PRB: COM Objects are not Being Released in Java

• SDK for Java, versions 1.51, 2.0, 2.01
• Microsoft Visual J++, versions 1.0, 1.1

SYMPTOMS

When setting a COM object reference to null and possibly attempting to force garbage collection by calling gc(), COM objects may not be released in a timely or predictable fashion.

CAUSE

If you do not call ComLib.release, normal garbage-collection will attempt to perform the release for you. However, this mechanism is not guaranteed due to the threading limitations of many COM objects. That is, many COM objects can be called only on the thread on which they were created. Because garbage collection occurs at unpredictable times, the required thread may have expired or may be no longer responding to messages by the time garbage-collection reclaims the object. In addition, this unpredictability can obscure true memory leaks and/or tie up important system resources.

RESOLUTION

It is recommended that you use explicit releases by calling ComLib.release in order to free COM objects in a timely and predictable fashion.

MORE INFORMATION

For example, if you have the following code that creates a COM object in Java:

   import excel8.*;
   import com.ms.com.*;

   _Global globXL=null;
   _Application appXL=null;

   try
   {
      globXL = (_Global)new Global();
      appXL = (_Application)globXL.getApplication();
   }
   catch (ComFailException e)
   {
      System.out.println(e.getMessage());
   }

   try
   {
      ComLib.release(appXL);
      appXL=null;
      ComLib.release(globXL);
      globXL=null;
   }
   catch (ComFailException e)
   {
      System.out.println(e.getMessage());
   }

REFERENCES

Please see the SDK2.0x for Java documentation for more information.

For the latest Knowledge Base articles and other support information on Visual J++ and the SDK for Java, see the following page on the Microsoft Technical Support site:

   http://support.microsoft.com/support/visualj/
   http://support.microsoft.com/support/java/