What is the difference between @ApplicationScoped and @Singleton scopes in CDI?

What is the difference between @ApplicationScoped and @Singleton scopes in CDI?

In CDI (Contexts and Dependency Injection for Java EE), both @ApplicationScoped and @Singleton are used to define beans with a long lifecycle, but they have different use cases and behavior:

  1. @ApplicationScoped:

    • @ApplicationScoped is a CDI scope that defines a bean with an application-wide lifecycle. There is only one instance of the bean per CDI application context, and it exists for the entire duration of the application.
    • This scope is suitable for objects that should be shared across all parts of the application, such as application-level configuration settings, caches, or other shared resources.
    • @ApplicationScoped beans are created when the application starts and destroyed when the application shuts down.

    Example:

    import javax.enterprise.context.ApplicationScoped; @ApplicationScoped public class AppConfig { // ... }

  2. @Singleton:

    • @Singleton is a CDI scope that defines a bean with a singleton lifecycle. There is only one instance of the bean per CDI application context, just like @ApplicationScoped. However, @Singleton allows you to specify additional options for concurrency control and eager initialization.
    • @Singleton beans can be configured as "container-managed" (default) or "bean-managed" singletons. Container-managed singletons are managed by the CDI container, while bean-managed singletons have more control over their lifecycle.
    • You can use @Lock annotations to control concurrency for @Singleton beans.

    Example:

    import javax.ejb.Singleton; @Singleton public class MySingletonBean { // ... }

In summary, the main differences between @ApplicationScoped and @Singleton in CDI are:

  • @ApplicationScoped is primarily used for objects that need to be shared across the entire application, whereas @Singleton is more flexible and can be used for shared objects with additional control options.
  • @Singleton provides more control over concurrency through annotations like @Lock.
  • @ApplicationScoped is generally simpler to use and sufficient for most shared resource scenarios, while @Singleton offers more advanced options for specific use cases.

The choice between these scopes depends on your application's requirements and whether you need the additional control and concurrency management features provided by @Singleton. For many applications, @ApplicationScoped is a suitable choice for shared resources.

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