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]).