Add the implementation of Onion-Architecture

onion-architecture/README.md

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+---
+title: "Onion Architecture in Java: A Layered Approach to Building Maintainable and Testable Applications"
+shortTitle: Onion Architecture
+description: "Learn how the Onion Architecture pattern promotes maintainability, testability, and separation of concerns in Java applications. Explore examples, benefits, and best practices."
+category: Architectural
+language: en
+tag:
+  - Decoupling
+  - Enterprise patterns
+  - Integration
+  - Microservices
+  - Scalability
+  - Security
+---
+## Intent of Microservices API Gateway Design Pattern
+
+In this project, the implementation demonstrates **Onion Architecture** with clear dependency direction: infrastructure and application depend on domain, but domain is independent.
+
+The central intent is to keep business rules in `domain`, orchestrate use cases in `application`, and isolate delivery/persistence/API details in `infrastructure`.
+
+Current implementation highlights:
+
+* `domain`: `Person`, `Category`, `PersonRepository`, `DomainException`
+* `application`: `SavePersonUseCase`, `GetPersonUseCase`, DTO records
+* `infrastructure`: Spring Boot REST controller, JPA entities, repository adapter, bean wiring
+
+## Also known as
+
+* Ports and Adapters Architecture
+* Hexagonal-style layering (conceptually related)
+* Dependency-rule-first architecture
+
+## Detailed Explanation of Onion Architecture Pattern with Real-World Examples
+
+Real-world example
+
+> Imagine a people-management service where business validation must stay consistent no matter how data is stored or exposed. In this codebase, `Person` and `Category` enforce invariants (e.g., age >= 18, required email/category), use cases coordinate behavior, and infrastructure adapts HTTP + JPA concerns. This allows the persistence or web layer to evolve without changing core domain rules.
+
+In plain words
+
+> The project keeps business logic in the center and treats frameworks as replaceable details around it.
+
+Wikipedia says
+
+> Onion Architecture is a software architecture pattern that emphasizes separation of concerns and dependency inversion by organizing code in concentric layers, with the domain model at the center.
+
+Sequence diagram
+
+![Onion Architecture class diagram](./etc/onion-architecture.png)
+
+Request flow in this implementation:
+
+1. Client calls REST endpoint in `PersonController` (`/api/persons`, `/api/persons/{id}`)
+2. Controller delegates to `SavePersonUseCase` or `GetPersonUseCase`
+3. Use case interacts with `PersonRepository` abstraction from `domain`
+4. `PersonRepositoryAdapter` maps domain <-> JPA and delegates to `SpringDataPersonRepository`
+5. Response DTO (`PersonResponse`) is returned to the client
+
+## Programmatic Example of Onion Architecture in Java
+
+This repository exposes a simple person API backed by use cases and domain models.
+
+Controller (infrastructure layer):
+
+```java
+@RestController
+@RequestMapping("/api")
+public class PersonController {
+
+    @GetMapping("/persons/{id}")
+    public ResponseEntity<PersonResponse> getPerson(@PathVariable Long id) {
+        var person = getPersonUseCase.execute(id);
+        return ResponseEntity.ok(person);
+    }
+
+    @GetMapping("/persons")
+    public ResponseEntity<List<PersonResponse>> getAllPersons() {
+        var persons = getPersonUseCase.executeAll();
+        return ResponseEntity.ok(persons);
+    }
+
+    @PostMapping("/persons")
+    public ResponseEntity<PersonResponse> savePerson(@RequestBody SavePersonCommand command) {
+        try {
+            var savedPerson = savePersonUseCase.execute(command);
+            return ResponseEntity.status(HttpStatus.OK).body(savedPerson);
+        } catch (DomainException e) {
+            return ResponseEntity.status(HttpStatus.BAD_REQUEST).build();
+        }
+    }
+}
+```
+
+Use case (application layer):
+
+```java
+public class SavePersonUseCase {
+
+    private final PersonRepository repository;
+
+    public PersonResponse execute(SavePersonCommand command) {
+        var category = new Category(command.categoryId(), command.categoryType());
+        var person = new Person(
+                null,
+                command.firstName(),
+                command.lastName(),
+                command.age(),
+                command.phoneNumber(),
+                command.email(),
+                category);
+
+        var savedPerson = repository.save(person);
+        return new PersonResponse(
+                savedPerson.getId(),
+                savedPerson.getFirstName(),
+                savedPerson.getLastName(),
+                savedPerson.getAge(),
+                savedPerson.getPhoneNumber(),
+                savedPerson.getEmail(),
+                savedPerson.getCategory().getId(),
+                savedPerson.getCategory().getType()
+        );
+    }
+}
+```
+
+Domain validation (domain layer):
+
+```java
+public class Person {
+    public Person(Long id, String firstName, String lastName, int age,
+                  String phoneNumber, String email, Category category) {
+        validateNames(firstName, lastName);
+        validateAge(age);
+        validatePhone(phoneNumber);
+        validateEmail(email);
+        validateCategory(category);
+        // assign fields...
+    }
+}
+```
+
+Repository adapter (infrastructure -> domain port):
+
+```java
+@Repository
+public class PersonRepositoryAdapter implements PersonRepository {
+
+    private final SpringDataPersonRepository repository;
+
+    @Override
+    public Optional<Person> findById(Long id) {
+        return repository.findById(id).map(this::mapToDomain);
+    }
+
+    @Override
+    public Person save(Person person) {
+        JpaPersonEntity savedEntity = repository.save(mapToEntity(person));
+        return mapToDomain(savedEntity);
+    }
+}
+```
+
+- **Maven 3.6.0** or higher
+
+### Build Steps
+
+1. **Navigate to the onion-architecture module directory:**
+   ```bash
+   cd java-design-patterns/onion-architecture
+   ```
+
+2. **Build all modules:**
+   ```bash
+   mvn clean package
+   ```
+   This will compile the `domain`, `application`, and `infrastructure` modules and package them into a Spring Boot executable JAR.
+
+3. **Run the Spring Boot application:**
+   ```bash
+   mvn -pl infrastructure spring-boot:run
+   ```
+   Alternatively, after building, run the JAR directly:
+   ```bash
+   java -jar infrastructure/target/infrastructure-1.26.0-SNAPSHOT.jar
+   ```
+
+### Accessing the API
+
+The application exposes REST endpoints at `http://localhost:8080/api`:
+There is a Postman collection available in the `etc/postman` folder for testing the API.
+
+- **Get all persons:**
+  ```bash
+  GET http://localhost:8080/api/persons
+  ```
+
+- **Get person by ID:**
+  ```bash
+  GET http://localhost:8080/api/persons/{id}
+  ```
+
+- **Create a new person:**
+  ```bash
+  POST http://localhost:8080/api/persons
+  Content-Type: application/json
+
+  {
+    "firstName": "John",
+    "lastName": "Doe",
+    "age": 30,
+    "phoneNumber": "555-1234",
+    "email": "john.doe@example.com",
+    "address": "123 Main St",
+    "categoryId": 1,
+    "categoryType": "individual"
+  }
+  ```
+
+### Run Tests
+
+To execute unit tests across all modules:
+
+```bash
+mvn clean test
+```
+
+To run tests for a specific module:
+
+```bash
+mvn -pl domain test
+mvn -pl application test
+mvn -pl infrastructure test
+```
+
+### Database
+
+The application uses an **H2 in-memory database** for demonstration purposes. Configuration is in `infrastructure/src/main/resources/application.properties`:
+
+- **JDBC URL:** `jdbc:h2:mem:testdb`
+- **Username:** `sa`
+- **Password:** `password`
+
+Sample data is initialized from `infrastructure/src/main/resources/data.sql` on application startup.
+
+## When to Use the Onion Architecture Pattern in Java
+
+* When domain rules must be stable and independent from frameworks.
+* When you want use cases to be testable without HTTP or database setup.
+* When infrastructure details (web, JPA, database) should be replaceable.
+* When dependency direction must be enforced from outer layers toward the domain core.
+
+## Onion Architecture Pattern Java Tutorials
+
+* [Clean Architecture with Spring Boot (Baeldung)](https://www.baeldung.com/spring-boot-clean-architecture)
+* [Hexagonal Architecture Explained (Cockburn)](https://alistair.cockburn.us/hexagonal-architecture)
+* [Spring Data JPA Reference](https://docs.spring.io/spring-data/jpa/reference/)
+
+## Benefits and Trade-offs of Microservices API Gateway Pattern
+
+Benefits:
+
+* Business validations are centralized in domain constructors (`Person`, `Category`).
+* Use cases stay independent from Spring, JPA, and transport concerns.
+* Repository abstraction (`PersonRepository`) keeps application logic persistence-agnostic.
+* Testability is strong across layers (domain, use case, adapter, controller tests).
+
+Trade-offs:
+
+* Additional mapping code between domain models, DTOs, and JPA entities.
+* More classes and modules than a simple CRUD-by-controller approach.
+* Requires discipline to avoid leaking infrastructure concerns into domain/application.
+
+## Real-World Applications of Microservices API Gateway Pattern in Java
+
+* People/contact management services with strict data validation.
+* Internal platforms where multiple delivery mechanisms (REST, batch, messaging) can share the same core domain.
+* Systems that need incremental infrastructure evolution while preserving business logic.
+
+## Related Java Design Patterns
+
+* [Repository](https://martinfowler.com/eaaCatalog/repository.html) - `PersonRepository` defines the domain-facing persistence contract.
+* [Adapter](https://refactoring.guru/design-patterns/adapter) - `PersonRepositoryAdapter` bridges domain model and Spring Data JPA.
+* [Dependency Injection](https://docs.spring.io/spring-framework/reference/core/beans/dependencies/factory-collaborators.html) - `ApplicationConfig` wires use case beans with repository implementations.
+
+## References and Credits
+
+* Project modules: `domain`, `application`, `infrastructure`
+* Java 21 + Maven multi-module setup
+* Spring Boot 3.3 (`spring-boot-starter-web`, `spring-boot-starter-data-jpa`, H2)
+* Layer-focused tests in each module validating domain invariants and use case behavior
+

onion-architecture/application/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+
+    The MIT License
+    Copyright © 2014-2022 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>onion-architecture</artifactId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>application</artifactId>
+    <name>Application</name>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.iluwatar</groupId>
+            <artifactId>domain</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+
+        <!-- JUnit 5 for testing -->
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+
+        <!-- Mockito for mocking -->
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-core</artifactId>
+            <scope>test</scope>
+        </dependency>
+
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-junit-jupiter</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>

onion-architecture/application/src/main/java/com/iluwatar/onion/application/dto/PersonResponse.java

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+/*
+ *
+ *  * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *  *
+ *  * The MIT License
+ *  * Copyright © 2014-2022 Ilkka Seppälä
+ *  *
+ *  * Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  * of this software and associated documentation files (the "Software"), to deal
+ *  * in the Software without restriction, including without limitation the rights
+ *  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  * copies of the Software, and to permit persons to whom the Software is
+ *  * furnished to do so, subject to the following conditions:
+ *  *
+ *  * The above copyright notice and this permission notice shall be included in
+ *  * all copies or substantial portions of the Software.
+ *  *
+ *  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ *  * THE SOFTWARE.
+ *
+ */
+package com.iluwatar.onion.application.dto;
+
+public record PersonResponse(
+    Long id,
+    String firstName,
+    String lastName,
+    int age,
+    String phoneNumber,
+    String email,
+    Long categoryId,
+    String categoryType) {}