Re: Class.forName().newInstance() vs new

From:

Ian Shef <invalid@avoiding.spam>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 14 Jun 2011 20:07:47 GMT

Message-ID:

<Xns9F04859078A43vaj4088ianshef@138.125.254.103>

Abu Yahya <abu_yahya@invalid.com> wrote in news:it07on$ilc$1
@speranza.aioe.org:

I have a class that instantiates a private member based on a value in a
configuration file.

The value is used to determine the class name.

I initially decided to use code similar to the following:

             // class name comes from property file
             String dbMgrClassName = props.getProperty("db.manager");
             Class<?> k = Class.forName(dbMgrClassName);
             db = (DB) k.newInstance();
             db.init(props);

However, a colleague pointed out that using "new" is faster than using
"Class.forName().newInstance()". So, I changed the code to the following:

             String dbMgrClassName = props.getProperty("db.manager");
             if (dbMgrClassName.equals("DB2"))
                 db = new DB2();
             else if (dbMgrClassName.equals("Oracle"))
                 db = new Oracle();
             else if (dbMgrClassName.equals("SQLServer"))
                 db = new SQLServer();
             else { // handle unrecognized
                 Class<?> k = Class.forName(dbMgrClassName);
                 db = (DB) k.newInstance();
             }
             db.init(props);

My question is: Does using if-else statements like the above really
improve performance?

Maybe yes, maybe no. If you don't have a benchmark that shows it to be an
issue, don't worry about it. Initializing the databas manager is probably
going to be a lot more preformance-consuming than anything else that you
are doing here.

What is the best approach to instantiate classes
whose type you don't know at compile time?

Well, actually you _do_ know the type for db, you just didn't show us
directly; apparently it is DB. What you don't know at compile time is the
implementation.
I prefer to use java.util.ServiceLoader myself. It means that you have to
have "a provider-configuration file in the resource directory META-
INF/services" instead of a property or a value in a separate configuration
file. This could be a good thing or a bad thing, depending upon your
application.