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<H1><A NAME="SECTION03590000000000000000"></A><A NAME="ss:Exc"></A><A NAME="9157"></A>
<BR>
Exceptions
</H1>
<P>
Exceptions are caught and thrown in exactly the same way as Java
exceptions. If an exception is defined in SIDL, Babel will generate
the code for it, and the exception can be thrown in Java. The only
difference is that SIDL exception constructor cannot take a String.
Instead, the message must be set with SIDL's <TT>setNote</TT>
method, the message is gotten with SIDL's <TT>getNote</TT> method. This
is important because SIDL exceptions inherit from the Java Class
<TT>Exception</TT>.<A NAME="9161"></A> The Java compiler <SPAN CLASS="textit">will not</SPAN> give an
error if getMessage is called, but the message returned will not have
been from SIDL.
<P>
<A NAME="9163"></A><A NAME="9164"></A>
The other problem is that regular Java exceptions cannot be passed on
by Babel. Of course, it's not possible to throw normal non-SIDL
exceptions from a SIDL Java function, the Java compiler will throw an
error. (Unless you have changed the Java method ``throws'' statement
outside the splicer blocks, which you should never do.)
However, Java runtime exceptions, such as
<TT>ArrayIndexOutOfBoundsException</TT> can be thrown. In this case, an
error message and stack trace are printed to stderr, the method
returns 0, the values of any out or inout arguments are set to NULL, and
the program proceeds.
<P>
Finally, SIDL Exceptions may be interfaces, where as Java exceptions
are always classes.
This means Babel allows you to throw an interface. However, in Java
we actually need to throw the interface's Wrapper class.
<P>
In this
example we have a class <TT>FibException</TT> which implements two
exception interfaces, <TT>NegativeValueException</TT> and
<TT>TooDeepException</TT>. These two Exceptions are thrown by a
babelized method named <TT>getFib</TT>. <TT>getFib</TT> is a standard
recursive Fibonacci number generating function, in which if something
goes wrong, it throws one of these two exceptions. First, server
side:
<P>
<BR>
<PRE CLASS="verbatim">public int getFib_Impl ( /*in*/ int n)
throws NegativeValueException.Wrapper, TooDeepException.Wrapper {
if (n < 0) {
FibException fex = new FibException();
NegativeValueException.Wrapper neg = (NegativeValueException.Wrapper)
NegativeValueException.Wrapper._cast(fex);
neg.setNote("n negative");
throw neg;
}
// .... Do Fibonacci stuff ....
}
</PRE></td></tr></table></blockquote>
<P>
You can see here some of the hoops you have to jump though to throw an
interface. First, since we cannot create an interface, or it's
Wrapper, directly, we first create a new <TT>FibException</TT> and cast it to
the interface we want. Secondly, we have to refer to the Wrapper's
full name in this case, because it is impossible to throw interfaces
in Java. Finally, as with all SIDL Exceptions, we use <TT>setNote</TT>
to set the exception's message, as we cannot pass in a message with the
constructor.
<P>
Next the client side:
<P>
<BR>
<PRE CLASS="verbatim">
try {
fib.getFib(-1);
} catch (NegativeValueException.Wrapper ex) {
System.err.println(ex.getNote());
} catch (TooDeepException.Wrapper ex) {
System.err.println(ex.getNote());
} catch (java.lang.Exception ex) {
if (((sidl.BaseInterface)ex).isType("sidl.SIDLException")) {
check(PASS, true, "Unexpected SIDL Exception thrown");
} else {
check(PASS, false, "Unexpected and unkown exception thrown");
}
}
</PRE></td></tr></table></blockquote>
<P>
In order to differentiate between the two different interfaces in this
case we must catch the Wrappers explicitly by their fully qualified
names. In the <TT>exceptions</TT> regression test we discover the types
of the Exceptions by calling the SIDL function <TT>isType</TT> on them.
However, because SIDL can cast between the two
interfaces, in this case <TT>isType</TT> would return true no matter what the
exception originally was. The final catch <TT>java.lang.Exception
ex</TT> should not ever be executed in out example code. <TT>getFib</TT> does not
throw any other kinds of exceptions, and babel cannot throw non-SIDL
Exceptions. This was included because it demonstrates the most basic
way to differentiate a SIDL exception from a Java exception.
<P>
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