Re: Generics: instantiating an object from a class name in configuration
Simon Brooke wrote:
Obviously one can vacuously 'bring this up to date' by changing line 4 to
4 Class<?> authenticatorClass;
Do not use TAB characters to indent Usenet posts; use a maximum of four spaces
per level for readability.
but I feel that the right thing to do must surely be to use
4 Class<Authenticator> authenticatorClass;
5
6 try {
7 authenticatorClass =
(Class<Authenticator>) Class.forName( v);
8 } catch (Exception e) {
Don't catch 'Exception', catch specific exceptions.
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
Class<Authenticator>'
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?
Excellent question, fully answered in the free chapter on generics
downloadable from
<http://java.sun.com/docs/books/effective/>
although of course you should study the entire book.
The gist is that sometimes you have to suppress warnings, but only when you
know for sure that it's safe to do so and document why it's safe with comments
in the code. So what you want is similar to:
public Authenticator instantiate( String name )
throws InitialisationException
{
Authenticator authenticator;
try
{
// ClassCastException caught so this is safe
@SuppressWarnings( "unchecked" )
Class <Authenticator> clazz =
(Class <Authenticator>) Class.forName( name );
authenticator = clazz.newInstance();
}
catch ( ClassNotFoundException ex )
{
String msg = "Could not find class \""+ name +"\". ";
logger.error( msg + ex.getLocalizedMessage(), ex );
throw new InitialisationException( msg, ex );
}
catch ( ClassCastException ex )
{
String msg = "Not a valid authenticator class \""+ name +"\". ";
logger.error( msg + ex.getLocalizedMessage(), ex );
throw new InitialisationException( msg, ex );
}
catch ( IllegalAccessException ex )
{
String msg =
"Could not access instance of \""+ name +"\". ";
logger.error( msg + ex.getLocalizedMessage(), ex );
throw new InitialisationException( msg, ex );
}
catch ( InstantiationException ex )
{
String msg =
"Could not instantiate authenticator from \""+ name +"\". ";
logger.error( msg + ex.getLocalizedMessage(), ex );
throw new InitialisationException( msg, ex );
}
assert authenticator != null;
return authenticator;
}
I usually use some sort of FubarMessage enum with "friendly" strings invoked
via 'toString()' to embody the fubar messages. You could even have a custom
'public String message( String name )' method in the enum to build each 'msg'.
--
Lew