Chapter 19: Comparison with Other Approaches

JBCT doesn’t exist in isolation. This chapter compares it to other architectural approaches you may encounter, helping you understand where JBCT fits and what it borrows from or rejects in other methodologies.

Traditional Layered Architecture

The most common Java backend architecture.

Structure

Controller Layer     → Receives HTTP requests, validates DTOs
    ↓
Service Layer        → Business logic, transactions, orchestration
    ↓
Repository Layer     → Database access, queries
    ↓
Entity Layer         → JPA entities, data structures

Example

@RestController
public class UserController {
    @Autowired private UserService userService;

    @PostMapping("/users")
    public ResponseEntity<UserDto> createUser(@Valid @RequestBody CreateUserDto dto) {
        User user = userService.createUser(dto);
        return ResponseEntity.ok(toDto(user));
    }
}

@Service
public class UserService {
    @Autowired private UserRepository userRepository;
    @Autowired private PasswordEncoder passwordEncoder;

    @Transactional
    public User createUser(CreateUserDto dto) {
        if (userRepository.existsByEmail(dto.getEmail())) {
            throw new EmailExistsException();
        }
        User user = new User();
        user.setEmail(dto.getEmail());
        user.setPassword(passwordEncoder.encode(dto.getPassword()));
        return userRepository.save(user);
    }
}

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    boolean existsByEmail(String email);
}

What JBCT Changes

Aspect	Layered	JBCT
Error handling	Exceptions	Result/Promise with Cause
Validation	@Valid + manual checks	Parse-don’t-validate
Business logic location	Service layer	Use case interface
Data flow	Mutable entities	Immutable value objects
Dependencies	Field injection	Constructor injection via factory

What JBCT Keeps

Separation of concerns (different responsibility per layer)
Controllers at the boundary
Repository pattern for data access

Verdict

JBCT refines layered architecture rather than replacing it. The layers still exist, but with explicit error handling and immutable data flow.

Hexagonal Architecture (Ports and Adapters)

Popularized by Alistair Cockburn. Core idea: business logic at the center, adapters at the edges.

Structure

                    ┌─────────────────────┐
   HTTP Adapter ───→│                     │←─── Database Adapter
                    │   Domain / Core     │
  Queue Adapter ───→│   (Business Logic)  │←─── External API Adapter
                    │                     │
                    └─────────────────────┘
                           ↑   ↑
                        Ports (Interfaces)

Example

// Port (interface defined by domain)
public interface UserRepository {
    Optional<User> findByEmail(Email email);
    void save(User user);
}

// Domain service (pure business logic)
public class UserRegistrationService {
    private final UserRepository userRepository;
    private final PasswordHasher passwordHasher;

    public User register(Email email, Password password) {
        if (userRepository.findByEmail(email).isPresent()) {
            throw new EmailAlreadyExistsException(email);
        }
        HashedPassword hashed = passwordHasher.hash(password);
        User user = new User(email, hashed);
        userRepository.save(user);
        return user;
    }
}

// Adapter (implements port)
public class JpaUserRepository implements UserRepository {
    private final JpaUserEntityRepository jpaRepo;

    @Override
    public Optional<User> findByEmail(Email email) {
        return jpaRepo.findByEmail(email.value())
            .map(this::toDomain);
    }
}

What JBCT Borrows

Ports concept → Step interfaces
Adapters concept → Adapter leaves
Domain at center → Use cases with pure business logic
Dependency inversion → Steps injected into use case factory

What JBCT Adds

Explicit error handling - Hexagonal doesn’t prescribe how to handle errors
Functional composition - Hexagonal uses imperative style
Typed failures - Cause types instead of exceptions
Structural patterns - Leaf, Sequencer, Fork-Join provide composition vocabulary

Key Difference

Hexagonal focuses on where code lives (inside vs outside the hexagon). JBCT focuses on how code composes (patterns, error handling, data flow).

Verdict

JBCT and Hexagonal are complementary. Use Hexagonal for high-level architecture, JBCT for implementation patterns within that architecture.

Clean Architecture

Uncle Bob’s architecture with explicit dependency rules.

Structure

┌───────────────────────────────────────────────┐
│              Frameworks & Drivers              │
│  ┌───────────────────────────────────────┐    │
│  │           Interface Adapters           │    │
│  │  ┌───────────────────────────────┐    │    │
│  │  │        Application Layer       │    │    │
│  │  │  ┌───────────────────────┐    │    │    │
│  │  │  │     Domain Layer      │    │    │    │
│  │  │  │     (Entities)        │    │    │    │
│  │  │  └───────────────────────┘    │    │    │
│  │  └───────────────────────────────┘    │    │
│  └───────────────────────────────────────┘    │
└───────────────────────────────────────────────┘

Dependencies point inward. Inner layers don’t know about outer layers.

Example

// Entity (innermost)
public class User {
    private UserId id;
    private Email email;
    private HashedPassword password;

    public boolean canLogin(Password attempt, PasswordHasher hasher) {
        return hasher.verify(attempt, this.password);
    }
}

// Use Case (application layer)
public class RegisterUserUseCase {
    private final UserGateway userGateway;
    private final PasswordHasher passwordHasher;

    public RegisterUserResponse execute(RegisterUserRequest request) {
        Email email = new Email(request.email);
        if (userGateway.existsByEmail(email)) {
            return RegisterUserResponse.failure("Email exists");
        }
        // ... rest of use case
    }
}

// Gateway interface (application layer, implemented by adapter)
public interface UserGateway {
    boolean existsByEmail(Email email);
    void save(User user);
}

What JBCT Borrows

Use case as first-class concept → UseCase interface
Dependency rule → Use cases don’t depend on adapters
Request/Response models → Request/ValidRequest/Response records

What JBCT Changes

Aspect	Clean Architecture	JBCT
Use case return	Response objects	Result/Promise
Error handling	Response codes or exceptions	Typed Cause
Entity behavior	Rich domain model	Value objects + pure functions
Composition	Imperative orchestration	Monadic chains

Key Difference

Clean Architecture prescribes dependency direction but not composition style. JBCT prescribes both.

Verdict

JBCT can be implemented within Clean Architecture. The layers map naturally:

Entities → Value objects
Use Cases → Use case interfaces
Interface Adapters → Adapter leaves
Frameworks → Spring configuration

Railway-Oriented Programming

Scott Wlaschin’s approach from F#, using “two-track” types for error handling.

Concept

Success Track:  ───●───────●───────●───────●─── → Success
                   │       │       │       │
                   ↓       ↓       ↓       ↓
Failure Track:  ───────────────────────────────→ Failure

Each function either stays on success track or switches to failure track.

Example (F# style in Java)

public Result<User> registerUser(String email, String password) {
    return validateEmail(email)           // Success or Failure
        .flatMap(this::validatePassword)  // Continue or stay failed
        .flatMap(this::checkUniqueness)   // Continue or stay failed
        .flatMap(this::createUser);       // Continue or stay failed
}

What JBCT Borrows

Two-track model → Result<T> is exactly this
flatMap for composition → Same chaining style
Errors as values → Cause instead of exceptions

What JBCT Adds

Four tracks, not two - T, Option, Result, Promise for different semantics
Structural patterns - Named patterns (Sequencer, Fork-Join) beyond just chaining
Aggregation - Result.all() for parallel validation
Async integration - Promise extends the model to async operations

Key Difference

ROP is a technique for error handling. JBCT is a complete methodology including project structure, testing, and team practices.

Verdict

JBCT incorporates ROP as its error handling model, then builds a full methodology around it.

vavr (formerly Javaslang)

Functional programming library for Java.

What vavr Provides

import io.vavr.control.Try;
import io.vavr.control.Either;
import io.vavr.control.Option;

// Try - captures exceptions
Try<Integer> result = Try.of(() -> Integer.parseInt(input));

// Either - success or failure with typed error
Either<Error, User> user = findUser(id);

// Option - presence or absence
Option<User> maybeUser = Option.of(nullableUser);

// Pattern matching
String message = Match(result).of(
    Case($Success($()), "Parsed"),
    Case($Failure($()), "Failed")
);

Comparison

Feature	vavr	Pragmatica Core
Option type	`Option<T>`	`Option<T>`
Error type	`Either<L, R>` or `Try<T>`	`Result<T>` with `Cause`
Async type	None (use CompletableFuture)	`Promise<T>`
Collections	Immutable collections	Uses Java collections
Pattern matching	Built-in DSL	Standard switch expressions
Tuples	Tuple1-8	Use records

Key Differences

vavr is a library, JBCT is a methodology - vavr provides types, JBCT provides patterns, structure, and practices.
Error typing - vavr’s Either has generic left type. Pragmatica Core’s Result always uses Cause, providing consistent error handling.
Async story - vavr doesn’t provide async primitives. JBCT’s Promise integrates error handling with async operations.
Simplicity - vavr includes many FP features (persistent collections, pattern matching DSL, streams). Pragmatica Core focuses on the minimum needed for JBCT.

When to Use vavr

You want immutable collections
You’re building a library with FP patterns
You need persistent data structures
You want pattern matching DSL

When to Use Pragmatica Core

You’re building backend services
You want a complete methodology (not just types)
You need integrated async support
You prefer simplicity over features

// Either for errors
fun divide(a: Int, b: Int): Either<DivisionError, Int> =
    if (b == 0) DivisionError.DivideByZero.left()
    else (a / b).right()

// Validated for accumulating errors
fun validateUser(name: String, age: Int): ValidatedNel<ValidationError, User> =
    ValidatedNel.applicative<ValidationError>().mapN(
        validateName(name),
        validateAge(age)
    ) { (n, a) -> User(n, a) }

// Effect for async
suspend fun fetchUser(id: UserId): Either<Error, User> =
    either { userRepository.findById(id).bind() }

Why Mention It?

Arrow-kt demonstrates that these patterns work well in a JVM language. Key insights:

Kotlin’s coroutines + Either = Similar to Promise<T>
Validated type = Similar to Result.all() accumulation
Typed errors = Same as JBCT’s Cause

Verdict

If you’re on Kotlin, Arrow-kt provides similar capabilities to JBCT. The concepts transfer, but the syntax differs due to language features (coroutines, extension functions, sealed classes).

Summary: Where JBCT Fits

                    Architectural Style
                           │
        ┌──────────────────┼──────────────────┐
        │                  │                  │
    Layered           Hexagonal           Clean
        │                  │                  │
        └──────────────────┼──────────────────┘
                           │
                    Implementation Style
                           │
        ┌──────────────────┼──────────────────┐
        │                  │                  │
   Imperative        Railway (ROP)         FP Library
   (exceptions)      (Result types)        (vavr, Arrow)
        │                  │                  │
        └──────────────────┼──────────────────┘
                           │
                         JBCT
                           │
                    ┌──────┴──────┐
                    │             │
              Pragmatica     Structural
              Lite Core      Patterns

JBCT:

Works within any architectural style (Layered, Hexagonal, Clean)
Uses railway-oriented error handling
Is simpler than full FP libraries
Provides structural patterns other approaches lack
Includes methodology beyond just types

Exercises

Map your architecture: Does your current project use Layered, Hexagonal, or Clean Architecture? Where would JBCT patterns fit?
Compare error handling: Take one exception-based method in your codebase. Rewrite it using ROP style with Result. What errors were implicit?
Evaluate vavr: If you use vavr, identify which features you actually use. Could you replace it with Pragmatica Core?
Cross-language patterns: If you have Kotlin services, compare how Arrow-kt’s Either compares to JBCT’s Result. Are the patterns similar?

Summary

JBCT is not revolutionary - it combines proven ideas:

Idea	Source
Ports and Adapters	Hexagonal Architecture
Use Cases as first-class	Clean Architecture
Errors as values	Railway-Oriented Programming
Functional types	vavr, Arrow-kt, Haskell
Parse don’t validate	Type-driven design

What JBCT adds:

Unified methodology - Architecture + implementation + testing
Structural patterns - Named, composable patterns
Team practices - Migration path, code review guidelines
AI optimization - Predictable code for AI collaboration

The goal isn’t originality - it’s unification.