3 months ago

While I don't recommend using Spring because of simplicity reasons, there are valid reasons to have Spring support. For example when building UI for an already existing Spring-based backend, or when gradually converting a legacy application to VoK.

### Use global service registry

The simplest way to go is to build a global service registry: a plain class with static getters which retrieve particular services from Spring (e.g. using ApplicationContext.getBean() call). To obtain the ApplicationContext just inject it into your UI and then simply call UI.getCurrent().getAppContext().getBean(). This approach has the following advantage over field injection approach:

1. You can call service getters from anywhere you want; you don't have to trouble yourself with constant worry whether the view/component in question was constructed by Spring and has everything injected properly. This fixes a lot of UninitializedPropertyAccessExceptions you might have.
2. You can even build the registry as a set of extension methods to a Services object as recommended by Vaadin-on-Kotlin. Please read Writing Services for more info.
3. You don't have to worry whether the injected Service Proxy is serializable and whether it will be reinjected upon deserialization (it won't, a constant cause of NullPointerExceptions).

### Injections

The harder way is to use injections. If you decide to go this way, your views must be constructed by Spring so that they have all dependencies injected properly. To achieve this, you must not use the autoViewProvider + @AutoView VoK mechanism (since AutoViewProvider constructs views simply by calling their constructors). Instead you must use SpringViewProvider and annotate your views with @SpringView.

Often you build custom components which depend on Spring services and use injection to obtain service instances. Such components must also be instantiated with Spring otherwise no injection is performed. To achieve this with the Kotlin DSL just write something like this: