COM allows client programs to access server programs, enabling integration between a variety of software components. Windows j.dll can be used as a calculation server by any COM client.
Servers provide functions, called methods, that can be accessed by a client. For example, the J server provides a method Do which executes a J sentence; and a method Get which retrieves the value of a variable. If a client runs the method Do'x=. 3+5', this causes the sentence to be executed in the J server. The client can then call method Get to retrieve the value of x.
COM Automation is an interface supported by many clients and servers. COM Automation gives a client an interface to a server that mimics the server's end-user interface. This allows a client to automate what would otherwise have to be done manually by a user. Although this is useful, the real power comes when the server has a general purpose programming environment that can be used to build complex services that can be used from a client. OLE Automation then provides easy access from the client to the full power of the server.
j.dll provides JDLLServer as a COM Automation server.