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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Data access object</span></span>

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</style><div role="note" class="hatnote navigation-not-searchable">This article is about the software design pattern. For the Microsoft library, see <a href="Jet_Data_Access_Objects" title="Jet Data Access Objects">Jet Data Access Objects</a>.</div>

<p>In <a href="Software" title="Software">software</a>, a <b>data access object</b> (<b>DAO</b>) is a pattern that provides an abstract <a href="Interface_(computer_science)" class="mw-redirect" title="Interface (computer science)">interface</a> to some type of <a href="Database" title="Database">database</a> or other <a href="Persistence_(computer_science)" title="Persistence (computer science)">persistence mechanism</a>. By mapping application calls to the persistence layer, the DAO provides data operations without exposing database details. This isolation supports the <a href="Single_responsibility_principle" class="mw-redirect" title="Single responsibility principle">single responsibility principle</a>. It separates the data access the application needs, in terms of domain-specific objects and data types (the DAO's public interface), from how these needs can be satisfied with a specific <a href="Database_Management_System" class="mw-redirect" title="Database Management System">DBMS</a> (the implementation of the DAO).

</p><p>Although this <a href="Design_pattern" title="Design pattern">design pattern</a> is applicable to most programming languages, most software with persistence needs, and most databases, it is traditionally associated with <a href="Java_Platform%2C_Enterprise_Edition" class="mw-redirect" title="Java Platform, Enterprise Edition">Java EE</a> applications and with <a href="Relational_databases" class="mw-redirect" title="Relational databases">relational databases</a> (accessed via the <a href="JDBC" class="mw-redirect" title="JDBC">JDBC</a> API because of its origin in <a href="Sun_Microsystems" title="Sun Microsystems">Sun Microsystems</a>' best practice guidelines<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> "Core J2EE Patterns".

</p><p>This object can be found in the <a href="Data_access_layer" title="Data access layer">Data Access layer</a> of the <a href="Multitier_architecture" title="Multitier architecture">3-Tier Architecture</a>.

</p><p>There are various ways in which this object can be implemented:

</p>

<ul><li>One DAO for each table.</li>

<li>One DAO for all the tables for a particular DBMS.</li>

<li>Where the SELECT query is limited only to its target table and cannot incorporate JOINS, UNIONS, subqueries and Common Table Expressions (CTEs)</li>

<li>Where the SELECT query can contain anything that the DBMS allows.</li></ul>

<div class="mw-heading mw-heading2"><h2 id="Advantages">Advantages</h2></div>

<p>Using data access objects (DAOs) offers a clear advantage: it separates two parts of an application that don't need to know about each other. This separation allows them to evolve independently. If business logic changes, it can rely on a consistent DAO interface. Meanwhile, modifications to persistence logic won't affect DAO clients.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>

</p><p>All details of storage are hidden from the rest of the application (see <a href="Information_hiding" title="Information hiding">information hiding</a>). <a href="Unit_testing" title="Unit testing">Unit testing</a> code is facilitated by substituting a <a href="Test_double" title="Test double">test double</a> for the DAO in the test, thereby making the tests independent of the persistence layer.

</p><p>In the context of the <a href="Java_(programming_language)" title="Java (programming language)">Java</a> programming language, DAO can be implemented in various ways. This can range from a fairly simple interface that separates data access from the application logic, to frameworks and commercial products.

</p><p>Technologies like <a href="Java_Persistence_API" class="mw-redirect" title="Java Persistence API">Java Persistence API</a> and <a href="Enterprise_JavaBeans" class="mw-redirect" title="Enterprise JavaBeans">Enterprise JavaBeans</a> come built into application servers and can be used in applications that use a Java EE application server. Commercial products such as <a href="TopLink" class="mw-redirect" title="TopLink">TopLink</a> are available based on <a href="Object%E2%80%93relational_mapping" title="Object–relational mapping">object–relational mapping</a> (ORM). Popular open source ORM software includes <a href="Doctrine_(PHP)" title="Doctrine (PHP)">Doctrine</a>, <a href="Hibernate_(Java)" class="mw-redirect" title="Hibernate (Java)">Hibernate</a>, <a href="IBATIS" class="mw-redirect" title="IBATIS">iBATIS</a> and JPA implementations such as <a href="Apache_OpenJPA" title="Apache OpenJPA">Apache OpenJPA</a>.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

</p>

<div class="mw-heading mw-heading2"><h2 id="Disadvantages">Disadvantages</h2></div>

<p>Potential disadvantages of using DAO include <a href="Leaky_abstraction" title="Leaky abstraction">leaky abstraction</a>, <a href="Duplicate_code" title="Duplicate code">code duplication</a>, and <a href="Abstraction_inversion" title="Abstraction inversion">abstraction inversion</a>. In particular, the abstraction of the DAO as a regular Java object can obscure the high cost of each database access. Developers may inadvertently make multiple database queries to retrieve information that could be returned in a single operation. If an application requires multiple DAOs, the same create, read, update, and delete code may have to be written for each DAO.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>

</p><p>Note that these disadvantages only appear when you have a separate DAO for each table and the SELECT query is prevented from accessing anything other than the target table.

</p>

<div class="mw-heading mw-heading2"><h2 id="Tools_and_frameworks">Tools and frameworks</h2></div>

<ul><li>ODB compiler-based <a href="Object%E2%80%93relational_mapping" title="Object–relational mapping">object–relational mapping</a> (ORM) system for C++</li>

<li>ORMLite: Lightweight <a href="Object%E2%80%93relational_mapping" title="Object–relational mapping">object–relational mapping</a> (ORM) framework in Java for JDBC and Android<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup></li>

<li><a href="Entity_Framework" title="Entity Framework">Microsoft Entity Framework</a></li>

<li>DBIx::Class <a href="Object%E2%80%93relational_mapping" title="Object–relational mapping">object–relational mapping</a> (ORM) module for <a href="Perl" title="Perl">Perl</a></li>

<li>TuxORM: Simple <a href="Object%E2%80%93relational_mapping" title="Object–relational mapping">object–relational mapping</a> (ORM) library in Java for JDBC</li>

<li><a href="Persist_(Java_tool)" title="Persist (Java tool)">Persist (Java tool)</a> Java-based <a href="Object%E2%80%93relational_mapping" title="Object–relational mapping">object–relational mapping</a> and data access object tool</li></ul>

<ul><li><a href="Create%2C_read%2C_update_and_delete" title="Create, read, update and delete">Create, read, update and delete</a> (CRUD)</li>

<li><a href="Data_access_layer" title="Data access layer">Data access layer</a></li>

<li><a href="Service_Data_Objects" title="Service Data Objects">Service Data Objects</a></li>

<li><a href="Object%E2%80%93relational_mapping" title="Object–relational mapping">Object–relational mapping</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.oracle.com/technetwork/java/dataaccessobject-138824.html">"Core J2EE Patterns - Data Access Objects"</a>. Sun Microsystems Inc. 2007-08-02.</cite></span>

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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.digitalocean.com/community/tutorials/dao-design-pattern">"Data Access Object(DAO) Design Pattern"</a>. <i>DigitalOcean</i>. 2022-08-03<span class="reference-accessdate">. Retrieved <span class="nowrap">2024-07-08</span></span>.</cite></span>

</li>

<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.geeksforgeeks.org/data-access-object-pattern/#8-advantages-of-the-data-access-object-designdao-pattern">"Data Access Object(DAO) Design Pattern"</a>. <i>GeeksforGeeks</i>. 2017-08-26<span class="reference-accessdate">. Retrieved <span class="nowrap">2024-07-08</span></span>.</cite></span>

</li>

<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.geeksforgeeks.org/data-access-object-pattern/">"Data Access Object(DAO) Design Pattern"</a>. <i>GeeksforGeeks</i>. 2017-08-26<span class="reference-accessdate">. Retrieved <span class="nowrap">2024-01-29</span></span>.</cite></span>

</li>

<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text">See <a rel="nofollow" class="external free" href="http://www.ibm.com/developerworks/java/library/j-genericdao/index.html">http://www.ibm.com/developerworks/java/library/j-genericdao/index.html</a> for workarounds</span>

</li>

<li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text"><cite id="CITEREFHodgsonReid2015" class="citation book cs1">Hodgson, Kyle; Reid, Darren (2015-01-23). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=x9hpBgAAQBAJ&q=%22ORMLite%22&pg=PA129"><i>ServiceStack 4 Cookbook</i></a>. Packt Publishing Ltd. p. Chapter 4. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>9781783986576</bdi><span class="reference-accessdate">. Retrieved <span class="nowrap">22 June</span> 2016</span>.</cite></span>

</li>

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</style><div id="Software_design_patterns225" style="font-size:114%;margin:0 4em"><a href="Software_design_pattern" title="Software design pattern">Software design patterns</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Design_Patterns" title="Design Patterns">Gang of Four<br>patterns</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Creational_pattern" title="Creational pattern">Creational</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Abstract_factory_pattern" title="Abstract factory pattern">Abstract factory</a></li>

<li><a href="Builder_pattern" title="Builder pattern">Builder</a></li>

<li><a href="Factory_method_pattern" title="Factory method pattern">Factory method</a></li>

<li><a href="Prototype_pattern" title="Prototype pattern">Prototype</a></li>

<li><a href="Singleton_pattern" title="Singleton pattern">Singleton</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Structural_pattern" title="Structural pattern">Structural</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Adapter_pattern" title="Adapter pattern">Adapter</a></li>

<li><a href="Bridge_pattern" title="Bridge pattern">Bridge</a></li>

<li><a href="Composite_pattern" title="Composite pattern">Composite</a></li>

<li><a href="Decorator_pattern" title="Decorator pattern">Decorator</a></li>

<li><a href="Facade_pattern" title="Facade pattern">Facade</a></li>

<li><a href="Flyweight_pattern" title="Flyweight pattern">Flyweight</a></li>

<li><a href="Proxy_pattern" title="Proxy pattern">Proxy</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Behavioral_pattern" title="Behavioral pattern">Behavioral</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Chain-of-responsibility_pattern" title="Chain-of-responsibility pattern">Chain of responsibility</a></li>

<li><a href="Command_pattern" title="Command pattern">Command</a></li>

<li><a href="Interpreter_pattern" title="Interpreter pattern">Interpreter</a></li>

<li><a href="Iterator_pattern" title="Iterator pattern">Iterator</a></li>

<li><a href="Mediator_pattern" title="Mediator pattern">Mediator</a></li>

<li><a href="Memento_pattern" title="Memento pattern">Memento</a></li>

<li><a href="Observer_pattern" title="Observer pattern">Observer</a></li>

<li><a href="State_pattern" title="State pattern">State</a></li>

<li><a href="Strategy_pattern" title="Strategy pattern">Strategy</a></li>

<li><a href="Template_method_pattern" title="Template method pattern">Template method</a></li>

<li><a href="Visitor_pattern" title="Visitor pattern">Visitor</a></li></ul>

</div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Concurrency_pattern" title="Concurrency pattern">Concurrency<br>patterns</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Active_object" title="Active object">Active object</a></li>

<li><a href="Balking_pattern" title="Balking pattern">Balking</a></li>

<li><a href="Binding_properties_pattern" title="Binding properties pattern">Binding properties</a></li>

<li><a href="Double-checked_locking" title="Double-checked locking">Double-checked locking</a></li>

<li><a href="Asynchronous_method_invocation" title="Asynchronous method invocation">Event-based asynchronous</a></li>

<li><a href="Guarded_suspension" title="Guarded suspension">Guarded suspension</a></li>

<li><a href="Monitor_(synchronization)" title="Monitor (synchronization)">Monitor</a></li>

<li><a href="Proactor_pattern" title="Proactor pattern">Proactor</a></li>

<li><a href="Reactor_pattern" title="Reactor pattern">Reactor</a></li>

<li><a href="Readers%E2%80%93writer_lock" title="Readers–writer lock">Read–write lock</a></li>

<li><a href="Scheduler_pattern" class="mw-redirect" title="Scheduler pattern">Scheduler</a></li>

<li><a href="Scheduled-task_pattern" title="Scheduled-task pattern">Scheduled-task pattern</a></li>

<li><a href="Semaphore_(programming)" title="Semaphore (programming)">Semaphore</a></li>

<li><a href="Thread_pool" title="Thread pool">Thread pool</a></li>

<li><a href="Thread-local_storage" title="Thread-local storage">Thread-local storage</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Architectural_pattern" title="Architectural pattern">Architectural<br>patterns</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Front_controller" title="Front controller">Front controller</a></li>

<li><a href="Interceptor_pattern" title="Interceptor pattern">Interceptor</a></li>

<li><a href="Specification_pattern" title="Specification pattern">Specification</a></li>

<li><a href="Publish%E2%80%93subscribe_pattern" title="Publish–subscribe pattern">Publish–subscribe</a></li>

<li><a href="Naked_objects" title="Naked objects">Naked objects</a></li>

<li><a href="Service_locator_pattern" title="Service locator pattern">Service locator</a></li>

<li><a href="Active_record_pattern" title="Active record pattern">Active record</a></li>

<li><a href="Identity_map_pattern" title="Identity map pattern">Identity map</a></li>

<li><a href="Data_transfer_object" title="Data transfer object">Data transfer object</a></li>

<li><a href="Inversion_of_control" title="Inversion of control">Inversion of control</a></li>

<li><a href="JSP_model_2_architecture" title="JSP model 2 architecture">Model 2</a></li>

<li><a href="Broker_pattern" title="Broker pattern">Broker</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Other<br>patterns</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Blackboard_design_pattern" class="mw-redirect" title="Blackboard design pattern">Blackboard</a></li>

<li><a href="Business_delegate_pattern" title="Business delegate pattern">Business delegate</a></li>

<li><a href="Composite_entity_pattern" title="Composite entity pattern">Composite entity</a></li>

<li><a href="Dependency_injection" title="Dependency injection">Dependency injection</a></li>

<li><a href="Guard_(computer_science)" title="Guard (computer science)">Guard clause</a></li>

<li><a href="Intercepting_filter_pattern" title="Intercepting filter pattern">Intercepting filter</a></li>

<li><a href="Lazy_loading" title="Lazy loading">Lazy loading</a></li>

<li><a href="Mock_object" title="Mock object">Mock object</a></li>

<li><a href="Null_object_pattern" title="Null object pattern">Null object</a></li>

<li><a href="Object_pool_pattern" title="Object pool pattern">Object pool</a></li>

<li><a href="Servant_(design_pattern)" title="Servant (design pattern)">Servant</a></li>

<li><a href="Type_Tunnel_pattern" class="mw-redirect" title="Type Tunnel pattern">Type tunnel</a></li>

<li><a href="Method_chaining" title="Method chaining">Method chaining</a></li>

<li><a href="Delegation_pattern" title="Delegation pattern">Delegation</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Books</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><i><a href="Design_Patterns" title="Design Patterns">Design Patterns</a></i></li>

<li><i><a href="Enterprise_Integration_Patterns" title="Enterprise Integration Patterns">Enterprise Integration Patterns</a></i></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">People</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Christopher_Alexander" title="Christopher Alexander">Christopher Alexander</a></li>

<li><a href="Erich_Gamma" title="Erich Gamma">Erich Gamma</a></li>

<li><a href="Ralph_Johnson_(computer_scientist)" title="Ralph Johnson (computer scientist)">Ralph Johnson</a></li>

<li><a href="John_Vlissides" title="John Vlissides">John Vlissides</a></li>

<li><a href="Grady_Booch" title="Grady Booch">Grady Booch</a></li>

<li><a href="Ward_Cunningham" title="Ward Cunningham">Ward Cunningham</a></li>

<li><a href="Martin_Fowler_(software_engineer)" title="Martin Fowler (software engineer)">Martin Fowler</a></li>

<li><a href="Robert_C._Martin" title="Robert C. Martin">Robert Martin</a></li>

<li><a href="Jim_Coplien" title="Jim Coplien">Jim Coplien</a></li>

<li><a href="Douglas_C._Schmidt" title="Douglas C. Schmidt">Douglas Schmidt</a></li>

<li><a href="Linda_Rising" title="Linda Rising">Linda Rising</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Communities</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="The_Hillside_Group" title="The Hillside Group">The Hillside Group</a></li>

<li><a href="Portland_Pattern_Repository" title="Portland Pattern Repository">Portland Pattern Repository</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">See also</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Anti-pattern" title="Anti-pattern">Anti-pattern</a></li>

<li><a href="Architectural_pattern" title="Architectural pattern">Architectural pattern</a></li></ul>

</div></td></tr></tbody></table></div></div><div><div class="zim-footer">

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