Patron Singleton

.idea/.gitignore

+# Default ignored files
+/shelf/
+/workspace.xml

.idea/codeStyles/Project.xml

+<component name="ProjectCodeStyleConfiguration">
+  <code_scheme name="Project" version="173">
+    <ScalaCodeStyleSettings>
+      <option name="MULTILINE_STRING_CLOSING_QUOTES_ON_NEW_LINE" value="true" />
+    </ScalaCodeStyleSettings>
+  </code_scheme>
+</component>
\ No newline at end of file

.idea/codeStyles/codeStyleConfig.xml

+<component name="ProjectCodeStyleConfiguration">
+  <state>
+    <option name="PREFERRED_PROJECT_CODE_STYLE" value="Default" />
+  </state>
+</component>
\ No newline at end of file

.idea/compiler.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="CompilerConfiguration">
+    <annotationProcessing>
+      <profile name="Maven default annotation processors profile" enabled="true">
+        <sourceOutputDir name="target/generated-sources/annotations" />
+        <sourceTestOutputDir name="target/generated-test-sources/test-annotations" />
+        <outputRelativeToContentRoot value="true" />
+        <module name="designpatterns" />
+      </profile>
+    </annotationProcessing>
+  </component>
+</project>
\ No newline at end of file

.idea/encodings.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="Encoding">
+    <file url="file://$PROJECT_DIR$/src/main/java" charset="UTF-8" />
+    <file url="file://$PROJECT_DIR$/src/main/resources" charset="UTF-8" />
+  </component>
+</project>
\ No newline at end of file

.idea/jarRepositories.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="RemoteRepositoriesConfiguration">
+    <remote-repository>
+      <option name="id" value="central" />
+      <option name="name" value="Central Repository" />
+      <option name="url" value="https://repo.maven.apache.org/maven2" />
+    </remote-repository>
+    <remote-repository>
+      <option name="id" value="central" />
+      <option name="name" value="Maven Central repository" />
+      <option name="url" value="https://repo1.maven.org/maven2" />
+    </remote-repository>
+    <remote-repository>
+      <option name="id" value="jboss.community" />
+      <option name="name" value="JBoss Community repository" />
+      <option name="url" value="https://repository.jboss.org/nexus/content/repositories/public/" />
+    </remote-repository>
+  </component>
+</project>
\ No newline at end of file

.idea/misc.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ExternalStorageConfigurationManager" enabled="true" />
+  <component name="MavenProjectsManager">
+    <option name="originalFiles">
+      <list>
+        <option value="$PROJECT_DIR$/pom.xml" />
+      </list>
+    </option>
+  </component>
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_1_8" default="true" project-jdk-name="1.8" project-jdk-type="JavaSDK">
+    <output url="file://$PROJECT_DIR$/out" />
+  </component>
+</project>
\ No newline at end of file

pom.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.example</groupId>
+    <artifactId>designpatterns</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <properties>
+        <maven.compiler.source>8</maven.compiler.source>
+        <maven.compiler.target>8</maven.compiler.target>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    </properties>
+
+</project>
\ No newline at end of file

src/main/java/org/example/designpatterns/singleton/App.java

+package org.example.designpatterns.singleton;
+
+import org.example.designpatterns.singleton.model.Connection;
+
+public class App {
+
+    public static void main(String[] args) {
+        // Instanciación por constructor prohíbido por ser "private"
+        // Connection c = new Connection();
+        Connection c = Connection.getInstancia();
+        c.conectar();
+        c.desconectar();
+
+        boolean rpta = c instanceof Connection;
+        System.out.println(rpta);
+
+
+        // Spring Framework gestiona sus beans como Singleton por defecto
+    }
+}

src/main/java/org/example/designpatterns/singleton/model/Connection.java

+package org.example.designpatterns.singleton.model;
+
+public class Connection {
+
+    // Declaración
+    private static Connection instancia;
+
+    // Para evitar instancia mediante operador "new"
+    private Connection() {}
+
+    // Para obtener la instancia unicamente por este método
+    // La palabra reservada "static" hace posible el acceso mediante Clase.metodo
+    public static Connection getInstancia() {
+        if(instancia == null)
+            instancia = new Connection();
+        return instancia;
+    }
+
+    // Método de prueba
+    public void conectar(){
+        System.out.println("Me conecté a la BD");
+    }
+
+    // Método de prueba
+    public void desconectar(){
+        System.out.println("Me desconecté a la BD");
+    }
+}