Other Technologies for Interoperability

There are several other methods used in different scenarios that can be used easily to allow interoperability between Java and .NET applications.

Data Interoperability

The state and data around applications can be stashed in a database from which either application can read according to defined standards. For example a Java-based application can write information to a database table, and a .NET-based application detecting an update to the table can read the values and establish its own state. It isn't a very efficient methodology but can be used where other methods fail or when the information needs to be stored anyway.

Presentation Layer Interoperability

In many cases, interoperability is a necessity when merging existing applications, as opposed to simply integrating them. In many cases, these applications may have had their presentation layers implemented using different technologies such as ASP.NET Web forms and JSP pages. There are few technology overlaps between the two that would allow for effective interoperability (for example page state or application state cannot be shared), but HTML/HTTP standards facilitate interoperability. For example, HTTP POST calls are standardized and handled in a standard way, so either can POST messages to each other and have them transparently managed.

Code Level Interoperability

A product called Visual MainWin for Java EE (VMW4JEE) from Mainsoft is an innovative solution for porting code that also allows unique levels of interoperability. It is covered in more detail in Chapter 16, "Migrating .NET Applications to Java." VMW4JEE is a plug-in for Visual Studio.NET that allows the developer to code and debug in C# or VB.NET and compile it to Java bytecode for deployment to a Java EE application server. It also allows for proxy generation to Java classes (in JAR files) or to Enterprise Java Beans (EJBs) so that they can be invoked directly from .NET, adding a whole new layer of interoperability. Functionality that is defined within a Java application running on Java EE now doesn't need to have an interface specifically developed to allow for interoperability, with the typical case being that it is wrapped by a Web service. Instead, the compiled Java classes can be imported to the C# project, VMW4JEE generates proxies for calling them, and the .NET coder can simply use them.