[jira] [Created] (JENA-1029) Add a Jena-wide subsystem lifecycle

["Andy Seaborne (JIRA)" <ji...@apache.org>]

Andy Seaborne created JENA-1029:
-----------------------------------

             Summary: Add a Jena-wide subsystem lifecycle
                 Key: JENA-1029
                 URL: https://issues.apache.org/jira/browse/JENA-1029
             Project: Apache Jena
          Issue Type: Improvement
    Affects Versions: Jena 3.0.0
            Reporter: Andy Seaborne


A subsystem lifecyclefor Jena would provide the hook for:

* basic wiring together e.g. wiring RIOT into Jena core
* initialization e.g. {{TDB.init()}} becomes redundant
* starting and stopping in large systems, e.g. starting and stopping Fuseki2 when in a Tomcat server

This proposal is as simple as possible. It is for system bootstrap and basic lifecycle. It is not intended to work for a mixture of jars from different Jena releases.

*Sub-system Interface*

{code:title=JenaSubsystemLifecycle|borderStyle=solid}
public interface JenaSubsystemLifecycle {
    public void start() ;
    public void stop() ;
}
{code}

{{stop}} would not be called normally as part of JVM exit (code can do that itself anyway). It is in support of start-stop-(re)start and only called if there is a hook for such a cycle. {{stop}} is more of a placeholder for the moment.

*Registry*

There is a single registry:

{code:title=JenaSubsystemRegistry|borderStyle=solid}
public class JenaSubsystemRegistry {
    public static void add(JenaSubsystemLifecycle module) ;
    public static boolean isRegistered(JenaSubsystemLifecycle module) ;
    public static void remove(JenaSubsystemLifecycle module) ;
    public static int size() ;
    public static boolean isEmpty() ;
    public static void forEach(Consumer<JenaSubsystemLifecycle> action) ;
}
{code}

*Ensuring initialization*

Jena core provides a "system init" that is cheap to call if initialization has already occurred. It should attempt the first call  as early as possible.

When initializing, it runs a {{ServiceLoader}} cycle to discover any {{JenaSubsystemLifecycle}} implementations described in a {{META-INF/services/...}} file and populates the registry. Then the registry is 
used to call {{start()}} in some unspecified order.



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