Feature/bael 6101 singleton (#13325)

* BAEL-6101: singleton bean and pattern example

* BAEL-6101: change package

* BAEL-6101: refactor

* BAEL-6101: refactor

* BAEL-6101: PR comment fix

spring-boot-modules/spring-boot-3/src/main/java/com/baeldung/sample/singleton/SingletonBeanConfig.java

@@ -0,0 +1,29 @@
+package com.baeldung.sample.singleton;
+
+import org.springframework.beans.factory.config.ConfigurableBeanFactory;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Scope;
+
+@Configuration
+public class SingletonBeanConfig {
+
+    @Bean
+    @Scope(value = ConfigurableBeanFactory.SCOPE_SINGLETON)
+    public SingletonBean singletonBean() {
+        return new SingletonBean();
+    }
+
+    @Bean
+    @Scope(value = ConfigurableBeanFactory.SCOPE_SINGLETON)
+    public SingletonBean anotherSingletonBean() {
+        return new SingletonBean();
+    }
+
+    static class SingletonBean {
+        public String getValue() {
+            return "test";
+        }
+    }
+
+}

spring-boot-modules/spring-boot-3/src/main/java/com/baeldung/sample/singleton/ThreadSafeSingleInstance.java

@@ -0,0 +1,24 @@
+package com.baeldung.sample.singleton;
+
+public final class ThreadSafeSingleInstance {
+
+    private static volatile ThreadSafeSingleInstance instance = null;
+
+    private ThreadSafeSingleInstance() {}
+
+    public static ThreadSafeSingleInstance getInstance() {
+        if (instance == null) {
+            synchronized(ThreadSafeSingleInstance.class) {
+                if (instance == null) {
+                    instance = new ThreadSafeSingleInstance();
+                }
+            }
+        }
+        return instance;
+    }
+
+    public String getValue() {
+        return "test";
+    }
+
+}

spring-boot-modules/spring-boot-3/src/test/java/com/baeldung/sample/singleton/SingletonBeanUnitTest.java

@@ -0,0 +1,35 @@
+package com.baeldung.sample.singleton;
+
+import static org.junit.jupiter.api.Assertions.assertNotSame;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Qualifier;
+import org.springframework.boot.test.context.SpringBootTest;
+
+@SpringBootTest
+class SingletonBeanUnitTest {
+
+    @Autowired
+    @Qualifier("singletonBean")
+    private SingletonBeanConfig.SingletonBean beanOne;
+
+    @Autowired
+    @Qualifier("singletonBean")
+    private SingletonBeanConfig.SingletonBean beanTwo;
+
+    @Autowired
+    @Qualifier("anotherSingletonBean")
+    private SingletonBeanConfig.SingletonBean beanThree;
+
+    @Test
+    void givenTwoBeansWithSameId_whenInjectingThem_thenSameInstancesAreReturned() {
+        assertSame(beanOne, beanTwo);
+    }
+
+    @Test
+    void givenTwoBeansWithDifferentId_whenInjectingThem_thenDifferentInstancesAreReturned() {
+        assertNotSame(beanOne, beanThree);
+    }
+
+}

spring-boot-modules/spring-boot-3/src/test/java/com/baeldung/sample/singleton/ThreadSafeSingleInstanceUnitTest.java

@@ -0,0 +1,16 @@
+package com.baeldung.sample.singleton;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+class ThreadSafeSingleInstanceUnitTest {
+
+    @Test
+    void givenTwoSingletonInstances_whenGettingThem_thenSameInstancesAreReturned() {
+        ThreadSafeSingleInstance instanceOne = ThreadSafeSingleInstance.getInstance();
+        ThreadSafeSingleInstance instanceTwo = ThreadSafeSingleInstance.getInstance();
+        assertSame(instanceOne, instanceTwo);
+    }
+
+}