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‎content/posts/java-extension-points.md‎

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---
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author: "Phong Nguyen"
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title: "Java OSGi Service"
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date: "2025-02-27"
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description: "Java: Eclipse Extensions and Extension Points"
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tags: ["java","eclipse"] #tags search
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FAcategories: ["syntax"] #The category of the post, similar to tags but usually for broader classification.
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FAseries: ["Themes Guide"] #indicates that this post is part of a series of related posts
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aliases: ["migrate-from-jekyl"] #Alternative URLs or paths that can be used to access this post, useful for redirects from old posts or similar content.
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ShowToc: true # Determines whether to display the Table of Contents (TOC) for the post.
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TocOpen: true # Controls whether the TOC is expanded when the post is loaded.
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weight: 12 # The order in which the post appears in a list of posts. Lower numbers make the post appear earlier.
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---
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Explain how to use the OSGi Service
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[Refer1](https://www.vogella.com/tutorials/EclipseExtensionPoint/article.html)
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[Refer2](https://help.eclipse.org/latest/index.jsp?topic=%2Forg.eclipse.pde.doc.user%2Fguide%2Ftools%2Feditors%2Fmanifest_editor%2Fextension_points.htm)
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[Refer3](https://vogella.com/blog/getting-started-with-osgi-declarative-services-2024/)<br>
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## 1. Introduction
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- **Extensions** are the central mechanism for contribute behavior to the platform. We used it to contribute functionality to a certain type of API.
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- **Extension points** define new functions points for the platform that other plug-ins can plug into.
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- **E.g.** When you want to create modular that can be added or removed at runtime, you can use the OSGi service.
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- These extensions and extension points are defined via the `plugin.xml` file.
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![image](/images/java_extensions_extensionpoints.png)<br>
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## 2. Using Extension points/ Extensions
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## 2.1. Create an EXTENSION POINT
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- A plugin which declares an **extension point** must declare the extension point in its plugin.xml.
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- It will contain code to evaluate the **extensions**.
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```xml
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<?xml version="1.0" encoding="UTF-8"?>
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<?eclipse version="3.4"?>
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<plugin>
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<extension-point id="com.extensionpoint.greeters"
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name="Greeters"
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schema="schema/com.extensionpoint.greeters.exsd" />
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</plugin>
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```
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- It has three attributes:
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- `id`: a required attr, simple name id
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- `name`: a required attr, sime name string
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- `schema`: optional attr, describe
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- **Examples**: Create a plugin named `com.extensionpoint.definition` contained the a extension point.
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```XML
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//MANIFEST
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Manifest-Version: 1.0
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Bundle-ManifestVersion: 2
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Bundle-Name: A Plugin for the Extension Point Definition
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Bundle-SymbolicName: com.extensionpoint.definition;singleton:=true
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Bundle-Version: 1.0.0.qualifier
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Export-Package: com.extensionpoint.definition
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Bundle-Vendor: Phong
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Automatic-Module-Name: com.extensionpoint.definition
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Bundle-RequiredExecutionEnvironment: JavaSE-21
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Import-Package: org.eclipse.core.runtime;version="[3.7.0,4.0.0)",
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org.eclipse.e4.core.di.annotations;version="[1.6.0,2.0.0)"
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```
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```XML
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// plugin.xml
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<?xml version="1.0" encoding="UTF-8"?>
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<?eclipse version="3.4"?>
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<plugin>
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<extension-point id="com.vogella.extensionpoint.definition.greeter" name="Greeter (Extension Point Name)" schema="schema/com.vogella.extensionpoint.definition.greeter.exsd"/>
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</plugin>
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```
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// exsd
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<?xml version='1.0' encoding='UTF-8'?>
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<!-- Schema file written by PDE -->
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<schema targetNamespace="com.vogella.extensionpoint.definition" xmlns="http://www.w3.org/2001/XMLSchema">
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<annotation>
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<appinfo>
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<meta.schema plugin="com.vogella.extensionpoint.definition" id="com.vogella.extensionpoint.definition.greeter.id" name="Greeter (Extensio Point Name)"/>
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</appinfo>
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<documentation>
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[Enter description of this extension point.]
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</documentation>
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</annotation>
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<element name="extension">
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<annotation>
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<appinfo>
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<meta.element />
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</appinfo>
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</annotation>
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<complexType>
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<choice minOccurs="1" maxOccurs="unbounded">
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<element ref="client"/>
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</choice>
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<attribute name="point" type="string" use="required">
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<annotation>
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<documentation>
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</documentation>
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</annotation>
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</attribute>
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<attribute name="id" type="string">
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<annotation>
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<documentation>
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</documentation>
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</annotation>
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</attribute>
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<attribute name="name" type="string">
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<annotation>
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<documentation>
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</documentation>
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<appinfo>
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<meta.attribute translatable="true"/>
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</appinfo>
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</annotation>
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</attribute>
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</complexType>
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</element>
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<element name="client">
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<complexType>
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<attribute name="class" type="string">
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<annotation>
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<documentation>
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</documentation>
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<appinfo>
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<meta.attribute kind="java"/>
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</appinfo>
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</annotation>
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</attribute>
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</complexType>
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</element>
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<annotation>
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<appinfo>
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<meta.section type="since"/>
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</appinfo>
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<documentation>
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[Enter the first release in which this extension point appears.]
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</documentation>
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</annotation>
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<annotation>
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<appinfo>
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<meta.section type="examples"/>
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</appinfo>
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<documentation>
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[Enter extension point usage example here.]
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</documentation>
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</annotation>
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<annotation>
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<appinfo>
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<meta.section type="apiinfo"/>
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</appinfo>
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<documentation>
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[Enter API information here.]
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</documentation>
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</annotation>
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<annotation>
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<appinfo>
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<meta.section type="implementation"/>
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</appinfo>
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<documentation>
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[Enter information about supplied implementation of this extension point.]
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</documentation>
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</annotation>
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</schema>
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```xml
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```
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```java
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// Evaluation class
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package com.extensionpoint.definition;
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import org.eclipse.core.runtime.CoreException;
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import org.eclipse.core.runtime.IConfigurationElement;
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import org.eclipse.core.runtime.IExtensionRegistry;
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import org.eclipse.core.runtime.ISafeRunnable;
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import org.eclipse.core.runtime.SafeRunner;
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import org.eclipse.e4.core.di.annotations.Execute;
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public class EvaluateContributionsHandler {
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private static final String IGREETER_ID =
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"com.extensionpoint.definition.greeter";
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@Execute
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public void execute(IExtensionRegistry registry) {
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IConfigurationElement[] config =
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registry.getConfigurationElementsFor(IGREETER_ID);
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try {
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for (IConfigurationElement e : config) {
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System.out.println("Evaluating extension");
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final Object o =
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e.createExecutableExtension("class");
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if (o instanceof IGreeter) {
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executeExtension(o);
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}
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}
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} catch (CoreException ex) {
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System.out.println(ex.getMessage());
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}
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}
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private void executeExtension(final Object o) {
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ISafeRunnable runnable = new ISafeRunnable() {
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@Override
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public void handleException(Throwable e) {
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System.out.println("Exception in client");
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}
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@Override
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public void run() throws Exception {
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((IGreeter) o).greet();
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}
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};
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SafeRunner.run(runnable);
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}
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}
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```
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```java
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// Attribute class
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package com.vogella.extensionpoint.definition;
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public interface IGreeter {
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void greet();
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}
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```
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<br>
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## 2.2. Create/adding EXTENSIONS to EXTENSION POINTS
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- **Examples**: Create a plugin named `com.extension.contribution` contained the a extension.
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```XML
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// MANIFEST
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Manifest-Version: 1.0
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Bundle-ManifestVersion: 2
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Bundle-Name: Contribution
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Bundle-SymbolicName: com.vogella.extensionpoint.contribution;singleton:=true
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Bundle-Version: 1.0.0.qualifier
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Require-Bundle: org.eclipse.core.runtime;bundle-version="3.32.0",
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com.vogella.extensionpoint.definition;bundle-version="1.0.0"
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Bundle-Vendor: VOGELLA
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Automatic-Module-Name: com.vogella.extensionpoint.contribution
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Bundle-RequiredExecutionEnvironment: JavaSE-21
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```
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```XML
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// plugin.xml
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```
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```java
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// The client class that implemented attribute class
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package com.vogella.extensionpoint.contribution;
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import com.vogella.extensionpoint.definition.IGreeter;
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public class GreeterExensionContribution implements IGreeter {
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@Override
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public void greet() {
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System.err.println("This call from GreeterExensionContribution");
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}
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}
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```
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## 2.3. Accessing EXTENSIONS
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- The information about the available extensions point and the provided extensions are stored in a class of type `IExtensionRegistry` .
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- We can use the dependency injection mechanism to get the `IExtensionRegistry` class that injected.
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- `IExtensionRegistry` contains all extensions of all extensions point.
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```java
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// The following will read all existing extensions
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// for the defined ID of the extension point
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Platform.getExtensionRegistry().
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getConfigurationElementsFor("yourextensionpointid");
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```
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```java
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IConfigurationElement[] elements = Platform.getExtensionRegistry()
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.getConfigurationElementsFor("com.renesas.cdt.devicesupport.api.builderDeviceSupport");
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for (IConfigurationElement ele : elements) {
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}
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```

‎static/images/java_extensions_extensionpoints.png‎

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