Chapter 7: Using Java and COBOL Together

This chapter introduces the different ways that Java and COBOL applications can call each other.


There are a number of ways of using COBOL and Java together. You can use the Interface Mapping Toolkit, the wizards and GUI tools to create interfaces to COBOL from Java, or you can hand code the calls using the supplied support libraries. Java itself provides access to other technologies, such as Remote Method Invocation (RMI) and Java Naming and Directory Interface (JNDI) which enable you to distribute objects across different machines.

You can use COBOL and Java together by:

Although Enterprise Server for Windows is required to run legacy programs exposed using the Interface Mapping Toolkit, you can choose to use Enterprise Server for Windows or Application Server for Net Express (or for Server Express) for running the other types of applications that use both COBOL and Java programs.

The Java language defines its own data types, which are different to the ones used in COBOL. The COBOL run-time system automatically converts between COBOL and Java types whenever you call Java from COBOL or COBOL from Java. Where a COBOL data type cannot be translated directly (for example, edited fields), it is translated to a string. Where a COBOL data type cannot be translated directly (for example, edited fields), it is under some circumstances translated to a string. See the chapter Java Data Types

Setting Up the Environment for Java and COBOL

You need a Java run-time system on any machine which is going to execute Java applications. If you are going to develop mixed Java and COBOL applications, you will also need a Java development environment. You can use either the Java Software Development Kit available from Sun, or any Java IDE which is based on either the Sun or Microsoft run-time environments listed below.

Net Express currently supports the following Java run-time systems on Windows:

Before you start writing COBOL and Java programs which interact, you need to set up the following environment variables for the COBOL and Java run-time systems:

Compiling COBOL Programs that Call Java

Any COBOL program which is going to call Java must be compiled with the following directive:


This does two things:

Compiling COBOL Programs that are used from the CobolBean interface

Any COBOL program which is going to be called from Java using CobolBean.cobcall*() methods methods should be compiled with the DATA-CONTEXT compiler directive. This enables the runtime system to create new application storage areas for each instance of a CobolBean that is created.

Compiling Java Programs in Net Express

When you use the Net Express IDE to create OO COBOL classes for use with Java, the Class Wizard adds the Java wrapper classes for OO COBOL to your Net Express project. The Java classes are compiled by the IDE when you rebuild your project. To set up this support edit mfj.cfg, in your net express\base\bin directory, to contain the full path and filename of your Java compiler.

If your net express\base\bin directory does not contain a copy of mfj.cfg, you will need to create it. Net Express creates mfj.cfg file the first time you compile a Java program in the Net Express IDE, providing that it can find the location of a Java compiler in your PC's registry or on the PATH environment variable.

In addition to specifying the Java compiler to use, you can use mfj.cfg to specify any Java compiler command line arguments. If you edited mfj.cfg to contain the following:

d:\jdk\bin\javac.exe -verbose

every time you compiled a Java program from the Net Express IDE, Net Express would use javac.exe in the d:\jdk\bin directory, specifying the -verbose option.

Linking to the Multi-threaded Run-time System

All COBOL programs for use with Java must be linked with the multi-threaded run-time system.

On Net Express, click Multi-threaded on the Link tab of the Project Build Settings dialog box. If you are debugging programs, click Settings on the Animate menu, and check Use multi-threaded runtime.

Unsupported JVM Errors

When you call COBOL from Java, the OO COBOL Java support loads one of several cbljvm_*.dll modules to interface to the Java Virtual Machine (JVM). The exact file loaded depends on the JVM you are using, and is selected by querying the name of the JVM the Java program is running under.

If your JVM is not one of those listed as supported, you will get a fatal "Unsupported JVM" error. You can force the loading of a particular JVM by setting a Java system property, called com.microfocus.cobol.cobjvm, to the name of one of the supported JVMs. For example, to load the support for the Sun JVM, contained in cbljvm_sun.dll, set this property to "sun".

You can set this property on the command line that runs your Java program, as follows:

java -Dcom.microfocus.cobol.cobjvm=name class

where name is the name of the support module you want to use. For example, to run Java program myclass with the support for the Sun JVM:

java -Dcom.microfocus.cobol.cobjvm=sun myclass

If you get the "unsupported JVM" error, try the Sun JVM first as many JVMs are rebadged from Sun.

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