Re: Generics: instantiating an object from a class name in configuration

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Fri, 09 Jul 2010 20:13:17 -0400
On 09-07-2010 19:07, Simon Brooke wrote:

OK, here's a problem which many people must have encountered, and there
must be a 'best practice' solution.

I have a thing which is configurable by plugging other things into it.
Obviously the other things I plug in must conform to specific interfaces,
but what thing I actually plug in is determined at run-time by reading
the name of the plugin class from a configuration file. An example (in
Java 1.4) is as follows:

1 String v = config.getValueAsString( "authenticator_class");
3 if (v != null) {
4 Class authenticatorClass;
6 try {
7 authenticatorClass = Class.forName( v);
8 } catch (ClassNotFoundException c) {
9 throw new InitialisationException(
10 "Could not find class [" + v
11 + "]", c);
12 }
14 try {
15 authenticator =
16 (Authenticator) authenticatorClass
17 .newInstance();
18 } catch (ClassCastException e) {
19 throw new InitialisationException(
20 "Not a valid authenticator class", e);
21 } catch (InstantiationException f) {
12 throw new InitialisationException(
23 "Could not instantiate authenticator", f);
24 }
25 }

Obviously one can vacuously 'bring this up to date' by changing line 4 to

4 Class<?> authenticatorClass;

but I feel that the right thing to do must surely be to use

4 Class<Authenticator> authenticatorClass;
6 try {
7 authenticatorClass =
                (Class<Authenticator>) Class.forName( v);
8 } catch (Exception e) {

then if the class specified did not inherit from Authenticator a
ClassCastException would be caught at line 8, and the second try/catch
block might become redundant. However, if I do that, Java 1.6 gives me a
warning at line 7:

'Type safety: Unchecked cast from Class<capture#1-of ?> to

Eclipse offers to fix this by adding an @SuppressWarnings clause, but I'm
not sure I want to suppress warnings...

What is the preferred pattern in Java 1.5/1.6, and why?

Have you tried:

authenticatorClass = Class<Authenticator>.forName( v);



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