Stage 20: compare singleton and prototype lookups
Learning path / Stage 20 of 37
Ask Spring for the same bean twice and observe whether it returns the same object.
Download the complete Stage 20 project · Open the project guide
Before you start
Use JDK 21 or newer and Maven 3.9 or newer. Verify with java -version and mvn -version. Extract this project into its own folder; do not merge its files with another stage.
This lesson reuses the product catalog and Todo responsibilities from Stages 8–14. Only the feature described below is new. Read the ordered explanation before opening the complete files.
What we change and why
Compare references, not class names
Both ReportDraft objects have the same Java class. The == comparison checks whether two references identify the exact same object.
Use the default singleton
SharedCatalog has one shared instance per bean definition per context. The lookup does not create a fresh instance.
Request a prototype
@Scope("prototype") requests a new instance on each resolution of that bean. It does not mean one instance per web request.
Run this example
Open a terminal in the extracted folder containing pom.xml. Run one command at a time.
mvn test
mvn spring-boot:runThis is a console lesson. It does not open a web port. Application.main starts a Spring context, calls the demonstration and closes the context. Read the terminal, not a browser. Spring Boot’s Maven plugin is used as a convenient launcher; these examples show Spring Framework directly.
Expected result
Singleton references equal: true
Prototype references equal: falseObject identifiers and elapsed times can vary. Compare the relationship between values rather than copying an identifier.
Understand each complete file
Create the following files at exactly these paths, or use the ZIP. Source files include their imports. The test checks the behaviour described in the lesson.
pom.xml
<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 https://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>4.1.1</version><relativePath/></parent>
<groupId>com.edupoly</groupId><artifactId>spring-stage-20-scopes</artifactId><version>1.0.0</version>
<properties><java.version>21</java.version></properties>
<dependencies>
<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter</artifactId></dependency>
<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><scope>test</scope></dependency>
</dependencies>
<build><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId></plugin></plugins></build>
</project>
src/main/java/com/edupoly/lesson/SharedCatalog.java
package com.edupoly.lesson;
import org.springframework.stereotype.Component;
@Component public class SharedCatalog { }
src/main/java/com/edupoly/lesson/ReportDraft.java
package com.edupoly.lesson;
import org.springframework.stereotype.Component;
import org.springframework.context.annotation.Scope;
@Component @Scope("prototype")
public class ReportDraft { }
src/main/java/com/edupoly/lesson/Lesson.java
package com.edupoly.lesson;
import org.springframework.context.ApplicationContext;
import org.springframework.stereotype.Component;
@Component public class Lesson {
private final ApplicationContext context;
public Lesson(ApplicationContext context) { this.context = context; }
public void run() {
System.out.println("Singleton references equal: " + (context.getBean(SharedCatalog.class) == context.getBean(SharedCatalog.class)));
System.out.println("Prototype references equal: " + (context.getBean(ReportDraft.class) == context.getBean(ReportDraft.class)));
}
}
src/main/java/com/edupoly/lesson/Application.java
package com.edupoly.lesson;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
public class Application {
public static void main(String[] args) throws Exception {
try (var context = new AnnotationConfigApplicationContext(AppConfig.class)) {
context.getBean(Lesson.class).run();
}
}
}
src/main/java/com/edupoly/lesson/AppConfig.java
package com.edupoly.lesson;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
@Configuration
@ComponentScan(basePackages = "com.edupoly.lesson")
public class AppConfig { }
src/test/java/com/edupoly/lesson/LessonTest.java
package com.edupoly.lesson;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
class LessonTest {
@Test void scopeControlsIdentity() {
try (var context = new AnnotationConfigApplicationContext(AppConfig.class)) {
assertSame(context.getBean(SharedCatalog.class), context.getBean(SharedCatalog.class));
assertNotSame(context.getBean(ReportDraft.class), context.getBean(ReportDraft.class));
}
}
}
Try a failure, then correct it
Remove @Scope("prototype") from ReportDraft. The second comparison becomes true. Restore it and explain the change.
Words you can now explain
| Word | Meaning in this example |
|---|---|
| Singleton | One shared SharedCatalog bean per definition in this context. |
| Prototype | Each getBean(ReportDraft.class) resolves a new ReportDraft. |
Practice assignment
Perform three prototype lookups and confirm all three references differ. Explain why singleton scope does not make mutable fields thread-safe.
Concepts used in this stage
Use the right column for a quick reminder, or open a concept above for its full explanation.