What are Spring bean scopes and how do singleton and prototype differ?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Know the default

   Beans are singleton scoped unless you declare otherwise, giving one shared instance per ApplicationContext.

2. Step 2

   Declare the scope

   Annotate a bean with @Scope("prototype") or use @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE) to change behaviour.

3. Step 3

   Understand instantiation timing

   Singletons are created eagerly at startup; prototypes are created lazily on each request or injection.

4. Step 4

   Mind the lifecycle gap

   Spring fully manages singleton destruction but does NOT call destroy callbacks on prototype beans.

5. Step 5

   Handle scope mixing

   Injecting a prototype into a singleton needs ObjectProvider, @Lookup, or a proxy so a fresh instance is fetched each time.

Code Example

@Configuration
public class AppConfig {

  @Bean // singleton by default: one shared instance
  public MetricsRegistry metricsRegistry() {
    return new MetricsRegistry();
  }

  @Bean
  @Scope("prototype") // new instance on every request
  public ReportBuilder reportBuilder() {
    return new ReportBuilder();
  }
}

@Service
public class ReportService {

  private final ObjectProvider<ReportBuilder> builderProvider;

  public ReportService(ObjectProvider<ReportBuilder> builderProvider) {
    this.builderProvider = builderProvider;
  }

  public Report build() {
    ReportBuilder builder = builderProvider.getObject(); // fresh each call
    return builder.compile();
  }
}

Follow-up Questions

• Why does Spring not call the destroy callback on prototype beans?
• How do you inject a prototype bean into a singleton correctly?
• What are the request and session scopes used for?
• Is a Spring singleton the same as the singleton design pattern?
• How do you make a singleton bean thread-safe?