Use @ConfigurationProperties over non-managed @Bean

Question

I would like to benefit from @ConfigurationProperties fantastic facilities without needing to expose the bean into my context. It is not a problem of @Pri

Answer 1

Being that this DataSource is private to a class, and that containing class can be/is inside the Spring context, you can have a @ConfigurationProperties class

@ConfigurationProperties("com.foo.bar.datasource.hikari")
public class HikariConfiguration { ... }

Which, by registering it via @EnableConfigurationProperties, is available for autowiring

@EnableConfigurationProperties(HikariConfiguration.class)
@SpringBootApplication
public class Application { ... }

And thus can be autowired in the containing class

@Component
class MyClass {
   private final HikariConfiguration hikariConfiguration;  
   private DataSource springDatasource;

   MyClass(final HikariConfiguration hikariConfiguration) {
      this.hikariConfiguration = hikariConfiguration;
   }

   ...

   private DataSource privateSingletonDataSource() {
      if (Objects.isNull(this.springDatasource)) {
         this.springDatasource = buildDataSource(this.hikariConfiguration);
      }

      return this.springDatasource;
   }
}

buildDataSource will manually construct the DataSource instance.
Remember that you need to take care of synchronization when building the DataSource.

---

Final response is that you cannot re-use DataSourceProperties. You can't even extend it to change the properties' prefix. Only a single instance of it can exist inside the context.
The best you can do is mimic what Spring does.

Having

com.non.exposed.datasource.hikari.url=testUrl
com.non.exposed.datasource.hikari.username=testUsername
com.non.exposed.datasource.hikari.password=testPassword
...

You can define a new @ConfigurationProperties class

@ConfigurationProperties("com.non.exposed.datasource")
public class NonExposedProperties {
    private final Map<String, String> hikari = new HashMap<>(8);

    public Map<String, String> getHikari() {
        return hikari;
    }
}

Then, autowire this properties class in your @Configuration/@Component class.
Follow in-code comments.

@Configuration
public class CustomConfiguration {
    private final NonExposedProperties nonExposedProperties;
    private DataSource dataSource;

    CustomConfiguration(final NonExposedProperties nonExposedProperties) {
        this.nonExposedProperties= nonExposedProperties;
    }

    public DataSource dataSource() {
        if (Objects.isNull(dataSource)) {
            // Create a standalone instance of DataSourceProperties
            final DataSourceProperties dataSourceProperties = new DataSourceProperties();

            // Use the NonExposedProperties "hikari" Map as properties' source. It will be
            // {
            //    url      -> testUrl
            //    username -> testUsername
            //    password -> testPassword
            //    ... other properties
            // }
            final ConfigurationPropertySource source = new MapConfigurationPropertySource(nonExposedProperties.getHikari());

            // Bind those properties to the DataSourceProperties instance
            final BindResult<DataSourceProperties> binded =
                    new Binder(source).bind(
                            ConfigurationPropertyName.EMPTY,
                            Bindable.ofInstance(dataSourceProperties)
                    );

            // Retrieve the binded instance (it's not a new one, it's the same as before)
            dataSource = binded.get().initializeDataSourceBuilder().build();
        }

        // Return the constructed HikariDataSource
        return dataSource;
    }
}