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 <time datetime="2021-05-28T21:36:41+09:00">2021-05-28</time> by Dennis in 
&nbsp;<a href="/categories/tech" rel="category tag">tech</a>


&nbsp;<a href="/tags/java" rel="tag">java</a>, <a href="/tags/design-pattern" rel="tag">Design Pattern</a>


 </header>
 (Java 디자인 패턴 스터디 모집 중 : <a href="https://github.com/bluedskim/javaDesignPatterns">https://github.com/bluedskim/javaDesignPatterns</a>)
<h2 id="observer-패턴-다이어그램">Observer 패턴 다이어그램</h2>
<img src="https://dev-to-uploads.s3.amazonaws.com/i/0umqa0oz6wf95h6aza4j.jpeg" alt="Design Pattern: The Observer Pattern" title="Design Pattern: The Observer Pattern">
(출처:<a href="https://dev.to/danlee0528/design-pattern-the-observer-pattern-3oha">https://dev.to/danlee0528/design-pattern-the-observer-pattern-3oha</a>)
<h2 id="해결하려는-문제--알림의-주체는-누구인가">해결하려는 문제 : 알림의 주체는 누구인가?</h2>
<ul>
<li>
push vs poll
어떤 이벤트가 발생하면 진행되어야 할 후속 이벤트들이 있다면 이벤트를 발생하는쪽이 알려줘야(Push) 하는가? 아니면 이벤트를 수신하는 쪽이 이벤트를 관찰(Poll)하고 있어야 하는가?
</li>
<li>
알려줘야 할 대상(Subscriber)들을 소스코드에 하드코딩하여 관리해야 하는가
</li>
</ul>
<h2 id="특징용도">특징/용도</h2>
<ol>
<li>publisher-subscriber(pub-sub)이라고도 한다.</li>
<li>객체들은 one(publisher)-to-many(subscriber) 관계를 가진다.</li>
<li>subscriber는 publisher알지만 publisher는 subscriber를 알지 못한다. 즉 관계는 단방향이다.</li>
</ol>
<h2 id="고려사항">고려사항</h2>
<ol>
<li>event driven아키텍처의 분산 이벤트 관리에서 많이 사용된다(<a href="https://en.wikipedia.org/wiki/Observer_pattern">https://en.wikipedia.org/wiki/Observer_pattern</a>)</li>
<li>동시성 처리를 위해 observer가 subject-events를 수신할 때 멀티쓰레드로 동작할 필요도 있다.</li>
</ol>
<h2 id="클래스-다이어그램">클래스 다이어그램</h2>
<img src="https://www.plantuml.com/plantuml/png/fLB1IWCn4BtdAq91sZsuU--XL5JemMgnNv1kfhlHJR8aiokB2WgzAOX7GJrxLF3YBzNjFvZMGhUo5S6vP9YyUNapBrsBp60c4teqic1oC7Hs-pARlBUc7rUVTm_bvCaZ91OSACWK2of1Zi3OzHB6cbj2K602ln75mBYxY15Jsmw2I62SZVnGos2KPC3jgP312SlP7bWHgG531ACi6Mlj4be-tvUJj_BcSVfwJRSuGsWr2XT-l-zptgJDNgyjPSFf5ShXnom4EZ0w1OF5RinK10V28YWmX8G9ivOUmfdTYK251JqdrCKwV9YbgJP8xVUvGEHQSCeuhoTkrPTf6YRUgjs0r3dyXy42phMtXcY4YgXokKSkgsFDsnKaoxcME8oXhoqUGmXgGR1YJDUU_Q5YFtTu2ZbwLRPeAU8FkYgBzlrE_Pf9LRRlNtGscLTlajxUQ7ON57TV_0i0" alt="Observer 패턴(김동석)" title="Observer 패턴(김동석)">
<h2 id="소스">소스</h2>
<ul>
<li>PropertyChangeSupport 를 사용한 observer 패턴 샘플</li>
<li>observable(Subject, Publisher) : NewsAgency.java
<div>
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</code></pre></td>
<td>
<pre><code data-lang="java">package net.dskim.desingpattern.observer;

import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;

/**
 * Subject(Publisher)
 */
public class NewsAgency {
	private String previousNews;

	private PropertyChangeSupport support;

	public NewsAgency() {
		support = new PropertyChangeSupport(this);
	}

	public void addPropertyChangeListener(PropertyChangeListener pcl) {
		support.addPropertyChangeListener(pcl);
	}

	public void removePropertyChangeListener(PropertyChangeListener pcl) {
		support.removePropertyChangeListener(pcl);
	}

	public void setNews(String news) {
		support.firePropertyChange("news", this.previousNews, news);
		this.previousNews = news;

	}
}</code></pre></td></tr></tbody></table>
</div></li>
<li>observer(Listener, Subscriber) : NewsChannel.java
<div>
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</code></pre></td>
<td>
<pre><code data-lang="java">package net.dskim.desingpattern.observer;

import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;

/**
 * Listener(Subscriber)
 */
public class NewsChannel implements PropertyChangeListener {

	private String news;

	@Override
	public void propertyChange(PropertyChangeEvent evt) {
		news = (String) evt.getNewValue();
	}

	public String getNews() {
		return news;
	}
}</code></pre></td></tr></tbody></table>
</div></li>
<li>client : ObserverTest.java
<div>
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</code></pre></td>
<td>
<pre><code data-lang="java">package net.dskim.desingpattern.observer;

import static org.junit.jupiter.api.Assertions.assertEquals;

import org.junit.jupiter.api.Test;

import lombok.extern.slf4j.Slf4j;

@Slf4j
public class ObserverTest {

	@Test
	public void whenChangingPCLNewsAgencyState_thenONewsChannelNotified() {
		String newsName = "뉴스 제목";

		NewsAgency observable = new NewsAgency();
		NewsChannel observer1 = new NewsChannel();
		NewsChannel observer2 = new NewsChannel();

		observable.addPropertyChangeListener(observer1);
		observable.addPropertyChangeListener(observer2);
		observable.setNews(newsName);

		log.info("observer.getNews()={}", observer1.getNews());
		assertEquals(observer1.getNews(), newsName);
		assertEquals(observer2.getNews(), newsName);
	}
}</code></pre></td></tr></tbody></table>
</div></li>
</ul>
<h2 id="참고">참고</h2>
<ul>
<li><a href="https://refactoring.guru/design-patterns/observer">refactoring.guru : Observer</a></li>
<li><a href="https://github.com/eugenp/tutorials/tree/master/patterns/design-patterns-behavioral/src/main/java/com/baeldung/observer">baeldung : The Observer Pattern in Java</a></li>
<li><a href="https://en.wikipedia.org/wiki/Observer_pattern">wikipedia : Observer pattern</a></li>
</ul>


 

 
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(Java 디자인 패턴 스터디 모집 중 : https://github.com/bluedskim/javaDesignPatterns) Observer 패턴 다이어그램 (출처:https://dev.to/danlee0528/design-pattern-the

Java 디자인 패턴-09.Observer

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