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Handling Java Exceptions

If a Java method throws an exception, e.g. by using throw new Exception("...") and the exception is not caught by Java then it is passed on as a Prolog exception. The thrown term is a global reference to the Exception object. The following example code illustrates how to handle Java exceptions in Prolog:

exception_example(JVM, ...) :-
   catch(
         %% Call Java method that may raise an exception
         jasper_call(JVM, ...),
         Excp,
         (
           ( is_java_exception(JVM, Excp)
           -> print_exception_info(JVM, Excp)
           ; throw(Excp)       % pass non-Java exceptions to caller
           )
         )
        ).

is_java_exception(_JVM, Thing) :- var(Thing), !, fail.
is_java_exception(_JVM, Thing) :-
   Thing = java_exception(_),      % misc error in Java/Prolog glue
   !.
is_java_exception(JVM, Thing) :-
   jasper_is_object(JVM, Thing),
   jasper_is_instance_of(JVM, Thing, 'java/lang/Throwable').


print_exception_info(_JVM, java_exception(Message)) :- !,
   format(user_error, '~NJasper exception: ~w~n', [Message]).
print_exception_info(JVM, Excp) :-
   /*
   // Approximate Java code
   {
      String messageChars = excp.getMessage();
   }
   */
   jasper_call(JVM,
               method('java/lang/Throwable', 'getMessage', [instance]),
               get_message(+object('java/lang/Throwable'), [-chars]),
               get_message(Excp, MessageChars)),

   /*
   // Approximate Java code
   {
      StringWriter stringWriter = new StringWriter();
      PrintWriter printWriter =  new PrintWriter(stringWriter);
      excp.printStackTrace(printWriter);
      printWriter.close();
      stackTraceChars = StringWriter.toString();
   }
   */

   jasper_new_object(JVM, 'java/io/StringWriter',
                     init, init, StringWriter),
   jasper_new_object(JVM, 'java/io/PrintWriter',
                     init(+object('java/io/Writer')),
                     init(StringWriter), PrintWriter),
   jasper_call(JVM,
               method('java/lang/Throwable', 'printStackTrace', [instance]),
               print_stack_trace(+object('java/lang/Throwable'),
                                 +object('java/io/PrintWriter')),
               print_stack_trace(Excp, PrintWriter)),
   jasper_call(JVM,
               method('java/io/PrintWriter','close',[instance]),
               close(+object('java/io/PrintWriter')),
               close(PrintWriter)),
   jasper_call(JVM,
               method('java/io/StringWriter','toString',[instance]),
               to_string(+object('java/io/StringWriter'),[-chars]),
               to_string(StringWriter, StackTraceChars)),

   jasper_delete_local_ref(JVM, PrintWriter),
   jasper_delete_local_ref(JVM, StringWriter),
   %% ! exceptions are thrown as global references
   jasper_delete_global_ref(JVM, Excp),

   format(user_error, '~NJava Exception: ~s\nStackTrace: ~s~n',
          [MessageChars, StackTraceChars]).