Spring Boot Compatibility

How is Across different from Spring Boot?

Across builds much more on top of Spring Framework than on Spring Boot. Across applications execute as a Spring Boot application and use some features provided by Spring Boot (like embedded webserver, YAML configuration and devtools).

Auto-configuration, however, is not supported, which means simply adding a starter to your project will not be enough. Many available configuration classes from Spring Boot will also not work in an Across application, because they are setup for a single ApplicationContext, whereas Across uses a different hierarchy.

See the across-autoconfigure documentation for the list of supported starters, and more information on how to make others compatible.

There is often no actual limitation to using the starter libraries, but some effort might be required to get the right configuration. We can offer commercial support for this if required.

How is Across different from Spring Framework?

Across builds on top of Spring framework and tries to stay as compatible as possible. There are some important differences, however.

ApplicationContext hierarchy

Across builds an entire ApplicationContext hierarchy where every module is its own ApplicationContext. A medium Across application easily has 5 to 10 ApplicationContext instances. As a simplified comparison: many classic Spring Framework web applications have two ApplicationContext instances, a Spring Boot application usually has only one.

You can use all features of Spring framework or related libraries in Across applications, but due to the ApplicationContext hierarchy, sometimes additional setup is required. Beans are only shared between contexts if they are exposed, and a module ApplicationContext can only access a bean from another module if the other module has already started (unless an Across-specific mechanism like @PostRefresh is being used).

ApplicationContext and implicit bean ordering

In a single ApplicationContext, you cannot rely on any bean ordering unless you set it explicitly. Beans are only considered to be ordered if they have an @Order annotation, or if they implement the Ordered interface.

In an Across application, inter-module ordering must be reliable and is determined by the dependencies that modules have on each other. The actual order of modules is determined at bootstrap time and never changed after. This ordering is used extensively throughout an Across application:

  • Modules are started in order (and stopped in reverse order).
  • Installers are executed in module order, no matter during which bootstrap phase.
  • Event handlers are executed in module order.
  • @RefreshableCollection always returns the list in module order.

Explicit ordering using the @Order annotation can be used to order beans regardless of module dependencies. An alternative @OrderInModule is available if you only want to influence relative ordering of beans within a module.

Events

Across uses its own event bus for publishing events. You can simply use the Spring ApplicationEventPublisher and @EventListener annotations, but how events are dispatched is subtly different in order to ensure they get handled in Across module order.

Can I use Spring Boot devtools with Across applications?

Absolutely.

Support for devtools is present when using @AcrossApplication to configure your application. The Across Initializr projects always add devtools to their project structure.