2. Getting Started with GNAT-AJIS

This chapter summarizes GNAT-AJIS’s basic capabilities and illustrates how to use the GNAT-AJIS tools for some simple applications.

2.1. Introduction

GNAT-AJIS (GNAT Ada-Java Interfacing Suite) is a collection of GNAT add-on tools for developing mixed-language Ada / Java applications where the Java components run on a JVM and the Ada components are compiled natively. Through GNAT-AJIS you can realize the following scenarios:

  1. In a Java application, invoke subprograms from natively-compiled Ada packages (i.e., either interface with an existing Ada API, or implement Java native methods in Ada);
  1. In a natively compiled Ada program, access methods and fields from Java classes or objects.

GNAT-AJIS addresses these scenarios through an Ada binding to the JNI services and ‘binding generator’ tools that automate the generation of the necessary ‘glue code’:

ada2java

Takes an Ada package specification as input and produces one or more Java classes, with native methods corresponding to the Ada subprograms. This allows you to call Ada from Java.

2.2. GNAT-AJIS Installation Structure

Installing the GNAT-AJIS tools results in the following directory structure. [1]

$GNATAJIS_INSTALL_DIR/
   bin/
      ada2java (Solaris, Linux) or ada2java.exe (Windows) -- executable
   include/
      ajis/
         Various `.ads` and `.adb` files
      gnatjni/
         Various `.ads` and `.adb` files
   lib/
      libajis.so (Solaris, Linux) or ajis.dll (Windows)
      libgnatjni.so (Solaris, Linux) or gnatjni.dll (Windows)
      ajis.jar
      ajis/
         Various `.ali` files
      gnat/
         Various files
      gnatjni/
         Various `.ali` files

2.3. GNAT-AJIS / GNAT Compatibility

ada2java is based on ASIS and requires a compatible version of the GNAT compiler. To check the status of your installation, run ada2java with the -v switch; this will indicate the version of the GNAT compiler that was used to build the GNAT-AJIS suite. Your GNAT-AJIS installation is compatible with that GNAT version:

  • Running ada2java requires using that specific GNAT version;
  • On the other hand, the generated Ada files may be compiled with that version or any later one.

The gnatjni and ajis libraries have been prebuilt for a specific version of GNAT. If you need to compile them for some other version of GNAT, you can rebuild the libraries manually:

gprbuild -P ajis.gpr -XExternal_Build=false -XObject_Dir=<some-dir>

where <some-dir> is a local directory where the temporary objects will be placed.

2.4. A Simple Example: Calling Ada from Java

This section illustrates how to invoke an Ada subprogram (compiled natively) from Java running on a JVM. In summary, the steps are as follows:

  • Make sure that the relevant environment variables are properly defined.
  • Write a package specification for the subprogram(s) to be called from Java, and a corresponding package body.
  • Invoke the GNAT-AJIS tool ada2java on the Ada package spec, to produce the corresponding Java classes (source files) and the necessary JNI ‘glue’ code (additional Ada source files). Providing the -L libname switch will cause a project file to be generated, which will help to automate some of the processing.
  • Invoke the Java compiler javac on the Java source files;
  • Invoke gprbuild on the project file generated by ada2java; this will compile the Ada files into a shared library (Solaris, Linux) or dll (Windows);
  • Invoke the Java interpreter to run a Java main class that invokes methods from the Java classes generated by ada2java.

These steps will now be described in detail.

2.4.1. Environment Setup

Since you will be using both the Ada and Java toolsets, you need to ensure that several environment variables are set. You can automate this step by defining these variables in a shell script / batch file. For convenience you will also find it useful to define an environment variable that ‘points to’ the root directory for the GNAT-AJIS tool installation. The description below assumes that GNATAJIS_INSTALL_DIR has this role.

PATH

Must contain the directories for the GNAT tools and for the GNAT-AJIS tools. The latter will be in the $GNATAJIS_INSTALL_DIR/bin directory. On Windows, it needs to contain the directory where the shared libraries are generated, typically ./lib although you can override this.

LD_LIBRARY_PATH

On Solaris and Linux, must contain the directories where your native libraries will reside (generally the ./lib subdirectory). This variable is not needed on Windows.

CLASSPATH

Must contain $GNATAJIS_INSTALL_DIR/lib/ajis.jar, which is the parent directory of the com.adacore.ajis Java package.

ADA_PROJECT_PATH

Must contain $GNATAJIS_INSTALL_DIR/lib/gnat, the directory that holds the GNAT project files needed for building applications with GNAT-AJIS.

2.4.2. An Ada Package

Assume that you would like to invoke an Ada procedure that displays the text Hello from Ada, followed by an integer value passed to Ada from Java. Declare a procedure Hello in a package spec Hello_Pkg (file hello_pkg.ads) and implement the body (file hello_pkg.adb):

package Hello_Pkg is
   procedure Hello (Item : in Integer);
end Hello_Pkg;

with Ada.Text_IO; use Ada.Text_IO;
package body Hello_Pkg is
   procedure Hello (Item : in Integer) is
   begin
      Put_Line("Hello from Ada: " & Integer'Image(Item));
   end Hello;
end Hello_Pkg;

2.4.3. Invoking ada2java

Change to the directory containing the Ada source files, and invoke the command

ada2java hello_pkg.ads -L hello_proj

This will generate a number of files and directories, including:

Hello_Pkg/
   Hello_Pkg_Package.java

Ada2Java/
   Library.java

hello_proj.gpr

Specs and bodies for the JNI_Binding package hierarchy

These have the following significance:

Directory Hello_Pkg
In the absence of an option that specifies the output directory for the generated Java file, ada2java creates a new directory with the same name as the Ada input unit and places the Java file in this directory.
File Hello_Pkg_Package.java

ada2java generates a Java source file with native method(s) corresponding to the visible subprogram(s) in the Ada package. (In general ada2java may generate several Java source files, based on the contents of the Ada package spec. In this example only one Java file is produced.) The name of this file is the same as the Ada unit, with _Package appended (since the input file is a package, rather than a procedure or function). The casing of the file name is the same as that specified on the Ada unit declaration.

Ada parameters are mapped to Java types; here Ada’s Integer corresponds to the Java type int.

In skeletal form, here is the Java class that is generated:

package Hello_Pkg;

public final class Hello_Pkg_Package {

   static public void Hello (int Item){...}
   ...
}

Directory Ada2Java and file Library.java

ada2java generates the boilerplate file Library.java to automate the library load step.

File hello_proj.gpr
This is a GNAT project file that automates building the application and loading the dynamic library.

ada2java generates also hello_proj.source_dirs.txt and hello_proj.library_interface.txt files to allow building custom hello_proj.gpr file when the generated file requires modifications.

Specs and bodies for the JNI_Binding package hierarchy
These files provide various ‘boilerplate’ packages as well as the package containing the ‘glue code’ procedure whose signature complies with the required JNI protocol and which invokes the Hello procedure supplied in the original Hello_Pkg package.

2.4.4. Compiling the Java class

Invoke the Java compiler on the generated Java class:

$ javac Hello_Pkg/Hello_Pkg_Package.java

This will generate the classfile Hello_Pkg_Package.class in the Hello_Pkg directory.

2.4.5. Building the Application

Run gprbuild, using the project file generated by ada2java at an earlier step:

$ gprbuild -p -P hello_proj.gpr

This will generate a dynamic library – libhello_proj.so (Solaris, Linux) or hello_proj.dll – in the subdirectory ./lib of the current directory, and will produce the necessary object files in the ./obj subdirectory. The two subdirectories will be created if they do not already exist.

The dynamic library will be loaded automatically at run-time, by one of the generated Java classes.

2.4.6. Running the Program

Write a main Java class, for example a file Hello.java:

import Hello_Pkg.Hello_Pkg_Package;

public class Hello{
    public static void main(String[] args){
       Hello_Pkg_Package.Hello(100);
    }
}

Compile this class:

$ javac Hello.java

Run the Java program:

$ java Hello

This will produce the following output:

Hello from Ada: 100

Note that the library produced earlier must be locatable when the program is executed. On Solaris and Linux the directory containing the library would be specified by LD_LIBRARY_PATH. On Windows, that directory would be specified by the PATH environment variable. However, for the purpose of this introduction, you could simply copy or move the library to the same location as the “Hello.class” file.


Footnotes

[1]

For simplicity, Unix-style notation is used throughout this manual in depicting directories and other host system conventions. For Windows, please make the relevant transformations (e.g. \ for / in path names.