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| <h2 id="index">Why JDO ?<a id="Why_JDO_"></a></h2> |
| <div class="sectionbody"> |
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| <p>The majority of applications need to persist (or store) data during |
| their lifecycle. There are many ways of doing this with an application |
| written in Java.</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>If your datastore is RDBMS you can handle the persistence (and |
| retrieval) of data yourself using <strong>JDBC</strong>. Obviously with this route you |
| have the burden of having to write the persistence layer yourself. This |
| gives much control, but also creates significant work, both in writing |
| the code but also in testing and maintenance.</p> |
| </li> |
| <li> |
| <p>You can use <strong>JDO</strong>, a standardised persistence API. With <strong>JDO</strong> you can |
| develop plain old java objects (POJOs) and persist them as they are |
| transparently. This requires very little work from the developer. It |
| allows persistence to any type of datastore in principle, being designed |
| with flexibility and datastore agnositicity in mind. This has been a |
| standard since 2002 (JDO1), being upgraded in 2006 (JDO2) and is in the |
| process of being developed further (JDO2.1) by Apache JDO</p> |
| </li> |
| <li> |
| <p>You can use <strong>JPA</strong>, a standardised persistence API, and part of the |
| EJB3 specification. This also allows you to to develop plain old Java |
| objects (POJOs) and persist them using a standardised API. It’s |
| specification is not as mature or as feature rich as the JDO API, nor |
| does it provide the flexibility of using any type of datastore. This was |
| released in 2006 (JPA1) to supercede EJB2. It really only allows |
| persistence to RDBMS datastores. If you want to persist to other |
| datastores you should consider JDO.</p> |
| </li> |
| <li> |
| <p><em>If you are stuck with using an EJB2.* architecture you could use |
| Entity Beans. This means that you hand off your objects to the EJB part |
| of the J2EE server. This simplifies things for the developer in some |
| respect but places major restrictions in that your objects have to be |
| Entity Beans.</em></p> |
| </li> |
| <li> |
| <p>You can also use a proprietary persistence API (e.g Hibernates own |
| API, TopLinks own API, iBatis, Castor etc). The disadvantages of going |
| this route are that you cannot easily swap to an alternative |
| implementation of the API if you hit problems with your software choice.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>To give a <em>guide</em>, here are a few important consideration points when |
| choosing a persistence layer for your application.</p> |
| </div> |
| <table class="tableblock frame-all grid-cols spread"> |
| <colgroup> |
| <col style="width: 16.6666%;"> |
| <col style="width: 16.6666%;"> |
| <col style="width: 16.6666%;"> |
| <col style="width: 16.6666%;"> |
| <col style="width: 16.6666%;"> |
| <col style="width: 16.667%;"> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">Feature</th> |
| <th class="tableblock halign-left valign-top">JDBC</th> |
| <th class="tableblock halign-left valign-top">JDO</th> |
| <th class="tableblock halign-left valign-top">JPA</th> |
| <th class="tableblock halign-left valign-top">EJB2</th> |
| <th class="tableblock halign-left valign-top">Custom ORM</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Standards-Driven</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Choice of datastores</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Support POJOs</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Usable in J2SE</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Usable in J2EE</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Out of box implementation (1)</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Simple to unit test</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Dynamic queries</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span> (2)</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Comprehensive ORM</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_warning_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_warning_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Primary Key generation</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span> (2)</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Supports inherited objects</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span> |
| (2)</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Schema Creation</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_error_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Existing schema</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><span class="image"><img src="images/icon_success_sml.png" alt="image"></span></p></td> |
| </tr> |
| </tbody> |
| </table> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p>refers to whether it is necessary to write the persistence yourself |
| (e.g as with JDBC) or whether you can just persist by simple calls.</p> |
| </li> |
| <li> |
| <p>requires the developer to write this layer.</p> |
| </li> |
| </ol> |
| </div> |
| </div> |
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