"Diethelm Guallar, Gonzalo" wrote:
> 1. A "system" object that must be alive while the server is
> processing requests; a Singleton, used by all interactions.
> The question is, what is a good way to handle (1)? Where do I
> put the line
>
> System system = new System(...);
You answered your question yourself - when your object is a
signleton it is created when getInstance() is called, this is
the first time any service is requested from your object.
Here is an example of Singleton patter for you :
(you must have seen many of them in Turbine sources)
public class System
{
// clients must use getInstance() to get an instnce of this class
private System()
{
// initialize the System
}
// the single instance of this class
private static instance;
public static System getInstance()
{
// make sure you don't synchronize duing each request for instance
// synchronization takes time, and the instance usually exists.
if(instance == null)
{
// acquire a sandbox (e.g. Tomcat context) wide lock
// to prevent two threads from performing initialization concurrently
synchronized(System.class)
{
// check instance again, because other thread could enter the
// monitor AFTER the above if. checking again will be usually
// faster than extraneous calling the constructor. On the other
// hand calling the constructor twice might break something.
if(instance == null)
{
// initialize the System now
instance = new Instance();
}
}
}
// return the valid instance to the client.
return instance;
}
}
every time you need to call a method from your System:
System.getInstance().method(args);
and the pattern guarantees that System will be always initialized, once per life of
the VM.
Rafal
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