10.18.8.1 Jasper Method Call Example

We begin with a small example.

                             
// Simple.java
     
     import se.sics.jasper.*;
     
     public class Simple {
       private String instanceDatum = "this is instance data";
     
       static int simpleMethod(int value) {
         return value*42;
       }
     
       public String getInstanceData(String arg) {
         return instanceDatum + arg;
       }
     }

Compile Simple.java (UNIX):

     % javac -deprecation \
       -classpath <installdir>/lib/sicstus-4.3.1/bin/jasper.jar Simple.java

Under Windows this may look like (the command should go on a single line):

     C:\> c:\jdk1.2.2\bin\javac -deprecation
      -classpath "D:\Program Files\SICStus Prolog 4.3.1\bin\jasper.jar" Simple.java

The option ‘-deprecation’ is always a good idea, it makes javac warn if your code use deprecated methods.

                                
% simple.pl
     
     :- use_module(library(jasper)).
     main :-
        %% Replace '/my/java/dir' below with the path containing
        %% 'Simple.class', e.g. to look in the current directory use
        %% classpath(['.']).
        %% You can also use the CLASSPATH environment variable and call
        %% jasper_initialize(JVM)
        %% Under Windows it may look like classpath(['C:/MyTest'])
        jasper_initialize([classpath(['/my/java/dir'])],JVM),
     
        format('Calling a static method...~n',[]),
        jasper_call(JVM,
                    method('Simple','simpleMethod',[static]), % Which method
                    simple_method(+integer,[-integer]), % Types of arguments
                    simple_method(42,X)), % The arguments.
        format('simpleMethod(~w) = ~w~n',[42,X]),
     
        format('Creating an object...~n',[]),
        jasper_new_object(JVM, 'Simple', init, init, Object),
     
        format('Calling an instance method on ~w...~n',[Object]),
        jasper_call(JVM,
                    method('Simple','getInstanceData',[instance]),
                    %% first arg is the instance to call
                    get_instance_data(+object('Simple'), +string,[-string]),
                    get_instance_data(Object, 'foobar', X1)),
        format('getInstanceData(~w) = ~w~n',['foobar',X1]).

Then, run SICStus:

     % echo "[simple],main." | sicstus
     SICStus 4.3.1 ...
     Licensed to SICS
     % consulting /home1/jojo/simple.pl...
     [...]
     % consulted /home1/jojo/simple.pl in module user, 100 msec 26644 bytes
     Calling a static method...
     simpleMethod(42) = 1764
     Creating an object...
     Calling and instance method on $java_object(135057576)...
     getInstanceData(foobar) = this is instance datafoobar

This example performed three things.

• The static method simpleMethod was called with argument '42', and returned the square of '42', '1764'.
• An object of class Simple was created.
• The method getInstanceData was executed on the object just created. The method took an atom as an argument and appended the atom to a string stored as a field in the object, yielding "this is instance datafoobar".