Contracts

A lightweight, type-safe Java library for dependency inversion and inversion of control.

Overview

Contracts provides a modern approach to decoupling API definitions from their implementations using a Contract-based system. This library enables clean architecture by separating what you need (the contract) from how it’s provided (the implementation), without requiring reflection, annotations, or dependency injection frameworks.

Key Features

• Type-Safe Contracts: Generic-based contracts eliminate casting and provide compile-time safety
• Zero Reflection: No reflection, annotations, or bytecode manipulation required
• Java Module System: Full JPMS support with strong encapsulation
• Lifecycle Management: Built-in reference counting and resource management
• Framework Agnostic: Works standalone or alongside Spring Boot, Guice, or other DI frameworks
• SOLID Principles: Designed around dependency inversion, interface segregation, and single responsibility
• Security Focused: OpenSSF Best Practices compliant with contract-level access control
• Multi-Module Architecture: Clean separation between API (contracts-api), implementation (contracts-impl), and testing (contracts-test)

Quick Start

The Contract pattern has three simple steps:

1. Author a Contract

Define a contract as a unique key that represents an agreement between providers and consumers:

// Define a contract for a service - each contract is a unique key
public static final Contract<WeatherService> WEATHER_SERVICE = 
    Contract.create("WeatherService");

2. Bind (Promise) an Implementation

A provider binds an implementation to the contract using a Promisor:

// Bind an implementation to the contract
GlobalContracts.bindContract(WEATHER_SERVICE, () -> new WeatherServiceImpl());

// Or with a singleton pattern
GlobalContracts.bindContract(WEATHER_SERVICE, 
    GlobalContracts.singleton(() -> new WeatherServiceImpl()));

3. Claim (Consume) the Implementation

Consumers claim the implementation through the contract:

// Claim the implementation - type-safe, no casting needed
WeatherService service = GlobalContracts.claimContract(WEATHER_SERVICE);
String forecast = service.getForecast();

Core Concepts

Contracts

A Contract<T> is a unique key that establishes an agreement between a provider (who binds) and a consumer (who claims). Each contract instance is unique by identity, ensuring proper encapsulation.

Promisors

A Promisor<T> is a functional interface that provides the implementation. It supports:

• Simple factories: Return a new instance on each claim
• Singletons: Return the same instance (via GlobalContracts.singleton())
• Lifecycle management: Reference-counted resources (via GlobalContracts.lifeCycle())

Global vs Local Contracts

• GlobalContracts: Singleton instance for application-wide contracts
• Contracts.createContracts(): Create isolated contract repositories for testing or modularity

  Advanced Features

Reference Counting & Lifecycle Management

// Lifecycle-managed promisor with automatic open/close
Promisor<Database> dbPromisor = GlobalContracts.lifeCycle(() -> new Database());
AutoClose unbind = GlobalContracts.bindContract(DB_CONTRACT, dbPromisor);

// When done, unbind to decrement reference count
unbind.close();

Contract Configuration

// Create a contract with custom configuration
Contract<Service> contract = Contract.create(Service.class, builder -> {
    builder.name("MyService")
           .replaceable(true);  // Allow rebinding
});

Partners (Aggregated Contracts)

Create contract repositories that delegate to parent repositories for shared contracts.

Project Structure

The library is organized into multiple Gradle submodules:

• contracts-api: Core API interfaces and contracts (no dependencies)
• contracts-impl: Default implementation of the Contracts system
• contracts-test: Reusable test utilities for testing contract-based code
• contracts-smoke: Smoke tests for validation
• Root module: Aggregates API and implementation for easy consumption

Installation

Gradle

dependencies {
    implementation 'io.github.jonloucks.contracts:contracts:2.5.2'
}

Maven

<dependency>
    <groupId>io.github.jonloucks.contracts</groupId>
    <artifactId>contracts</artifactId>
    <version>2.5.2</version>
</dependency>

Use Cases

• Plugin Architectures: Define extension points without tight coupling
• Testing: Easily mock dependencies by binding test implementations
• Modular Applications: Decouple modules using contracts as boundaries
• Library Development: Provide APIs without exposing implementations
• Configuration Management: Bind different implementations based on environment

  Documentation & Resources

• Javadoc API Documentation
• Test Coverage Report
• License
• Contributing Guidelines
• Code of Conduct
• Coding Standards
• Security Policy
• Pull Request Template

Badges

Requirements

• Deployment: Java 11+ (LTS)
• Compilation: Java 9+ language level
• Module System: Full JPMS support

Design Principles

The library adheres to these core principles:

1. SOLID Principles: Dependency inversion, interface segregation, single responsibility
2. Security First: OpenSSF Best Practices with contract-level access control
3. Strong Encapsulation: Proper data hiding and module boundaries
4. Universal Compatibility: Use anywhere, anytime across many codebases
5. Type Safety: No casting needed, no unchecked surprises
6. Privacy by Default: Contracts are only visible to the author unless explicitly shared
7. LTS Support: Deploys on oldest supported LTS Java version (currently 11) with JPMS
8. Minimal Language Requirements: Compiles with Java 9 language level
9. Zero Magic: No reflection, injection, or annotations required
10. Testability: Promotes black-box testing, leading to test reuse and maintainability
11. Framework Neutral: Works standalone or with Spring Boot, Guice, etc.

Why Contracts?

Traditional dependency injection frameworks often require:

• Reflection and runtime classpath scanning
• Framework-specific annotations
• Complex configuration
• Tight coupling to the DI container

Contracts offers a simpler alternative:

• ✅ Compile-time type safety
• ✅ Explicit, readable code
• ✅ No framework lock-in
• ✅ Works with Java modules
• ✅ Zero runtime overhead from reflection
• ✅ Perfect for libraries that need to avoid DI framework dependencies

Building from Source

# Build the project
./gradlew build

# Run all tests with coverage
./gradlew check jacocoTestReport

# Publish to local Maven repository
./gradlew publishToMavenLocal

License

See LICENSE.md for details.

Contributing

Contributions are welcome! Please read CONTRIBUTING.md and CODE_OF_CONDUCT.md before submitting pull requests.