bundles/org.eclipse.pde.doc.user/concepts/extension.htm - platform/eclipse.platform.common - Git at Google

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 <h1>Extensions and Extension Points</h1>

 <p>A basic rule for building modular software systems is to avoid tight
 coupling between components.  If components are tightly integrated,
 it becomes difficult to assemble the pieces into
 different configurations or to replace a component with a different implementation
 without causing a ripple of changes across the system.</p>

 <p>Loose coupling in Eclipse is achieved partially through the mechanism
 of <strong>extensions</strong> and <strong>extension points</strong>.
 The simplest metaphor for describing extensions and extension points is electrical
 outlets. The outlet, or socket,
 is the extension point; the plug, or light bulb that connects to it, the
 extension. As with electric outlets, extension points come in a wide
 variety of shapes and sizes, and only the extensions that are designed
 for that particular extension point will fit.</p>

 <p>When a plug-in wants to allow other plug-ins to extend or customize
 portions of its functionality, it will declare an extension
 point.  The extension point declares a contract, typically a combination
 of XML markup and Java interfaces, that extensions must conform to.
 Plug-ins that want to connect to that extension point must implement
 that contract in their extension.  The key attribute is that the plug-in
 being extended knows nothing about the plug-in that is connecting to
 it beyond the scope of that extension point contract. This allows plug-ins
 built by different individuals or companies to interact seamlessly, even
 without their knowing much about one another.</p>

 <p>The Eclipse Platform has many applications of the extension
 and extension point concept.  Some extensions are entirely
 <i>declarative</i>; that is, they contribute no code at all.  For example,
 one extension point provides customized key bindings,
 and another defines custom file annotations, called <i>markers</i>;
 neither of these extension points requires any code on behalf of the extension.
 </p>

 <p>Another category of extension points is for overriding the default behavior of
 a component.  For example, the Java development tools include a
 code formatter but also supply an extension point for third-party
 code formatters to be plugged in.  The resources plug-in has an
 extension point that allows certain plug-ins to replace the implementation
 of basic file operations, such as moving and deletion. </p>

 <p>Yet another category
 of extension points is used to group related elements in the user
 interface.  For example, extension points for providing views,
 editors, and wizards to the UI allow the base UI plug-in to group
 common features, such as
 putting all import wizards into a single dialog,
 and to define a consistent way of presenting UI contributions from a wide
 variety of other plug-ins.</p>

 <h3 class="related">Related references</h3>
 <a href="../guide/tools/editors/manifest_editor/extensions.htm">Extensions  (Plug-in Editor)</a><br>
 <a href="../guide/tools/editors/manifest_editor/extension_points.htm">Extension Points (Plug-in Editor)</a>

 <h3 class="related">Related concepts</h3>
 <a href="plugin.htm">Plug-in</a>

 </body>
 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
	"http://www.w3.org/TR/html4/loose.dtd">
	<html lang="en">
	<head>
	<meta name="copyright" content="Copyright (c) IBM Corporation and others 2009. This page is made available under license. For full details see the LEGAL in the documentation book that contains this page." >
	<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
	<title>Extensions and Extension Points</title>
	<link rel="stylesheet" href="../book.css" charset="ISO-8859-1" type="text/css">
	<script language="JavaScript" src="PLUGINS_ROOT/org.eclipse.help/livehelp.js" type="text/javascript"> </script>
	</head>

	<body>
	<h1>Extensions and Extension Points</h1>

	<p>A basic rule for building modular software systems is to avoid tight
	coupling between components. If components are tightly integrated,
	it becomes difficult to assemble the pieces into
	different configurations or to replace a component with a different implementation
	without causing a ripple of changes across the system.</p>

	<p>Loose coupling in Eclipse is achieved partially through the mechanism
	of <strong>extensions</strong> and <strong>extension points</strong>.
	The simplest metaphor for describing extensions and extension points is electrical
	outlets. The outlet, or socket,
	is the extension point; the plug, or light bulb that connects to it, the
	extension. As with electric outlets, extension points come in a wide
	variety of shapes and sizes, and only the extensions that are designed
	for that particular extension point will fit.</p>

	<p>When a plug-in wants to allow other plug-ins to extend or customize
	portions of its functionality, it will declare an extension
	point. The extension point declares a contract, typically a combination
	of XML markup and Java interfaces, that extensions must conform to.
	Plug-ins that want to connect to that extension point must implement
	that contract in their extension. The key attribute is that the plug-in
	being extended knows nothing about the plug-in that is connecting to
	it beyond the scope of that extension point contract. This allows plug-ins
	built by different individuals or companies to interact seamlessly, even
	without their knowing much about one another.</p>

	<p>The Eclipse Platform has many applications of the extension
	and extension point concept. Some extensions are entirely
	<i>declarative</i>; that is, they contribute no code at all. For example,
	one extension point provides customized key bindings,
	and another defines custom file annotations, called <i>markers</i>;
	neither of these extension points requires any code on behalf of the extension.
	</p>

	<p>Another category of extension points is for overriding the default behavior of
	a component. For example, the Java development tools include a
	code formatter but also supply an extension point for third-party
	code formatters to be plugged in. The resources plug-in has an
	extension point that allows certain plug-ins to replace the implementation
	of basic file operations, such as moving and deletion. </p>

	<p>Yet another category
	of extension points is used to group related elements in the user
	interface. For example, extension points for providing views,
	editors, and wizards to the UI allow the base UI plug-in to group
	common features, such as
	putting all import wizards into a single dialog,
	and to define a consistent way of presenting UI contributions from a wide
	variety of other plug-ins.</p>

	<h3 class="related">Related references</h3>
	<a href="../guide/tools/editors/manifest_editor/extensions.htm">Extensions (Plug-in Editor)</a><br>
	<a href="../guide/tools/editors/manifest_editor/extension_points.htm">Extension Points (Plug-in Editor)</a>

	<h3 class="related">Related concepts</h3>
	<a href="plugin.htm">Plug-in</a>

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