Interacting with modules

On this page, you will:

Use the ObjectMapper component from the AcrossWebModule.
Learn how to use events to modify a menu from DebugWebModule.

Using components from modules

In previous chapters you have seen how to add modules to your application descriptor. Each module can define several components, but you will never be able to use these components outside of your module.

Why is this?

By default, Across will create an ApplicationContext per module.

Any Spring bean, for example a @Component or @Service, defined in this module is only visible inside that module.

A module can expose certain components or beans, so they can be used in any other module.

AcrossWebModule will expose certain objects, like the ObjectMapper class. Doing this, you can wire this bean in any other module when you require object conversion.

First, modify your application descriptor to include AcrossWebModule.

Adding AcrossWebModule to the application descriptor

@AcrossApplication(
		modules = {
				AcrossWebModule.NAME (1)
		}
)
public class DemoApplication
{

}

1	Add `AcrossWebModule` to your application descriptor.

In the following example we will wire the ObjectMapper to convert a User object to JSON and back again.

Modify your StartupLogger class to the one below.

A component which converts to JSON and back to a User object.

@Component
@Slf4j
@RequiredArgsConstructor
public class StartupLogger {

    private final ObjectMapper objectMapper; (1)

    @Bean
    public Void dummyBean() throws IOException {
        User user = User.builder().firstName("Jane").lastName("Doe").build();  (2)
        String json = objectMapper.writeValueAsString( user );  (3)
        LOG.info("This user in json: {}", json );
        LOG.info( "The json converted back into a user object: {}", objectMapper.readValue(json, User.class) );  (4)
        return null;
    }

    @Data  (5)
    @Builder  (6)
    @NoArgsConstructor  (7)
    @AllArgsConstructor  (8)
    private static final class User {
        private String firstName;
        private String lastName;
    }
}

1	We use the Jackson `ObjectMapper` bean, which is exposed by `AcrossWebModule` by default.
2	We use a builder to create a `User` object.
3	We use the objectMapper to convert this user to a a JSON string.
4	The JSON string is converted back to a User class.
5	We require getters/setters, so that the `ObjectMapper` can find the fields when mapping from JSON to a `User` object.
6	We use a builder to quickly create a `User` object.
7	The `@NoArgsConstructor` is required so that the `ObjectMapper` can create a `User` object when converting from JSON to a `User` object.
8	The `@AllArgsConstructor` is required by the `@Builder`.

Now run the application.

$ mvn spring-boot:run

In the console you should see the following:

INFO --- [ost-startStop-1] c.e.demo.application.StartupLogger       : This user in json: {"firstName":"Jane","lastName":"Doe"}
INFO --- [ost-startStop-1] c.e.demo.application.StartupLogger       : The json converted back into a user object: StartupLogger.User(firstName=Jane, lastName=Doe)

Working with events

Since all components are scoped to the current module, you need a way to communicate between modules.

Using events, one module can send out an event and all other modules that are listening to this event, can act upon it.

In the following example, we will listen to the DebugMenuEvent from the DebugWebModule to modify the menu.

Modify your StartupLogger class to the one below.

A component which listens to the DebugMenuEvent and modifies the menu.

@Component
public class StartupLogger {

    @EventListener (1)
    public void buildMainMenu(DebugMenuEvent event) {  (2)
        event.builder()
                .group("/across-docs", "Documentation").and()  (3)
                .item("/across-docs/site", "Across documentation", "https://docs.across.dev")  (4)
                .and()
                .changeItemPath("/across", "/across-docs");  (5)
    }
}

1	We use the Spring’s `@EventListener` to catch events.
2	We are listening to events of type `DebugMenuEvent`, which is published when the menu is about to be built.
3	We create a new menu group called `Documentation`, in which we will add a new menu item.
4	We create a new menu item which links to our documentation.
5	We move all other paths under `/across` underneath this new menu group. In this case the Across Context Browser will move to the `Documentation` dropdown.

You can see an overview of events published by Across Web components is available here.

For a more in-depth overview of menu’s, visit the page Working with menus.

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