Currency Exchange API

A low-level, high-control RESTful API built with Java 25 for managing currency exchange rates and performing financial conversions. This project demonstrates manual infrastructure management, from database connection pooling and migrations to custom request body parsing and dependency injection.

Architecture & Data Flow

The application follows a tiered architecture to ensure a clear separation of concerns. Below is the end-to-end data flow from the client to the database.

Data Flow Diagram

sequenceDiagram
    participant Client
    participant Servlet as Controller (Servlet)
    participant Service as Service Layer
    participant DAO as Data Access Object
    participant DB as SQLite Database

    Client->>Servlet: HTTP Request (GET, POST, PATCH)
    Note over Servlet: Manual Parameter Parsing (HttpUtil)
    Servlet->>Service: Business Logic Execution
    Note over Service: Currency Conversion / Calculations
    Service->>DAO: Data Retrieval/Persistence Request
    DAO->>DB: SQL Execution via JDBC (HikariCP)
    DB-->>DAO: ResultSet
    Note over DAO: Manual Entity Mapping (RowMapper)
    DAO-->>Service: Domain Entity
    Service-->>Servlet: DTO (ExchangeValue) / Entity
    Note over Servlet: Manual JSON Serialization (Jackson)
    Servlet-->>Client: JSON Response + HTTP Status

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Key Technical Approaches & Challenges

Low-Level Database & Request Management

Unlike high-level frameworks, this project manages the entire lifecycle of a request manually. This includes:

• Manual Request Parsing: Handling complex scenarios like the PATCH method, where servlet containers do not automatically parse x-www-form-urlencoded bodies.
• Explicit Mapping: Manually mapping database ResultSet rows to Java Entities and then to DTOs for JSON output.

Manual Dependency Injection (DI)

To avoid the "Singleton anti-pattern" and improve testability, the project implements a manual DI container via AppContext.

• Decoupling: Services receive their DAOs through constructors rather than static getInstance() calls.
• Centralized Wiring: All components are instantiated and wired in a single location.

Application Lifecycle & Configuration

An AppContextListener serves as the application's entry point, managing the global configuration.

• Automated Migrations: Upon initialization, the listener triggers DatabaseManager to run Flyway migrations, ensuring the SQLite schema is always up-to-date.
• Resource Cleanup: It ensures the HikariCP connection pool is gracefully closed during application shutdown.

Global Exception Handling & Status Codes

A centralized ExceptionFilter manages application-wide errors, ensuring consistent API responses.

• Exception Mapping: Custom business exceptions (e.g., EntityNotFoundException, DataIntegrityViolationException) are mapped to precise HTTP status codes like 404 Not Found, 409 Conflict, or 400 Bad Request.

Persistence Layer & HikariCP

The project utilizes HikariCP for high-performance connection pooling with SQLite.

• SQLite Constraints: Mandatory configuration of PRAGMA foreign_keys = ON; is enforced at the connection level to maintain relational integrity.

Setup & Deployment

Prerequisites

• Java 25 JDK
• Maven 3.9+
• Servlet Container (e.g., Apache Tomcat 10+)

Environment Configuration

The application requires an environment variable to locate the database file:

• DB_DIR: Set this to the directory where the SQLite database file should be stored (e.g., /var/lib/app/data).

Building the Project

1. Clone the repository.
2. Navigate to the project root and run:

mvn clean package

This will generate a currency-exchange-web.war file in the currency-exchange-web/target/ directory.

Deployment

1. Copy the generated .war file to your servlet container's deployment directory (e.g., webapps/ for Tomcat).
2. Start the container.
3. The application will automatically perform database migrations and seed initial data upon startup.

Technologies Used

• Java 25 (Core language)
• Persistence: SQLite, JDBC, HikariCP, Flyway
• Web: Jakarta Servlet API, Jackson Databind
• Utilities: Lombok