Stage 21 · observe a prototype captured by a singleton

Stage 21: observe a prototype captured by a singleton

Understand why repeated service calls reuse one injected prototype.

Open this stage in the online learning path

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.

Shared ReportService → Stored draft reference → Same draft

What we change and why

Reuse the Stage 20 report draft
ReportDraft still has prototype scope. The new ReportService has default singleton scope.
Resolve the dependency once
Spring resolves ReportDraft while constructing the singleton service. The service then keeps that particular reference. Prototype scope has not changed; no new resolution happens in draftForOperation().
Compare two business operations
The two calls return the same reference.

Run this example

Open a terminal in the extracted folder containing pom.xml. Run one command at a time.

Terminal · Windows, macOS and Linux

mvn test
mvn spring-boot:run

This 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

Expected output · ignore framework startup messages

Two operations share a draft: true

Object 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

pom.xml · complete file

<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-21-prototype-injection</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/ReportDraft.java

src/main/java/com/edupoly/lesson/ReportDraft.java · complete file

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/ReportService.java

src/main/java/com/edupoly/lesson/ReportService.java · complete file

package com.edupoly.lesson;

import org.springframework.stereotype.Service;
@Service public class ReportService {
    private final ReportDraft draft;
    public ReportService(ReportDraft draft) { this.draft = draft; }
    public ReportDraft draftForOperation() { return draft; }
}

src/main/java/com/edupoly/lesson/Lesson.java

src/main/java/com/edupoly/lesson/Lesson.java · complete file

package com.edupoly.lesson;

import org.springframework.stereotype.Component;
@Component public class Lesson {
    private final ReportService reports;
    public Lesson(ReportService reports) { this.reports = reports; }
    public void run() {
        ReportDraft first = reports.draftForOperation();
        ReportDraft second = reports.draftForOperation();
        System.out.println("Two operations share a draft: " + (first == second));
    }
}

src/main/java/com/edupoly/lesson/Application.java

src/main/java/com/edupoly/lesson/Application.java · complete file

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

src/main/java/com/edupoly/lesson/AppConfig.java · complete file

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

src/test/java/com/edupoly/lesson/LessonTest.java · complete file

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 operationsHaveExpectedDraftIdentity() {
        try (var context = new AnnotationConfigApplicationContext(AppConfig.class)) {
            var service = context.getBean(ReportService.class);
            assertSame(service.draftForOperation(), service.draftForOperation());
        }
    }
}

Try a failure, then correct it

Change the service to create new ReportDraft() itself. The identity changes, but the draft bypasses Spring configuration and lifecycle processing. Restore container-based resolution.

Words you can now explain

Word	Meaning in this example
Prototype dependency	A fresh object is produced whenever this bean is resolved.
Singleton service	One service is reused by all calls.
Dependency resolution	The constructor dependency is resolved when the singleton service is created.

Practice assignment

Explain where the one prototype object is created and why a second service method call does not create another.

Read the official documentation →