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The Singleton Pattern
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‎pom.xml‎

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<properties>
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<maven.compiler.source>21</maven.compiler.source>
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<maven.compiler.target>21</maven.compiler.target>
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<maven.compiler.release>21</maven.compiler.release>
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<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
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<junit.version>5.8.2</junit.version>
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</properties>
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<dependencies>
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<artifactId>guava</artifactId>
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<version>33.4.0-jre</version>
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</dependency>
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<!-- JUnit 5 -->
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-api</artifactId>
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<version>${junit.version}</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-engine</artifactId>
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<version>${junit.version}</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.apache.commons</groupId>
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<artifactId>commons-lang3</artifactId>
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<version>3.12.0</version>
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<scope>test</scope>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<!-- sse the updated Maven compiler plugin to support Java 21 -->
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-compiler-plugin</artifactId>
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<version>3.11.0</version>
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<configuration>
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<release>${maven.compiler.release}</release>
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<showWarnings>true</showWarnings>
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<showDeprecation>true</showDeprecation>
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</configuration>
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</plugin>
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<!-- plugin for test -->
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-surefire-plugin</artifactId>
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<version>3.1.2</version>
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</plugin>
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<!-- if you need to use the preview feature -->
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-compiler-plugin</artifactId>
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<version>3.11.0</version>
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<configuration>
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<compilerArgs>
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<arg>--enable-preview</arg>
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</compilerArgs>
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</configuration>
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</plugin>
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</plugins>
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</build>
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<!-- if you need to use Java 21 preview features -->
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<profiles>
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<profile>
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<id>enable-preview</id>
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<activation>
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<property>
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<name>enablePreview</name>
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</property>
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</activation>
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<build>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-compiler-plugin</artifactId>
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<configuration>
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<compilerArgs>
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<arg>--enable-preview</arg>
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</compilerArgs>
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</configuration>
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</plugin>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-surefire-plugin</artifactId>
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<configuration>
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<argLine>--enable-preview</argLine>
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</configuration>
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</plugin>
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</plugins>
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</build>
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</profile>
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</profiles>
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</project>

‎src/main/java/Singleton/DoubleCheckedLockingSingleton.java‎

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package Singleton;
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import java.io.Serial;
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import java.io.Serializable;
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public final class DoubleCheckedLockingSingleton implements Serializable {
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private static volatile DoubleCheckedLockingSingleton instance; //use volatile to prevent instruction reordering
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private static boolean initialized = false; //defense signs
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private DoubleCheckedLockingSingleton() {
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synchronized (DoubleCheckedLockingSingleton.class) {
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if (initialized) { //prevent reflection calls
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throw new RuntimeException("Prohibit creating instances through reflection");
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}
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initialized = true;
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}
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}
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public static DoubleCheckedLockingSingleton getInstance() {
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if (instance == null) { //first check
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synchronized (DoubleCheckedLockingSingleton.class) {
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if (instance == null) { //second check
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instance = new DoubleCheckedLockingSingleton();
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}
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}
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}
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return instance;
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}
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//return existing instances during deserialization
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@Serial
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private Object readResolve() {
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return getInstance();
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}
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}

‎src/main/java/Singleton/EnumSingleton.java‎

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package Singleton;
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public enum EnumSingleton {
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INSTANCE
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;
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public void doSomething() {
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// do sth...
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}
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}

‎src/main/java/Singleton/StaticInnerClassSingleton.java‎

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package Singleton;
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public final class StaticInnerClassSingleton {
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private StaticInnerClassSingleton() {}
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private static class SingletonHolder {
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private static final StaticInnerClassSingleton INSTANCE = new StaticInnerClassSingleton();
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}
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public static StaticInnerClassSingleton getInstance() {
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return SingletonHolder.INSTANCE;
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}
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}

‎src/main/java/common/utils/SerializationUtils.java‎

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package common.utils;
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import java.io.*;
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public class SerializationUtils {
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public static byte[] serialize(Object obj) throws IOException {
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ByteArrayOutputStream baos = new ByteArrayOutputStream();
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try (ObjectOutputStream oos = new ObjectOutputStream(baos)) {
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oos.writeObject(obj);
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}
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return baos.toByteArray();
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}
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@SuppressWarnings("unchecked")
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public static <T> T deserialize(byte[] bytes) throws IOException, ClassNotFoundException {
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try (ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(bytes))) {
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return (T) ois.readObject();
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}
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}
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}

‎src/test/java/singleton/DoubleCheckedLockingSingletonTest.java‎

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package singleton;
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import Singleton.DoubleCheckedLockingSingleton;
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import common.utils.SerializationUtils;
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import org.junit.jupiter.api.Test;
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import java.lang.reflect.Constructor;
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import static org.junit.jupiter.api.Assertions.*;
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public class DoubleCheckedLockingSingletonTest {
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@Test
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public void testSingletonInstance() {
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//Test ordinary obtaining instances
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DoubleCheckedLockingSingleton instance1 = DoubleCheckedLockingSingleton.getInstance();
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DoubleCheckedLockingSingleton instance2 = DoubleCheckedLockingSingleton.getInstance();
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assertNotNull(instance1);
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assertSame(instance1, instance2, "Two instances should be the same object");
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}
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@Test
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public void testReflectionAttack() {
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try {
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//Attempt to create an instance through reflection
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Constructor<DoubleCheckedLockingSingleton> constructor =
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DoubleCheckedLockingSingleton.class.getDeclaredConstructor();
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constructor.setAccessible(true);
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//First instance acquisition (normal method)
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DoubleCheckedLockingSingleton instance1 = DoubleCheckedLockingSingleton.getInstance();
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//Attempt to create a second instance through reflection
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assertThrows(RuntimeException.class, () -> {
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DoubleCheckedLockingSingleton instance2 = constructor.newInstance();
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}, "An exception should be thrown to prevent reflection from creating an instance");
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} catch (Exception e) {
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fail(STR."Reflection call failed: \{e.getMessage()}");
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}
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}
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@Test
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public void testSerialization() throws Exception {
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//Obtain a singleton instance
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DoubleCheckedLockingSingleton original = DoubleCheckedLockingSingleton.getInstance();
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//Serialize
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byte[] serialized = SerializationUtils.serialize(original);
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//Deserialize
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DoubleCheckedLockingSingleton deserialized =
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SerializationUtils.deserialize(serialized);
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//Verify that even after deserialization, it still remains the same instance
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assertSame(original, deserialized, "After deserialization, it should be the same instance");
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}
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}

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