Where does J# fit in the .NET world?

Microsoft's J# enables Java developers to easily jump from Java to the .NET platform. Take an inside look at J#.

The disputes between Microsoft and Sun Microsystems are famous. These led to the dismissal of Java from the Microsoft platform (but you can still install it as an add-on) and Microsoft dumping its Java clone Visual J++ product.

Microsoft decided to take another approach by luring existing Java developers to the .NET platform. One component of this strategy is the J# programming language. Let's take a closer look at this language and the many associated Microsoft tools.

The Java clone

In late 2001, Microsoft announced its JUMP initiative. The campaign's goal was to enable existing Java developers to easily jump from Java to the .NET platform. J# was a key component in the campaign.

The J# syntax is strikingly similar to Java—it's even identical in many instances. The following code sample contains a simple Java console application for reading the contents of a URL:

public class URLReader {
public static void main(String[] args) throws Exception {
URL builder = new URL("");
BufferedReader in = new BufferedReader(new InputStreamReader(builder.openStream()));
String inputLine;
while ((inputLine = in.readLine()) != null)
} }

One amazing aspect of this code is that it compiles with both the Sun Java and .NET J# compilers. This example utilizes base Java packages.

However, you may run into problems when the application gets more complicated. Here's a brief list of what J# doesn't include:

  • The ability to create class bytecode files.
  • The ability to develop Java applets for browsers.
  • The ability to create applications that will run in a Java Virtual Machine (JVM).
  • Java Native Interface or Remote Method Invocation (RMI) support.
  • Support for the*,*, and netscape.* packages.

The .NET Framework includes tools for working with your Java/J# code.

J# toolset

The .NET Framework includes the following features to help with J# development:

  • J# compiler to convert your Java language source code to Microsoft intermediate language (MSIL)
  • A converter to convert Java-language bytecode into MSIL
  • Class libraries that provide some of the capabilities of the JDK 1.1.4 packages that are in Visual J++ 6.0 (Visit Microsoft's site for a more detailed explanation of what packages/classes are supported.)
  • WFC and many of the* packages (,,,

The J# compiler (vjc.exe) and bytecode converter (jbimp.exe) are located in the .NET Framework installation directory. The default directory path for these files is:


J# files are saved with the .jsl file extension, but the compiler doesn't restrict input to only these files. For this reason, you can easily compile a Java source file with it. For example, I compiled the Java sample using the following line:


The J# compiler contains numerous options that you may utilize with command-line switches. This list contains a sample of these options:

  • /o—enable compiler optimization
  • /debug—emit debugging information
  • /help—display help and description of available command-line options
  • /out—write compiled output to specified file

The default compilation output (this may be changed) is an .exe file containing MSIL. This executable may be run from the command line.

Another command-line tool allows you to convert a compiled Java file (bytecode) to its MSIL equivalent. This is useful when the Java source code isn't available.

The command-line is only one option for J# development. The Visual Studio .NET IDE provides full J# support, so it's possible to build a full-featured application (albeit ASP.NET, Windows Forms, command-line, Web service, and so forth) using J#.

J# seems to be a Java clone, but C# has been called this as well. The C# syntax closely resembles both J# and Java.

Another avenue

C# provides another approach for Java developers wanting to jump into .NET development. Microsoft offers the Java Language Conversion Assistant (JLCA). It's a tool that converts Java code into C# for developers who want to move existing applications to the .NET Framework. It provides a quick, low-cost method of converting Java-language applications to C#. The current version is 3.0 beta, and it offers the following features:

  • The conversion of J2EE 1.3 and JDK 1.3 libraries, as well as Enterprise Java Beans, Java Authentication and Authorization Service, Java Cryptography Extension, Java Message Service, Java Naming and Directory Interface, and RMI.
  • Deep support for migrating SWING applications.
  • JavaServer Pages (JSP) and servlet application conversion.

It's fully integrated with Visual Studio .NET, so the developer doesn't have to leave the comforts of the IDE to perform the conversion. The JLCA helps you leverage existing investments, thus avoiding some of the risks of new development and decreasing your time to market. Once the conversion is complete, you can immediately begin using the power of the .NET Framework and the component-oriented programming to extend the code.

Here's a quick example of the conversion. The following Java code is used as the input:

class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!"); //Display the string.
} }

The C# produced by JCLA follows:

using System;
public class HelloWorld {
public static void  Main(System.String[] args) {
System.Console.Out.WriteLine("Hello World!");
} }

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About Tony Patton

Tony Patton has worn many hats over his 15+ years in the IT industry while witnessing many technologies come and go. He currently focuses on .NET and Web Development while trying to grasp the many facets of supporting such technologies in a productio...

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