Implementing the Domain Services (Hexagonal Architecture with Spring Boot — Part 3)

This is the third article of a 4 5 part series explaining Hexagonal Architecture and how to implement such an architecture using Spring Boot and TDD.

Part 2 of the video is live now:

Domain Services

At the end of implementing the REST API adaptor, we have mocked out two domain services GetStockPositionService and GetStockMarketValueService.

As you go through the code below, notice that we will write the domain services functionally, and we can test them without using the Spring framework for dependency injection. Testing domain services functionally and without the Spring framework enables very fast testing.

Also, notice that where the domain services need to call some adaptors, whether data repository or external services, a port to interface is used. Again, in the tests, we specify the required behaviour of these adaptors through these ports, and the domain services does not have to know any further details of these adaptors beyond their behaviours.

Let’s start with the test for GetStockPositionService, GetStockPositionServiceTest as shown here:

package com.example.hexademo.domain.service; // imports not shown class GetStockPositionServiceTest { private final StockPositionsRepository repository = mock(StockPositionsRepository.class); private final GetStockPositionService subject = new GetStockPositionService(repository); @Test void get() { // arrange String user = DomainModelFaker.fakeUser(); String symbol = DomainModelFaker.fakeStockSymbol(); StockPosition fakeStockPosition = DomainModelFaker.fakeStockPosition(user, symbol); when(repository.findOneByUserAndSymbol(user, symbol)).thenReturn(Mono.just(fakeStockPosition)); // act Mono<StockPosition> result = subject.get(user, symbol); // assert StepVerifier.create(result) .expectNext(fakeStockPosition) .verifyComplete(); } }

The repository required for this domain service is specified as an interface and provided directly to the service as a constructor parameter.

package com.example.hexademo.domain.service; // imports not shown @Service public class GetStockPositionService { private final StockPositionsRepository repository; public GetStockPositionService(StockPositionsRepository repository) { this.repository = repository; } public Mono<StockPosition> get( String user, String symbol ) { return repository.findOneByUserAndSymbol(user, symbol); } }

The other domain service GetStockMarketValueService and its test GetStockMarketValueServiceTest are written in a similar style.

package com.example.hexademo.domain.service; // imports not shown class GetStockMarketValueServiceTest { private GetStockMarketPricePort getStockMarketPricePort = mock(GetStockMarketPricePort.class); private GetStockMarketValueService subject = new GetStockMarketValueService(getStockMarketPricePort); @Test void get() { // arrange String symbol = DomainModelFaker.fakeStockSymbol(); BigDecimal fakeQuantity = DomainModelFaker.fakeQuantity(); BigDecimal fakePrice = DomainModelFaker.fakeAmount(); when(getStockMarketPricePort.get(symbol)).thenReturn(Mono.just(fakePrice)); // act Mono<BigDecimal> result = subject.get(symbol, fakeQuantity); // assert StepVerifier.create(result) .expectNextMatches(amount -> amount.equals(fakeQuantity.multiply(fakePrice))) .verifyComplete(); } }

package com.example.hexademo.domain.service; // imports not shown @Service public class GetStockMarketValueService { private final GetStockMarketPricePort getStockMarketPricePort; public GetStockMarketValueService(GetStockMarketPricePort getStockMarketPricePort) { this.getStockMarketPricePort = getStockMarketPricePort; } public Mono<BigDecimal> get( String symbol, BigDecimal quantity) { return getStockMarketPricePort.get(symbol).map(price -> price.multiply(quantity)); } }

GetStockMarketValueService requires a source for the current market price of the stock for calculating its result. For now, we’ll specify an port as an interface GetStockMarketPricePort for this service.

Back to Part 2

Continue to part 4