If it's part of a library for his app then it makes sense. If it's purely application logic then I see your point.
The hivemind singleton model only works if the developer accesses the object through hivemind. For a library, it may make perfect sense to want to ensure that noone can create more than one instance, regardless of the access method. (not all libraries are solely for hivemind/DI use) If he anticipates uses outside of hivemind AND wants to guarantee the singleton pattern he obviously has to do it himself. That said, I do think the package private instance methods and provider objects are overkill in this case. I made the suggestion about the providers early in this thread b/c I thought you wanted to store an instance of an interface in the session, which I later learned is not the case. You can probably get away with a single interface per DAO plus two implementations: The singleton itself and a proxy for the interface that can be loaded by hivemind. Then users can just inject the service (whose implementation is the proxy) and use it without worrying about any provider. On the other hand, the provider for all the interfaces provides a nice single-point way of providing an alternate DA layer. Just replace the implementation of the provider and the whole DA layer is set. Using individual services means replacing them all. -Mike On 3/19/06, James Carman <[EMAIL PROTECTED]> wrote: > I really think you're over-thinking here. What does the dao provider buy > you in a dependency injection-based system? HiveMind will make sure that > your DAOs are singletons and inject them into any object that needs them. > > > -----Original Message----- > From: Adam Zimowski [mailto:[EMAIL PROTECTED] > Sent: Sunday, March 19, 2006 11:53 AM > To: Tapestry users > Subject: Re: POJO dependency injection (with interface) into TAP4 > application > > I solved the problem *exactly* as I wanted originally, and as I > promised I'd post my solution, here it is. Let me just summarize: > > I'm building a large open source J2EE app. I'm building it off of > PostgreSQL backend, but I want the app to be completely unaware of the > DAO implementation, so in the future somebody can develop MySQL or any > other backend without changing a single line of code in the > application. > > Entire application uses DAO interfaces only, and Hivemind dependency > injection is used to provide the DAO provider (which in turn provides > references to DAOs). The DAO provider itself can be further enhanced > via Hivemind to inject specific DAO implementations (currently not > needed). > > Anyway, the beauty as I see it is that DAOs are now like a 3rd party > library from the application standpoind. If somebody develops MySQL > database for the app, they would provide DAO JAR, configure it an the > app is happy. > > Each DAO itself is a special kind of a singleton. Constructor is > private - nothing special, but getInstance() method has a package > scope. This is elegant because if an application developer will try to > incorrectly use specific DAO rather than the interface, he/she would > not even be able to instantiate one because getInstance() is only > visible from the package, and only the DAO provider object which lives > in the same package as all DAOs can instantiate it. Then, Hivemind > injects the provider... > > hivemodule.xml > > <service-point id="daoProvider" > interface="org.opendating.data.dao.IApplicationDAOProvider"/> > > <implementation service-id="daoProvider"> > <create-instance > class="org.opendating.data.dao.postgres.PostgresDAOProvider" > model="singleton"/> > </implementation> > > /* > * Created on Mar 18, 2006, 9:42:05 AM CST > * Created by Adam Zimowski > * > * Copyright (C) 2005-present OpenDating.org > * > * This program is free software; you can redistribute it and/or > * modify it under the terms of the GNU General Public License > * as published by the Free Software Foundation; either version 2 > * of the License, or (at your option) any later version. > * > * This program is distributed in the hope that it will be useful, > * but WITHOUT ANY WARRANTY; without even the implied warranty of > * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > * GNU General Public License for more details. > * > * You should have received a copy of the GNU General Public License > * along with this program; if not, write to the Free Software Foundation, > * or visit http://www.fsf.org > */ > package org.opendating.data.dao; > > > /** > * Defines all DAO components used by the application. The application uses > * this interface to lookup every DAO it needs, without consideration for a > * particullar implementation. The actual implementation of this provider is > * wired through Hivemind configuration microkernel. This approach allows to > * easily replace entire database back end, without changing a single line > of > * application code. For instance, one could port the database to MySQL > without > * writing a single stored procedure, and along with the new database could > * provide a JARred DAO library with a provider implementing this interface, > * and a series of DAOs conforming to their respective interfaces. A simple > * change in Hivemind configuration would then wire the application to > MySQL. > * > * @author Adam Zimowski > * @since 0.1 > * @.file [EMAIL PROTECTED] } > */ > public interface IApplicationDAOProvider { > > public abstract ILoginDAO getLoginDAO(); > > public abstract IDateDAO getDateDAO(); > > public abstract IMemberDAO getMemberDAO(); > > public abstract IPostalCodeDAO getPostalCodeDAO(); > > public abstract IProfileOptionsDAO getProfileOptionsDAO(); > > public abstract ISessionDAO getSessionDAO(); > > public abstract ISimpleProfileDAO getSimpleProfileDAO(); > > public abstract IStatsDAO getStatsDAO(); > } > > > /* > * Created on Mar 18, 2006, 9:39:09 AM CST > * Created by Adam Zimowski > * > * Copyright (C) 2005-present OpenDating.org > * > * This program is free software; you can redistribute it and/or > * modify it under the terms of the GNU General Public License > * as published by the Free Software Foundation; either version 2 > * of the License, or (at your option) any later version. > * > * This program is distributed in the hope that it will be useful, > * but WITHOUT ANY WARRANTY; without even the implied warranty of > * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > * GNU General Public License for more details. > * > * You should have received a copy of the GNU General Public License > * along with this program; if not, write to the Free Software Foundation, > * or visit http://www.fsf.org > */ > package org.opendating.data.dao.postgres; > > import org.apache.log4j.Logger; > import org.opendating.data.dao.IApplicationDAOProvider; > import org.opendating.data.dao.IDateDAO; > import org.opendating.data.dao.ILoginDAO; > import org.opendating.data.dao.IMemberDAO; > import org.opendating.data.dao.IPostalCodeDAO; > import org.opendating.data.dao.IProfileOptionsDAO; > import org.opendating.data.dao.ISessionDAO; > import org.opendating.data.dao.ISimpleProfileDAO; > import org.opendating.data.dao.IStatsDAO; > import org.opendating.util.log.LogEngine; > > public class PostgresDAOProvider implements IApplicationDAOProvider { > > private Logger _aLog = LogEngine.getAppLogger(); > > public PostgresDAOProvider() { > if(_aLog.isDebugEnabled()) > _aLog.debug("Creating PostgreSQL DAO Provider..."); > } > > /* (non-Javadoc) > * @see > org.opendating.data.dao.db.main.IMainDAOProvider#getLoginDAO() > */ > public ILoginDAO getLoginDAO() { > return LoginPgDAO.getInstance(); > } > > /* (non-Javadoc) > * @see org.opendating.data.dao.IApplicationDAOProvider#getDateDAO() > */ > public IDateDAO getDateDAO() { > return DatePgDAO.getInstance(); > } > > /* (non-Javadoc) > * @see > org.opendating.data.dao.IApplicationDAOProvider#getMemberDAO() > */ > public IMemberDAO getMemberDAO() { > return MemberPgDAO.getInstance(); > } > > /* (non-Javadoc) > * @see > org.opendating.data.dao.IApplicationDAOProvider#getPostalCodeDAO() > */ > public IPostalCodeDAO getPostalCodeDAO() { > return PostalCodePgDAO.getInstance(); > } > > /* (non-Javadoc) > * @see > org.opendating.data.dao.IApplicationDAOProvider#getProfileOptionsDAO() > */ > public IProfileOptionsDAO getProfileOptionsDAO() { > return ProfileOptionsPgDAO.getInstance(); > } > > /* (non-Javadoc) > * @see > org.opendating.data.dao.IApplicationDAOProvider#getSessionDAO() > */ > public ISessionDAO getSessionDAO() { > return SessionPgDAO.getInstance(); > } > > /* (non-Javadoc) > * @see > org.opendating.data.dao.IApplicationDAOProvider#getSimpleProfileDAO() > */ > public ISimpleProfileDAO getSimpleProfileDAO() { > return SimpleProfilePgDAO.getInstance(); > } > > /* (non-Javadoc) > * @see > org.opendating.data.dao.IApplicationDAOProvider#getStatsDAO() > */ > public IStatsDAO getStatsDAO() { > return StatsPgDAO.getInstance(); > } > } > > --------------- one of the dao's ------------------------- > > /* > * Created on Aug 24, 2005, 8:29:00 PM > * Created by Adam Zimowski > * > * Copyright (C) 2005-present OpenDating.org > * > * This program is free software; you can redistribute it and/or > * modify it under the terms of the GNU General Public License > * as published by the Free Software Foundation; either version 2 > * of the License, or (at your option) any later version. > * > * This program is distributed in the hope that it will be useful, > * but WITHOUT ANY WARRANTY; without even the implied warranty of > * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > * GNU General Public License for more details. > * > * You should have received a copy of the GNU General Public License > * along with this program; if not, write to the Free Software Foundation, > * or visit http://www.fsf.org > */ > package org.opendating.data.dao.postgres; > > import java.util.List; > > import org.opendating.data.SimpleEntity; > import org.opendating.data.dao.IDateDAO; > > > /** > * Provides data access to all the date-related information. > * > * @author Adam Zimowski > * @since 0.1 > * @.file [EMAIL PROTECTED] } > */ > public final class DatePgDAO extends SimpleEntityPgDAO implements IDateDAO { > > private static DatePgDAO _instance = null; > > /** > * Default constructor; instantiates this class, and calls the > underlying > * base. > * > * @since 0.1 > */ > private DatePgDAO() { > super(); > } > > static DatePgDAO getInstance() { > if(_instance == null) _instance = new DatePgDAO(); > return _instance; > } > > /* (non-Javadoc) > * @see org.opendating.data.dao.IDateDAO#getMonths() > */ > public List<SimpleEntity> getMonths() { > return getSimpleEntityList("f_get_qtn_months"); > } > } > > ----- Then, Tapestry objects simply implement this: ---------- > > /* > * Created on Mar 18, 2006, 2:31:06 PM CST > * Created by Adam Zimowski > * > * Copyright (C) 2005-present OpenDating.org > * > * This program is free software; you can redistribute it and/or > * modify it under the terms of the GNU General Public License > * as published by the Free Software Foundation; either version 2 > * of the License, or (at your option) any later version. > * > * This program is distributed in the hope that it will be useful, > * but WITHOUT ANY WARRANTY; without even the implied warranty of > * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > * GNU General Public License for more details. > * > * You should have received a copy of the GNU General Public License > * along with this program; if not, write to the Free Software Foundation, > * or visit http://www.fsf.org > */ > package org.opendating.tapestry; > > import org.apache.tapestry.annotations.InjectObject; > import org.opendating.data.dao.IApplicationDAOProvider; > > /** > * Provides easy access to the DAO provider for Tapestry based objects. > Since > * Tapestry is tightly integrated with Hivemind, it provides convinient > * annotations which can be used instead of plain registry lookup. Therefore > * for Tapestry pages and comonents it is easiest to simply implement this > * interface (without implementing the method), and call the getter to grab > * the provider. Tapestry and Hivemind will do the rest of the magic, and > * deliver a valid reference... > * > * @author Adam Zimowski > * @since 0.1 > * @.file [EMAIL PROTECTED] } > */ > public interface ITapestryDAOProvider { > > @InjectObject("service:org.opendating.daoProvider") > public abstract IApplicationDAOProvider getDAO(); > } > > Regards, > Adam > > --------------------------------------------------------------------- > To unsubscribe, e-mail: [EMAIL PROTECTED] > For additional commands, e-mail: [EMAIL PROTECTED] > > > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: [EMAIL PROTECTED] > For additional commands, e-mail: [EMAIL PROTECTED] > > --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
