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'https://www.twitter.com/martinfowler'>X (Twitter)</a></p> <p><a href = 'https://boardgamegeek.com/blog/13064/martins-7th-decade'>BGG</a></p> </div> </div> </nav> </nav> <main> <h1>Catalog of Patterns of Enterprise Application Architecture</h1> <div class = 'front-matter'> <div class = 'author'> <address class = 'name'><a href = '/' rel = 'author'>Martin Fowler</a></address> <p class = 'date'>05 March 2003</p> </div> <section class = 'introduction'> <p>Enterprise applications are about the display, manipulation, and storage of large amounts of often complex data; together with the support or automation of business processes with that data. Examples include reservation systems, financial systems, supply chain systems, and many others that run modern business. Enterprise applications have their own particular challenges and solutions, and they are different from embedded systems, control systems, telecoms, or desktop productivity software.</p> <p>The book <a href = '/books/eaa.html'>Patterns of Enterprise Application Architecture</a> collects together patterns that I, and my colleagues, have seen in these systems over the years. They include such topics as layering, structuring business logic, structuring a web user interface, linking in-memory data to a relational database, and handling session state in stateless environments. This site contains short summaries of these patterns, with deep links to the relevant chapters for the online eBook publication on oreilly.com (marked on this page with <a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420'><img src = '/external.svg' style = 'height: 0.8em'></img>).</a></p> </section> <div class = 'cover-image'><a href = '/books/eaa.html'><img src = '/books/eaa.jpg'></img></a></div> </div> <p class = 'chapter-title'>Domain Logic Patterns</p> <div class = 'pattern-list'> <div class = 'pattern'> <h2><a href = 'transactionScript.html'>Transaction Script</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch09.html#ch09lev1sec1'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Organizes business logic by procedures where each procedure handles a single request from the presentation.</p> </div> <div class = 'pattern'> <h2><a href = 'domainModel.html'>Domain Model</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch09.html#ch09lev1sec2'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An object model of the domain that incorporates both behavior and data.</p> </div> <div class = 'pattern'> <h2><a href = 'tableModule.html'>Table Module</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch09.html#ch09lev1sec3'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>A single instance that handles the business logic for all rows in a database table or view.</p> </div> <div class = 'pattern'> <h2><a href = 'serviceLayer.html'>Service Layer</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch09.html#ch09lev1sec4'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Defines an application's boundary with a layer of services that establishes a set of available operations and coordinates the application's response in each operation.</p> </div> </div> <p class = 'chapter-title'>Data Source Architectural Patterns</p> <div class = 'pattern-list'> <div class = 'pattern'> <h2><a href = 'tableDataGateway.html'>Table Data Gateway</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch10.html#ch10lev1sec1'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An object that acts as a gateway to a database table. One instance handles all the rows in the table.</p> </div> <div class = 'pattern'> <h2><a href = 'rowDataGateway.html'>Row Data Gateway</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch10.html#ch10lev1sec2'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An object that acts as a gateway to a single record in a data source. There is one instance per row.</p> </div> <div class = 'pattern'> <h2><a href = 'activeRecord.html'>Active Record</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch10.html#ch10lev1sec3'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An object that wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data.</p> </div> <div class = 'pattern'> <h2><a href = 'dataMapper.html'>Data Mapper</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch10.html#ch10lev1sec4'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>A layer of mappers that moves data between objects and a database while keeping them independent of each other and the mapper itself.</p> </div> </div> <p class = 'chapter-title'>Object-Relational Behavioral Patterns</p> <div class = 'pattern-list'> <div class = 'pattern'> <h2><a href = 'unitOfWork.html'>Unit of Work</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch11.html#ch11lev1sec1'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Maintains a list of objects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrency problems.</p> </div> <div class = 'pattern'> <h2><a href = 'identityMap.html'>Identity Map</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch11.html#ch11lev1sec2'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Ensures that each object gets loaded only once by keeping every loaded object in a map. Looks up objects using the map when referring to them.</p> </div> <div class = 'pattern'> <h2><a href = 'lazyLoad.html'>Lazy Load</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch11.html#ch11lev1sec3'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An object that doesn't contain all of the data you need but knows how to get it.</p> </div> </div> <p class = 'chapter-title'>Object-Relational Structural Patterns</p> <div class = 'pattern-list'> <div class = 'pattern'> <h2><a href = 'identityField.html'>Identity Field</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch12.html#ch12lev1sec1'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Saves a database ID field in an object to maintain identity between an in-memory object and a database row.</p> </div> <div class = 'pattern'> <h2><a href = 'inheritanceMappers.html'>Inheritance Mappers</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch12.html#ch12lev1sec10'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>A structure to organize database mappers that handle inheritance hierarchies.</p> </div> <div class = 'pattern'> <h2><a href = 'foreignKeyMapping.html'>Foreign Key Mapping</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch12.html#ch12lev1sec2'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Maps an association between objects to a foreign key reference between tables.</p> </div> <div class = 'pattern'> <h2><a href = 'associationTableMapping.html'>Association Table Mapping</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch12.html#ch12lev1sec3'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Saves an association as a table with foreign keys to the tables that are linked by the association.</p> </div> <div class = 'pattern'> <h2><a href = 'dependentMapping.html'>Dependent Mapping</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch12.html#ch12lev1sec4'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Has one class perform the database mapping for a child class.</p> </div> <div class = 'pattern'> <h2><a href = 'embeddedValue.html'>Embedded Value</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch12.html#ch12lev1sec5'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Maps an object into several fields of another object's table.</p> </div> <div class = 'pattern'> <h2><a href = 'serializedLOB.html'>Serialized LOB</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch12.html#ch12lev1sec6'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Saves a graph of objects by serializing them into a single large object (LOB), which it stores in a database field.</p> </div> <div class = 'pattern'> <h2><a href = 'singleTableInheritance.html'>Single Table Inheritance</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch12.html#ch12lev1sec7'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Represents an inheritance hierarchy of classes as a single table that has columns for all the fields of the various classes.</p> </div> <div class = 'pattern'> <h2><a href = 'classTableInheritance.html'>Class Table Inheritance</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch12.html#ch12lev1sec8'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Represents an inheritance hierarchy of classes with one table for each class.</p> </div> <div class = 'pattern'> <h2><a href = 'concreteTableInheritance.html'>Concrete Table Inheritance</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch12.html#ch12lev1sec9'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Represents an inheritance hierarchy of classes with one table per concrete class in the hierarchy.</p> </div> </div> <p class = 'chapter-title'>Object-Relational Metadata Mapping Patterns</p> <div class = 'pattern-list'> <div class = 'pattern'> <h2><a href = 'metadataMapping.html'>Metadata Mapping</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch13.html#ch13lev1sec1'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Holds details of object-relational mapping in metadata.</p> </div> <div class = 'pattern'> <h2><a href = 'queryObject.html'>Query Object</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch13.html#ch13lev1sec2'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An object that represents a database query.</p> </div> <div class = 'pattern'> <h2><a href = 'repository.html'>Repository</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch13.html#ch13lev1sec3'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Mediates between the domain and data mapping layers using a collection-like interface for accessing domain objects.</p> </div> </div> <p class = 'chapter-title'>Web Presentation Patterns</p> <div class = 'pattern-list'> <div class = 'pattern'> <h2><a href = 'modelViewController.html'>Model View Controller</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch14.html#ch14lev1sec1'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Splits user interface interaction into three distinct roles.</p> </div> <div class = 'pattern'> <h2><a href = 'pageController.html'>Page Controller</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch14.html#ch14lev1sec2'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An object that handles a request for a specific page or action on a Web site.</p> </div> <div class = 'pattern'> <h2><a href = 'frontController.html'>Front Controller</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch14.html#ch14lev1sec3'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>A controller that handles all requests for a Web site.</p> </div> <div class = 'pattern'> <h2><a href = 'templateView.html'>Template View</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch14.html#ch14lev1sec4'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Renders information into HTML by embedding markers in an HTML page.</p> </div> <div class = 'pattern'> <h2><a href = 'transformView.html'>Transform View</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch14.html#ch14lev1sec5'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>A view that processes domain data element by element and transforms it into HTML.</p> </div> <div class = 'pattern'> <h2><a href = 'twoStepView.html'>Two Step View</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch14.html#ch14lev1sec6'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Turns domain data into HTML in two steps: first by forming some kind of logical page, then rendering the logical page into HTML.</p> </div> <div class = 'pattern'> <h2><a href = 'applicationController.html'>Application Controller</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch14.html#ch14lev1sec7'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>A centralized point for handling screen navigation and the flow of an application.</p> </div> </div> <p class = 'chapter-title'>Distribution Patterns</p> <div class = 'pattern-list'> <div class = 'pattern'> <h2><a href = 'remoteFacade.html'>Remote Facade</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch15.html#ch15lev1sec1'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Provides a coarse-grained facade on fine-grained objects to improve efficiency over a network.</p> </div> <div class = 'pattern'> <h2><a href = 'dataTransferObject.html'>Data Transfer Object</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch15.html#ch15lev1sec2'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An object that carries data between processes in order to reduce the number of method calls.</p> </div> </div> <p class = 'chapter-title'>Offline Concurrency Patterns</p> <div class = 'pattern-list'> <div class = 'pattern'> <h2><a href = 'optimisticOfflineLock.html'>Optimistic Offline Lock</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch16.html#ch16lev1sec1'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Prevents conflicts between concurrent business transactions by detecting a conflict and rolling back the transaction.</p> </div> <div class = 'pattern'> <h2><a href = 'pessimisticOfflineLock.html'>Pessimistic Offline Lock</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch16.html#ch16lev1sec2'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Prevents conflicts between concurrent business transactions by allowing only one business transaction at a time to access data.</p> </div> <div class = 'pattern'> <h2><a href = 'coarseGrainedLock.html'>Coarse-Grained Lock</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch16.html#ch16lev1sec3'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Locks a set of related objects with a single lock.</p> </div> <div class = 'pattern'> <h2><a href = 'implicitLock.html'>Implicit Lock</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch16.html#ch16lev1sec4'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Allows framework or layer supertype code to acquire offline locks.</p> </div> </div> <p class = 'chapter-title'>Session State Patterns</p> <div class = 'pattern-list'> <div class = 'pattern'> <h2><a href = 'clientSessionState.html'>Client Session State</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch17.html#ch17lev1sec1'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Stores session state on the client.</p> </div> <div class = 'pattern'> <h2><a href = 'serverSessionState.html'>Server Session State</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch17.html#ch17lev1sec2'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Keeps the session state on a server system in a serialized form</p> </div> <div class = 'pattern'> <h2><a href = 'databaseSessionState.html'>Database Session State</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch17.html#ch17lev1sec3'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Stores session data as committed data in the database.</p> </div> </div> <p class = 'chapter-title'>Base Patterns</p> <div class = 'pattern-list'> <div class = 'pattern'> <h2><a href = 'gateway.html'>Gateway</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec1'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An object that encapsulates access to an external system or resource.</p> </div> <div class = 'pattern'> <h2><a href = 'serviceStub.html'>Service Stub</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec10'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Removes dependence upon problematic services during testing. WSDL</p> </div> <div class = 'pattern'> <h2><a href = 'recordSet.html'>Record Set</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec11'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An in-memory representation of tabular data.</p> </div> <div class = 'pattern'> <h2><a href = 'mapper.html'>Mapper</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec2'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>An object that sets up a communication between two independent objects.</p> </div> <div class = 'pattern'> <h2><a href = 'layerSupertype.html'>Layer Supertype</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec3'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>A type that acts as the supertype for all types in its layer.</p> </div> <div class = 'pattern'> <h2><a href = 'separatedInterface.html'>Separated Interface</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec4'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Defines an interface in a separate package from its implementation.</p> </div> <div class = 'pattern'> <h2><a href = 'registry.html'>Registry</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec5'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>A well-known object that other objects can use to find common objects and services.</p> </div> <div class = 'pattern'> <h2><a href = 'valueObject.html'>Value Object</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec6'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>A small simple object, like money or a date range, whose equality isn't based on identity.</p> </div> <div class = 'pattern'> <h2><a href = 'money.html'>Money</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec7'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Represents a monetary value.</p> </div> <div class = 'pattern'> <h2><a href = 'specialCase.html'>Special Case</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec8'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>A subclass that provides special behavior for particular cases.</p> </div> <div class = 'pattern'> <h2><a href = 'plugin.html'>Plugin</a>聽聽<a class = 'external' href = 'https://learning.oreilly.com/library/view/patterns-of-enterprise/0321127420/ch18.html#ch18lev1sec9'><img alt = 'link to chapter on oreilly.com' class = 'external' src = '/external.svg'></img></a></h2> <p class = 'intent'>Links classes during configuration rather than compilation.</p> </div> </div> <section class = 'end-matter'> <h2>Notes</h2> <p>This page had a design refresh in July 2024, but the content is still the same as its original 2003 publication.</p> <p>Many of these sketch diagrams in the patterns demonstrate the rather poor GIF output of Visio at that time. 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