/solid-java

name: solid-java
description: "Use when writing or refactoring Java code, structuring modules/[feature]/ layers, or defining interfaces/ contracts (SOLID, files within FUSE_SOLID_MAX_LINES)."
versions:
  java: "25"
user-invocable: true
references: references/solid-principles.md, references/single-responsibility.md, references/open-closed.md, references/liskov-substitution.md, references/interface-segregation.md, references/dependency-inversion.md, references/architecture-patterns.md, references/templates/module.md, references/templates/service.md, references/templates/interface.md, references/templates/repository.md, references/templates/error.md, references/templates/test.md
related-skills: solid-detection

<objective> SOLID Java enforces a modular architecture for Java 25+ (current LTS): every feature lives under `modules/[feature]/` (controllers, services, repositories, interfaces, models/DTOs) with shared code in `modules/core/`, interfaces live only in `modules/[feature]/interfaces/`, controllers stay thin and delegate to services, with files within `FUSE_SOLID_MAX_LINES`, records are used for DTOs and sealed types for restricted hierarchies, and every public method carries Javadoc.

Before writing any new code it requires a DRY check against `modules/core/services` and `modules/core/interfaces`. See `solid-principles.md` for the overview, the per-principle references for SRP/OCP/LSP/ISP/DIP detail, and the templates for module/service/interface/repository/error/test scaffolding. </objective>

SOLID Java - Modular Architecture

Agent Workflow (MANDATORY)

Before ANY implementation, spawn 3 agents in parallel, one `Agent` call each with a `name`:

1. **fuse-ai-pilot:explore-codebase** - Analyze existing architecture 2. **fuse-ai-pilot:research-expert** - Verify Java docs via Context7 3. **fuse-ai-pilot:sniper** - Post-implementation validation

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DRY - Reuse Before Creating (MANDATORY)

**Before writing ANY new code:** 1. **Grep the codebase** for similar interfaces, services, or logic 2. Check shared locations: `modules/core/services/`, `modules/core/interfaces/` 3. If similar code exists -> extend/reuse instead of duplicate 4. If code will be used by 2+ features -> create it in `modules/core/`

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Architecture (Modules MANDATORY)

| Layer | Location | |-------|----------| | Controllers | `modules/[feature]/controllers/` | | Services | `modules/[feature]/services/` | | Repositories | `modules/[feature]/repositories/` | | Interfaces | `modules/[feature]/interfaces/` | | Models/DTOs | `modules/[feature]/models/` | | Shared | `modules/core/{services,interfaces,models}/` |

**NEVER use flat `src/` structure - always `modules/[feature]/`**

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Critical Rules (MANDATORY)

| Rule | Value | |------|-------| | File limit | `FUSE_SOLID_MAX_LINES` | | Controllers | Thin, delegate to services | | Interfaces | `modules/[feature]/interfaces/` ONLY | | Javadoc | Every public method documented | | Records | Use for DTOs (Java 16+) | | Sealed | Use for restricted hierarchies (Java 17+) |

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Reference Guide

Concepts

| Topic | Reference | When to consult | |-------|-----------|-----------------| | **SOLID Overview** | [solid-principles.md](references/solid-principles.md) | Quick reference | | **SRP** | [single-responsibility.md](references/single-responsibility.md) | Fat classes | | **OCP** | [open-closed.md](references/open-closed.md) | Adding providers | | **LSP** | [liskov-substitution.md](references/liskov-substitution.md) | Contracts | | **ISP** | [interface-segregation.md](references/interface-segregation.md) | Fat interfaces | | **DIP** | [dependency-inversion.md](references/dependency-inversion.md) | Injection | | **Architecture** | [architecture-patterns.md](references/architecture-patterns.md) | Modular patterns |

Templates

| Template | When to use | |----------|-------------| | [module.md](references/templates/module.md) | Feature module structure | | [service.md](references/templates/service.md) | Business logic service | | [interface.md](references/templates/interface.md) | Contract definition | | [repository.md](references/templates/repository.md) | Data access layer | | [error.md](references/templates/error.md) | Custom exceptions | | [test.md](references/templates/test.md) | Unit tests with mocks |

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Forbidden

| Anti-Pattern | Fix | |--------------|-----| | Files over `FUSE_SOLID_MAX_LINES` | Split by responsibility | | Interfaces in impl files | Move to `interfaces/` directory | | `new ConcreteClass()` in services | Use dependency injection | | Flat `src/` structure | Use `modules/[feature]/` | | God classes | Split by responsibility |