Understanding the Differences Between @ApplicationScoped and @Singleton in CDI

What are the key differences between @ApplicationScoped and @Singleton in CDI?

In Contexts and Dependency Injection (CDI), both @ApplicationScoped and @Singleton are used to manage the lifecycle of beans, but they serve different purposes and behave differently in terms of instantiation and proxying.

import javax.enterprise.context.ApplicationScoped;
import javax.inject.Singleton;

@ApplicationScoped
public class AppConfiguration {
    // Application-scoped bean
}

@Singleton
public class GlobalService {
    // Singleton-scoped bean
}

Causes

• @ApplicationScoped beans are tied to the lifecycle of the application and are created once per application and can vary between different deployments or contexts.
• @Singleton beans, by contrast, are tied to the lifecycle of the JVM and exist as a single instance throughout the entire application runtime, independent of the application context.

Solutions

• Use @ApplicationScoped when you need a single instance per application but want to leverage the context lifecycle, enabling easier management and scoping of resources in a web application.
• Use @Singleton when you require a single instance shared across all applications, and you need tight control over the bean instance's lifecycle.

Mistake: Assuming @ApplicationScoped beans can have multiple instances.

Solution: @ApplicationScoped beans will only have one instance at a time per application, but they can be shared across different threads safely.

Mistake: Confusing @Singleton with application lifecycle management.

Solution: Remember that @Singleton is not scoped to the application but rather to the program as a whole, which remains throughout the JVM's lifecycle.