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Integrate your legacy applications with JNI

Learn how to integrate a legacy accounting application and a new sales force automation program using JNI

By Michael L. Perry and Itworld.com, JavaWorld.com, 03/16/01

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Page 3 of 5

Set up the environment

Now that we have the technology assessment and information models in place, we can assemble our development environment. (See Resources to download sample code.) Please note that the attached sample code does not implement Aphid's client/server behavior, its disconnected operation, or its XML transactions. In the sample, both the Aphid GUI and Beetle UI run on the same machine, which would not hold true in actual production.

We now identify four target modules. Both applications run independently, so they are represented as top-level modules, Aphid\App.class and BeetleUI.exe. To access the native code from a standalone Java program, JNI loads a DLL, and the Beetle business logic is extracted to a separate module, BeetleBL.dll. To localize the JNI support, we create a wrapper module, BeetleJNI.dll. The module relationship diagram appears in Figure 3. Notice that only the Beetle business logic module has direct access to the database.

Figure 3: Module relationship diagram
for both Aphid and Beetle. Beetle's busi
ness logic is extracted so that both appli-
cations can share the code.



Because Beetle was originally written in C, we will use C for both BeetleUI.exe and BeetleBL.dll. However, JNI code is slightly more readable when written in C++, so we will use C++ for BeetleJNI.dll. (I used Microsoft Visual C++ 6.0 and Borland JBuilder Foundation 3.5 to create those modules, though I attempted to avoid anything compiler-specific. You will need a C/C++ compiler, a Java 1.2.2 compiler and VM, and ODBC. Create an ODBC data source named BEETLE, which references the database "Beetle.mdb" included in the sample source code.)

Since we are starting with a legacy application, we already have a module called Beetle.exe. First, we want to rename that project BeetleUI.exe to reflect the new separation of user interface from business logic, and then we want to create empty projects for each of the other modules. We design a directory structure that separates C code from Java code, Java source files from Java class files, and the various C modules from the database (please see sample code).

One important consideration when developing a JNI project is that JNI searches for native modules within CLASSPATH. Therefore, we must create a separate directory called devpath for all the C modules and put it in our PATH environment variable. We then instruct the C/C++ compiler to write all output files into devpath. A quick reboot makes the new PATH take effect, and now we are ready to code.

Connect the modules

First, to instruct JNI to load our new module, we must place a call to the static function System.loadLibrary in an appropriate place. Because the class Beetle.Company is the top of the JNI-specific code, putting the call in its static initializer makes sense. The code is as follows: 

public class Company {
  ...
  static {
    System.loadLibrary("BeetleJNI");
  }
  ...
}


The Java runtime searches for BeetleJNI.dll in CLASSPATH and automatically matches up all native methods. But wait -- we haven't written any native methods.

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