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<H1>COM Apartments in JACOB</H1>
<p>
<H2>introduction</H2>
<p>
The COM model for Threading differs from the Java model.
In COM, each component can declare whether or not it support
multi-threading.
You can find some basic information about COM threading at:
<p>
<a href="http://www.execpc.com/~gopalan/com/com_threading.html">
http://www.execpc.com/~gopalan/com/com_threading.html</a>
<p>
<a href="www.microsoft.com/msj/0297/apartment/apartment.htm">
www.microsoft.com/msj/0297/apartment/apartment.htm</a>
<p>
<a href="http://www.cswl.com/whiteppr/white/multithreading.html">
http://www.cswl.com/whiteppr/white/multithreading.html
</a>
<p>
The term <b>Single Threaded Apartment (STA)</b> refers to a thread
where all COM objects created in that thread are
single-threaded. This can manifest itself in two ways:
<br>
Either all calls into that component are made from the same thread
that created the component
<br>
OR any call that is made from another thread gets serialized by COM.
This serialization of calls is done by using a Windows message loop and
posting messages to a hidden window (I'm not kidding). The way COM
achieves this is by requiring any other thread to make calls through
a local Proxy object rather than the original object (more on this
when we discuss the JACOB DispatchProxy class).
<p>
What does this mean for a Java application? If you are using a component
that declares itself as <b>ThreadingModel "Apartment"</b> (you can
find this out by looking in the registry under its CLSID), and you plan
to create, use and destroy this component in one thread - then you are
following the rules of an STA and you can declare the thread as an
STA thread.
<p>
On the other hand, if you need to make method calls from another thread
(e.g. in a servlet) then you have a few choices. Either you
create the component in its own STA, by extending
<code>com.jacob.com.STA</code>, and use the
<code>com.jacob.com.DispatchProxy</code> class to pass the Dispatch
pointer between threads, or you can declare your thread as an MTA
thread. In that case, COM will make
the cross-thread calls into the STA that is running your component.
If you create an Apartment threaded component in the MTA,
COM will automatically create an STA for you and put your
component in there, and then marshall all the calls.
<p>
This brings us to the notion of a <b>Main STA</b>. COM requires that
if there is any Apartment threaded component in your application, then
the first STA created is tagged as the <b>Main STA</b>. COM uses the
Main STA to create all the Apartment threaded components that are
created from an MTA thread. The problem is that if you have already
created an STA, then COM will pick that as the Main STA, and if you
ever exit that thread - the whole application will exit.
<H2>COM Threads in JACOB Prior to Version 1.7</H2>
<p>
Up until version 1.7 of JACOB, there was only one model available
in JACOB:
<ul>
<li>
Before version 1.6: All threads were automatically initialized as STAs.
</li>
<li>
In version 1.6: All threads were automatically initialized as MTAs.
</li>
</ul>
<p>
The reason for the change in default was that tagging a Java thread
as an STA can cause problems. Any Java Swing application, as well as
servlets and applets need to be able to make calls from multiple
threads. If you try to make COM method calls across STA threads - it
will fail!
<p>
In most cases, the default chosen by JACOB 1.6 (MTA) works fine, however
there are some notable exceptions that have caused people grief. One
such exception is in the case of MAPI. It turns out that if you try to
create a MAPI object from an MTA thread - it simply fails and exits.
This has caused some people to recompile JACOB 1.6 back with the STA
default.
<p>
There is another problem with MTA threads: when you are using Apartment
threaded components, we already noted that COM will create the
components in the Main STA. If one doesn't exist, COM will create it.
However, this means that <b>all</b> Apartment threaded components will
be created in the <b>same STA</b>. This creates a bottleneck, and a
dependency between unrelated components. Also, if that STA exits, then
all components are destroyed and the application will likely crash.
<H2>COM Threads in JACOB Version 1.7</H2>
<p>
In Version 1.7 we have added finer grained control to allow the
Java programmer to control how COM creates its components.
Unfortunately, this means that you need to have a pretty good
understanding of the dark and mystical subject of COM Apartments.
There are a few different cases you need to consider:
<H3>Default</H3>
<p>
If you simply run code that was created in Version 1.6 and ignore
the COM threading issue, then you will get the same behavior as in 1.6:
Each java thread will be an MTA thread, and all Apartment threaded
components will be created by COM in its own Main STA. This typically
works for most applications (exceptions noted above).
<H3>Create Your Own Apartment</H3>
<p>
To declare an MTA thread use the following template:
<br>
<pre>
<code>
ComThread.InitMTA();
...
...
ComThread.Release();
</pre>
</code>
<br>
If you want JACOB to create its own Main STA (rather than having COM
choose an STA for you), then you should use:
<br>
<code>
<pre>
Thread 1:
ComThread.InitMTA(true); // a true tells JACOB to create a Main STA
...
...
ComThread.Release();
...
Thread 2:
ComThread.InitMTA();
...
...
ComThread.Release();
...
...
ComThread.quitMainSTA();
</pre>
</code>
<br>
In this case, you can also create the Main STA explicitly:
<br>
<code>
<pre>
ComThread.startMainSTA();
...
...
Thread 1:
ComThread.InitMTA();
...
...
ComThread.Release();
...
Thread 2:
ComThread.InitMTA();
...
...
ComThread.Release();
...
...
ComThread.quitMainSTA();
</pre>
</code>
<br>
In the latter case, all Apartment threaded components will be created in
JACOB's main STA. This still has all the problems of components
sharing the same Main STA and creating a bottleneck. To avoid that,
you can also create STA threads yourself:
<br>
<code>
<pre>
ComThread.startMainSTA();
...
...
Thread 1:
ComThread.InitSTA();
...
...
ComThread.Release();
...
Thread 2:
ComThread.InitMTA();
...
...
ComThread.Release();
...
...
ComThread.quitMainSTA();
</pre>
</code>
<br>
In this example, thread 1 is an STA and thread 2 is an MTA. You could
omit the call to ComThread.startMainSTA(), but if you do, then COM will
make the first STA your main one, and then if you exit that thread,
the application will crash.
<p>
Actually, Thread 1 is <i>almost</i> an STA. It's lacking a windows
message loop. So, this type of STA is fine as long as you are creating
a component and using it in the same thread, and not makind event
callbacks.
<H3>JACOB's STA Class</H3>
<p>
If you want to create an true STA where you can create a component and
then let other threads call methods on it, then you need a windows
message loop. JACOB provides a class called:
<code>com.jacob.com.STA</code> which does exactly this.
<code>
<pre>
public class com.jacob.com.STA extends java.lang.Thread
{
public com.jacob.com.STA();
public boolean OnInit(); // you override this
public void OnQuit(); // you override this
public void quit(); // you can call this from ANY thread
}
</pre>
</code>
<p>
The STA class extends
<code>java.lang.Thread</code> and it provides you with two methods
that you can override: <code>OnInit</code> and <code>OnQuit</code>.
These methods are called from the thread's <code>run</code> method
so they will execute in the new thread. These methods allow you to
create COM components (Dispatch objects) and release them.
To create an STA, you subclass it and override the OnInit.
<p>
The <code>quit</code> method is the <b>only</b> other method that
can be called from any thread. This method uses the Win32 function
<code>PostThreadMessage</code> to force the STA's windows message loop
to exit, thereby terminating the thread.
<p>
You will then need to make calls into the component that is running
in the STA thread. If you simply try to make calls from another thread
on a Dispatch object created in the STA thread, you will get a COM
Exception. For more details see:
<a href="http://www.develop.com/effectivecom">
Don Box 'Effective COM' Rule 29</a>: Don't Access raw
interface pointers across apartment boundaries.
<H3>The DispatchProxy Class</H3>
Since you cannot call methods directly on a Dispatch object created
in another STA JACOB provides a method for the class that created
the Dispatch object to marshal it to your thread. This is done via
the <code>com.jacob.com.DispatchProxy</code> class.
<code>
<pre>
public class DispatchProxy extends JacobObject {
public DispatchProxy(Dispatch);
public Dispatch toDispatch();
public native void release();
public void finalize();
}
</code>
</pre>
<p>
This class works as follows: the thread that created the Dispatch
object constructs an instance of DispatchProxy(Dispatch) with the
Dispatch as a parameter. This instance can then be accessed from
another thread, which will invoke its <code>toDispatch</code> method
proxy as if it were local to your thread. COM will do the inter-thread
marshalling transparently.
<p>
The following example is part of samples/test/ScriptTest2.java in the
JACOB distribution. It shows how you can create the ScriptControl
in one STA thread and make method calls on it from another:
<code>
<pre>
import com.jacob.com.*;
import com.jacob.activeX.*;
class ScriptTest2 extends STA
{
public static ActiveXComponent sC;
public static Dispatch sControl = null;
public static DispatchProxy sCon = null;
public boolean OnInit()
{
try
{
System.out.println("OnInit");
System.out.println(Thread.currentThread());
String lang = "VBScript";
sC = new ActiveXComponent("ScriptControl");
sControl = (Dispatch)sC.getObject();
// sCon can be called from another thread
sCon = new DispatchProxy(sControl);
Dispatch.put(sControl, "Language", lang);
return true;
}
catch (Exception e)
{
e.printStackTrace();
return false;
}
}
public void OnQuit()
{
System.out.println("OnQuit");
}
public static void main(String args[]) throws Exception
{
try {
ComThread.InitSTA();
ScriptTest2 script = new ScriptTest2();
Thread.sleep(1000);
// get a thread-local Dispatch from sCon
Dispatch sc = sCon.toDispatch();
// call a method on the thread-local Dispatch obtained
// from the DispatchProxy. If you try to make the same
// method call on the sControl object - you will get a
// ComException.
Variant result = Dispatch.call(sc, "Eval", args[0]);
System.out.println("eval("+args[0]+") = "+ result);
script.quit();
System.out.println("called quit");
} catch (ComException e) {
e.printStackTrace();
}
finally
{
ComThread.Release();
}
}
}
</pre>
</code>
<p>
You can try to modify the <code>Dispatch.call</code> invocation in
the main thread to use <code>sControl</code> directly, and you will see
that it fails. Notice that once we construct the ScriptTest2 object
in the main thread, we sleep for a second to allow the other thread
time to initialize itself.
<p>
The STA thread calls <code>sCon = new DispatchProxy(sControl);</code>
to save a global reference to the DispatchProxy that represents the
<code>sControl</code> object. The main thread then calls:
<code>Dispatch sc = sCon.toDispatch();</code> to get a local Dispatch
proxy out of the DispatchProxy object.
<p>
At most <b>one(!)</b>
thread can call toDispatch(), and the call can be made only once.
This is because a IStream object is used to pass the proxy, and
it is only written once and closed when you read it.
If you need multiple threads to access a Dispatch pointer, then
create that many DispatchProxy objects. For more details please
refer to the Don Box reference above.
<H3>Recommended Procedure</H3>
<ul>
<li>
It is recommended that you always allow JACOB to manage the main STA
rather than letting COM create one on its own or tag one of yours.
</li>
<li>
Declare an STA thread using ComThread.InitSTA()
if all your
method calls for that component are going to come from the same thread.
</li>
<li>
If you want an STA thread that allows other threads to call into it,
use the <code>com.jacob.com.STA</code> class as outlined above.
</li>
<li>
If you have a COM component that declares its ThreadingModel as
"Free" or "Both", then use the MTA.
</li>
<li>
In most cases, if you need to make method calls from multiple threads,
you can simply
use MTA threads, and allow COM to create the components in
the Main STA. You should only create your own STA's and DispatchProxy
if you understand COM well enough to know when the MTA solution
will fail or have other shortcomings.
<p>
There are 3 examples in the samples/test directory that demonstrate
these cases:
<p>
ScriptTest.java - creates an STA for the ScriptControl component and
runs all its method calls from that STA.
<p>
ScriptTest2.java - creates a separate STA thread, and makes
method calls into the component from another thread using DispatchProxy.
<p>
ScriptTest3.java - creates a separate MTA thread, and makes method
calls into the component from another MTA thread. This is simpler
than ScriptTest2 for most applications.
<p>
<h3>Default Threading Model</h3>
If you create a new thread, and don't call
<code>ComThread.InitSTA()</code> or <code>ComThread.InitMTA()</code>
on it, then the first time your java code creates a JacobObject, it
will try to register itself with the ROT, and when it sees that the
current thread is not initialized, it will initialize it as MTA.
This means that the code to do this is no longer inside the native
jni code - it is now in the <code>com.jacob.com.ROT</code> class.
For more details on the ROT, see the
<a href="JacobComLifetime.html">Object Lifetime</a> document.