4-JACOB Project mailing-list FAQ

JACOB Project mailing-list FAQ
Donated by Massimiliano Bigatti (http://www.bigatti.it, max@bigatti.it)

See “Credits” for the fundamental work of others.
Updated version at: http://danadler.com/jacob/jacobfaq.html
This is revision 0.2 - (20 Sept 2002)

Jacob related

What is Jacob?
The Jacob project is a generic COM bridge for Java pioneered by Dan Adler (http://danadler.com). It is licensed as open-source on sourceforge.net.

How do I do this/that in Word, Excel, Project, ... ?
Let’s clarify: Jacob is only a bridge, it is an ancronym for JAva COm Bridge. It only implements a generic bridge to call COM components from Java.
It is not the Java interface of Microsoft Office nor it supports directly its COM components. To know more on calling Office with Java, see the “Microsoft Office related” section of this FAQ.

Do I need to compile Jacob to use it?
No, you need only to install it. You can install it as an extension (see “What I should do if IIS/other crashed using Jacob?”) or as a part of your application. To do the latter:

• put jacob.dll in a directory that Windows searches for libraries (Windows/System32 - or add your own directory to the PATH system variable);
  
• put jacob.jar in your CLASSPATH.
  

I did not get jacob.dll when downloading jacob_xx.jar
Turn “Show hidden files” on in your Windows Explorer.

If I use Jacob in my application do I have to install jacob.dll on client side?
Yes, if Jacob is used in your client code.
No, if your code uses Jacob on the server side.
You only have to install Jacob on the computer where the process that makes direct use of Jacob is running.

I’m getting an “UnsatisfiedLinkException” on com.jacob.com... What should I do?
Check if jacob.jar is in your CLASSPATH. Try to move it at the beginning of the list, maybe your CLASSPATH is too long and Java is ignoring the latest jars.

What I should do if IIS/other crashed using Jacob?
Try to install Jacob as a JRE extension. To do this: put jacob.dll in jre/bin and jacob.jar in jre/lib/ext. The jre directory is under your jdk installation, or Programs/JavaSoft if you’re using only the runtime environment.
It might also be a threading issue, see (“What should I do if my program hangs after the code has called Jacob?”).

What should I do if my program hangs after the code has called Jacob?
Maybe you’re experiencing threading problems. Upgrade to Jacob 1.7 and use the ComThread class to implement COM threading manually. See COM Object Lifetime ( http://danadler.com/jacob/JacobComLifetime.html) in Jacob.

Can I call COM component across machines or operating systems?
The answer to this question is No. The way jacob is implemented is using JNI
and a c++ DLL designed for the windows platform. COM is a binary standard
defined by Microsoft as it hasn’t been ported over (as far as I know) to any
other platforms yet. Jacob is only a java bridge for COM which defines how
components talk to each other when they are on the same machine. The DCOM
standard defines how components communicate accross networks, but Jacob has no support for DCOM.
Solutions:

• Commercial products, like J-Integra support the DCOM protocol natively.
  
• Use RMI. Make a wrapper for the Jacob classes, and RMI'ify it. It actually works, but the Jacob lead didn't want to make the classes implement serializable, making the whole operation slightly difficult.
  
• Use Web Services. Expose the stuff you'd like to have as a Web Service, and use SOAP and snacks on the Java side to access it.
  

When I downloaded Jacob I didn’t get javadocs, where can I find these?
There are not javadocs available for Jacob classes. However, Jacob is almost 1:1 compatible with Microsoft COM access for Java, you can check out the MS docs at http://www.microsoft.com/java/download/dl_sdk40.htm.

When should I call ComThread.release() in a JSP/Servlet environment?
Why not make a "worker thread" that do all the COM invocations for you? So
that you in a way marshal your requests to this thread or threads. You'd
need some way of "job control", but this could be very simple depending on
what you need.


I’m getting “An unexpected exception has been detected in native code outside the VM.” error, using Jacob in a Web Application running on Tomcat as a service. What should I do?
frode@coretrek.no says: “Are your COM component Apartment threaded?

The problem with this kind of set up is that when the thread responsible
for hosting your COM component goes down, the apartment goes down as well
and your COM component becomes invalid.

I have noticed that for the very first thread in Java, the "MAIN" thread,
there is a corresponding Win32 thread, and When the MAIN thread exists,
the Win32 thread is still alive, as you can see if you use the Windows
Task Manager and enable the "Threads#" column. The thread count does not
decrease after the MAIN exists. (When any other thread exits,
the thread count decrease).

When you run as an NT service, your entry point where you probably
create your object is _not_ the first (win32) thread that never really exits.
Does this make sense? Anyway, I solved this problem by making sure
that the thread in which you create the Dispatch object never exists.

Try adding the following code to the "end" of the thread where you
create your object to prevent the thread from exiting:

    Object o = new Object();

    synchronized(o) {

        while (true) {

            try {

                o.wait();   //  hang around forever

            } catch (InterruptedException e) {}

        }

    }

Hope this helps!”

COM related

How do I call a Visual Basic/COM component/DLL from Java?
You have to instantiate the COM component using either the ActiveX or Dispatch classes. When you have the object instantiated, you can call Dispatch.call() to invoke a single COM method passing the parameters the method needs. You can check out samples for this in the Jacob distribution (in the “samples” directory), the simplest (if you have MS Access installed) would probabily be Access.java

How do I setup a Jacob installation and make sure it works?
Try the following:
1) unzip jacobSrc.zip - the bundle downloaded from http://danadler.com/jacob.
2) cd to <jacob_unzipped>/samples/test
3) run prompt> set path=%path%;./MathProj
4) run prompt> regsvr32 MathProj/MathTest.dll
5) run prompt> javac -classpath ../../jacob.jar math.java
6) run prompt> java -classpath ../../jacob.jar;. math

What is the correct way to pass optional parameters?
Use the variant.noParam(), that makes it as if that argument is skipped.

How do I display a Visual COM component?
You can’t. Under “Limitations” on the Jacob homepage you can read “This release does not provide any support for hostng graphical ActiveX controls inside of Java components”.
You need to refer to other (commercial) products, like: Linar/Intrinsyc’s JIntegra suite (http://www.intrinsyc.com/products/bridging/jintegra.asp), and GenSym (note GenSym (http://www.gensym.com) has dropped support for their product)
There is also a (http://www.nothome.com/IECanvas/) project to run Internet Explorer inside a Java application, although I don’t know how stable it is.

I’m getting a “co-create ComFailException”, what should I do?
Maybe you need to register your DLL on the system, or you mispelled the name in the Java source code. To register a DLL use “REGSVR32 <filename>”. REGSVR32 ships with every copy of Windows. It should pop-up a message box saying the component was registered successfully.

I’m getting a “ComFailException: Can’t map name to dispid”, what should I do?
If the exception says “com.jacob.com.ComFailException: Can't map name to dispid”: maybe you’re trying to call method that didn't exist in the library...

Are COM Dates compatible with Java Dates? How can I convert one to each other?
You can use this http://groups.yahoo.com/group/jacob-project/message/97code. Jacobgen wrappers should handle this conversion automatically.

On ComThread.release() I get javaw.exe Application Error, The istruction at xxx referenced memory at 0x00000. What should I do?
That sometimes happens when your java app attempts to exit before COM
is finished. Try adding a sleep of a couple of seconds before the
java application exits. That usually cures it.

How do I create a COM component that will be called from Jacob?
You have to implement more functions and export them, preferably using
a .def file.

Building COM objects with C++ is more involved than I can go into
here. I advice you to get a decent book on creating COM objects in
C++ like Inside DCOM from Microsoft Press if you want to travel that
road. Tools like ATL generate the boilerplate code for you.

I think it would be easier if you used some high level language to
create COM components. Python would be my choice, an easy to use open
source scripting language (http://www.python.org), VB is very popular
but also expensive and proprietary, but maybe the easiest way to make
COM components.

Microsoft Components/Office rela