Service Locator Pattern

转自：http://blog.youkuaiyun.com/fitzwilliam/article/details/1539068

How JNDI lookups are accomplished can vary from vendor to vendor. In addition, repeated JNDI lookups can be expensive.

Each time a user needs a service retrieved from JNDI, they must populate the appropriate environment information, make a connection to the JNDI server and then look up the service. What is needed is a single point for creating and retrieving various services from JNDI. This is where the Service Locator pattern is useful. 

The Service Locator pattern abstracts away all JNDI lookups behind a simple-to-use interface. The Service Locator implementation used in this chapter provides methods for retrieving EJBHome interface and JDBC database connections. In addition, this Service Locator minimizes the number of JNDI lookups required by caching EJBHomes interfaces and the DataSource objects used to create JDBC connections the first time a user requests a specific EJB or JDBC connection.

 

The for the Service Locator pattern is shown below:

/*
 * The Service Locator pattern abstracts away the JNDI
 * Logic necessary for retrieving a JDBC Connection or EJBHome
 * interface
 *
 */
public class ServiceLocator {
    private static ServiceLocator serviceLocatorRef = null;
    private static HashMap ejbHomeCache = null;
    private static HashMap dataSourceCache = null;
   
    /*
     * Enumerating the different services avaialbe from the
     * Service Locator
     */
    public static final int COUNTER = 0;
    public static final int TITLEDAO = 1;
    public static final int EDITORDAO = 2;
    public static final int SUBSCRIPTIONDB = 3;
   
    /* The JNDI Names used to lookup a service */
    private static final String COUNTER_JNDINAME="counter/Counter";
    private static final String TITLEDAO_JNDINAME="title/TitleDAO";
    private static final String EDITORDAO_JNDINAME="editor/EditorDAO";
    private static final String SUBSCRIPTIONDB_JNDINAME="java:/subscriptionDS";
   
    /* References to each of the different EJB Home Interfaces */
   private static final Class COUNTERCLASSREF = CounterHome.class;
   private static final Class TITLECLASSREF = TitleDAOHome.class;
   private static final Class EDITORCLASSREF = EditorDAOHome.class;
  
   static {
       serviceLocatorRef = new ServiceLocator();
   }
  
   /* Private Constructor for the ServiceLocator */
   private ServiceLocator(){
       ejbHomeCache = new HashMap();
       dataSourceCache = new HashMap();
   }
  
   /*
    * The ServiceLocator is implemented as a Singleton.  The getInstance()
    * method will return the static reference to the ServiceLocator stored
    * inside of the ServiceLocator Class.
    */
   public static ServiceLocator getInstance(){
       return serviceLocatorRef;
   }
  
   /*
    * The getServiceName will retrieve the JNDI name for a requested
    * service.  The service is indicated by the ServiceId passed into
    * the method.
    */
   static private String getServiceName(int pServiceId) throws ServiceLocatorException{
      
       String serviceName = null;
      
       switch ( pServiceId ) {
        case COUNTER:
            serviceName = COUNTER_JNDINAME;
            break;
        case TITLEDAO:
            serviceName = TITLEDAO_JNDINAME;
            break;
        case EDITORDAO:
            serviceName = EDITORDAO_JNDINAME;
            break;
        case SUBSCRIPTIONDB:
            serviceName = SUBSCRIPTIONDB_JNDINAME;
            break;
        default:
            throw new ServiceLocatorException("Unable to locate the service requested in" +
                    "ServiceLocator.getServiceName() method.  ");
        }
       return serviceName;
   }
  
   /*
    * Returns the EJBHome Class reference for a requested service.
    * If the method can not make a match, it will throw a
    * ServiceLocatorException.
    */
    static private Class getEJBHomeRef(int pServiceId) throws ServiceLocatorException{
        Class homeRef = null;
       
        switch ( pServiceId ) {
            case COUNTER:
                homeRef = COUNTERCLASSREF;
                break;
            case TITLEDAO:
                homeRef = TITLECLASSREF;
                break;
            case EDITORDAO:
                homeRef = EDITORCLASSREF;
                break;
            default:
                throw new ServiceLocatorException("Unable to locate the service requested in" +
                        "ServiceLocator.getEJBHomeRef() method.  ");
        }
        return homeRef;
    }
   
    /*
     * The getEJBHome method will return an EJBHome interface for a
     * requested service.  If it can not find the requested EJB, it will
     * throw a ServiceLocatorException.
     *
     * The getEJBHome interface caches a requested EJBHome so that the first
     * time an EJB is requested, a home interface will be retrieved but then
     * be placed into a cache.
     */
    public EJBHome getEJBHome(int pServiceId) throws ServiceLocatorException{
        //Tring to find the JNDI Name for the requested service
        String serviceName = getServiceName(pServiceId);
        EJBHome ejbHome = null;
       
        try {
            //checking to see if I can find the EJBHome interface in cache
            if ( ejbHomeCache.containsKey(serviceName) ) {
                ejbHome = ( EJBHome ) ejbHomeCache.get(serviceName);
                return ejbHome;
            } else {
                //If I could not find the EJBHome interface in the cache, look it
                //up and the cache it.
                Context ctx = new InitialContext();
                Object jndiRef = ctx.lookup(serviceName);
               
                Object portableObj = PortableRemoteObject.narrow(jndiRef, getEJBHomeRef(pServiceId));
               
                ejbHome = ( EJBHome ) portableObj;
               
                ejbHomeCache.put(serviceName, ejbHome);
                return ejbHome;
            }
        } catch ( NamingException e ) {
            throw new ServiceLocatorException("Naming exception error in ServiceLocator.getEJBHome()", e);
        } catch (Exception e) {
            throw new ServiceLocatorException("General exception in ServcieLocator.getEJBHome", e);
        }
    }
   
    /*
     * The getDBConn() method will create a JDBC connection for the
     * requested database.  It too uses a caching algorithm to minimize
     * the number of JNDI hits that it must perform.
     */
    public Connection getDBConn(int pServiceId) throws ServiceLocatorException{
        //Getting the JNDI Service Name
        String serviceName = getServiceName(pServiceId);
        Connection conn = null;
       
        try {
            //Checking to see if the requested DataSource is in the Cache
            if ( dataSourceCache.containsKey(serviceName) ) {
                DataSource ds = ( DataSource ) dataSourceCache.get(serviceName);
                conn = ds.getConnection();
                return conn;
            } else {
                //The DataSource was not in the cache.  Retrieve it from JNDI
                //and put it in the cache.
                Context ctx = new InitialContext();
                DataSource newDataSource = ( DataSource ) ctx.lookup(serviceName);
                dataSourceCache.put(serviceName, newDataSource);
                conn = newDataSource.getConnection();
                return conn;
            }
        } catch ( SQLException e ) {
            throw new ServiceLocatorException("A SOL error has occured in ServiceLocator.getDBConn()", e);
        } catch ( NamingException e ) {
            throw new ServiceLocatorException("A JNDI Naming exception has occured in ServiceLocator.getDBConn()", e);
        } catch (Exception e) {
            throw new ServiceLocatorException("An exception has occured in ServiceLocator.getDBConn()", e);
        }
    }
} 

 

Implementing the ServiceLocator as a Singleton pattern is extremely efficient because the class is heavily used and does not need to maintain instance information.

The use of non-final static methods in the ServiceLocator is actually in violation of the EJB specification (see section 21.4.2). However, we can get away with this because we are not dependant on our cache being consistent across Java Virtual Machine, which is the reason that the restriction exists. You must also verify that your factory classes can be used by multiple threads concurrently without synchronization; this issue is not addressed in this specification. These two issues make our solution perform better but also make it slightly less portable. We could trade some performance back for portability by doing the caching in the bean instance instead.

 

From： 

J2EE Design Patterns Applied, Wrox

by Craig A.Berry, John Carnell, Matjaz B.Juric, Meeraj Moidoo Kunnumpurath, Nadia Nashi, Sasha Romano