Initializing the Prolog engine

Although the org.jpl7.JPL class provides a number of methods for initializing the Prolog engine from within Java, their use is not usually necessary: Prolog will be automatically initialized with default parameters at the first attempt to use it.

By default, the SWIPL install accessible via the default executable (i.e., the one that runs when one types swipl) will be initialized. From it, SWIPL will try to find/guess the resources and set-up the engine correctly.

So, most users will rely on automatic initialization. However, the following is a template for explicitly initializing the Prolog engine:

if (System.getenv("SWI_HOME_DIR") != null ||
        System.getenv("SWI_EXEC_FILE") != null ||
        System.getenv("SWIPL_BOOT_FILE") != null) {
   String init_swi_config =
            String.format("%s %s %s -g true -q --no-signals --no-packs",
                    System.getenv("SWI_EXEC_FILE") == null ? "swipl" :
                    System.getenv("SWIPL_BOOT_FILE") == null ? "" :
                            String.format("-x %s", System.getenv("SWIPL_BOOT_FILE")),
                    System.getenv("SWI_HOME_DIR") == null ? "" :
                            String.format("--home=%s", System.getenv("SWI_HOME_DIR")));
    System.out.println(String.format("\nSWIPL initialized with: %s", init_swi_config));

    JPL.setDefaultInitArgs(init_swi_config.split("\\s+"));    // initialize SWIPL engine
} else
    System.out.println("No explicit initialization done: no SWI_HOME_DIR, SWI_EXEC_FILE, or SWIPL_BOOT_FILE defined");

//JPL.setTraditional(); // most often not used
System.out.println("Prolog engine actual init args: " + Arrays.toString(Prolog.get_actual_init_args()));

(For an explanation on the various options used (-x, --home, -f, etc.) pelase refer to the set of available CLI options.)

The above template will initialize the Prolog engine if any of the three environment variables SWI_HOME_DIR, SWI_EXEC_FILE, or SWIPL_BOOT_FILE are provided.

The example system.Init (located in $SWI_HOME_DIR/doc/packages/examples/jpl/java/system) uses the above template and can be run as follows:

LD_LIBRARY_PATH=/usr/local/swipl-git/lib/swipl/lib/x86_64-linux/   \
   SWI_HOME_DIR=/usr/local/swipl-git/lib/swipl \
   SWI_EXEC_FILE=/usr/local/swipl-git/lib/swipl/bin/x86_64-linux/swipl \
   SWIPL_BOOT_FILE=/usr/local/swipl-git/lib/swipl/boot.prc \    
    java -cp ../:/usr/local/swipl-git/lib/swipl/lib/jpl.jar system.Init  

Note also that one may need to tell your development environment (e.g., IntelliJ or ECLIPSE) or applications to use the correct jpl.jar, by correctly setting the CLASSPATH variable.

Finally, if you want to call Java from Prolog using JPL, you need to add the directory holding the JVM shared objects to the dynamic linker search path:

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/lib/jvm/java-11-openjdk-amd64/lib/server/
an API between SWI-Prolog and the Java Virtual Machine - hosted on GitHub