that Dispatch = nothing using vba syntax, it's empty. The same dispatch that you receive with new Dispatch(). Unfortunately Jacob 1.17 does not provide any method to explicitly check whether the Dispatch is empty or not. So I see 3 possible solutions:

1) Use Variant.isNull() after receiving the Variant from COM call, before converting it to Dispatch. So it requires 1 additional line:

  Variant vsh = e.getProperty("ActiveSheet");
  if (vsh.isNull()) {
    System.out.println("Null dispatch received.");
  }
  ActiveXComponent sh = new ActiveXComponent(vsh.toDispatch());

2) Catch IllegalStateException when using suspected Dispatch for the first time.

3) Write a custom isNull(Dispatch d) function

public static boolean isNull(Dispatch d)
{
  try {
    Dispatch.call(d, "");
  }
  catch (IllegalStateException ise) {
    return true;
  }
  catch (ComFailException cfe) {
    // that's ok, we didn't expect this call to succeed
  }
  return false;
}

In the specific example in the question the call was just a mistake, because Excel has no ActiveSheet if no workbook is open or created.