Java Patterns

---
name: java-patterns
description: >
  Chooses and implements a design pattern in Java/Spring from the symptom that
  justifies it. Use when the request involves a design pattern, Strategy, Factory,
  Decorator, Builder, Specification, "how to structure this class", a growing
  if/switch chain, or refactoring a class with too many responsibilities.
disable-model-invocation: true
allowed-tools: Read, Write, Edit, Glob, Grep
---

# Java Patterns

A pattern is not a rule. A rule says *always*; a pattern says *when*. That's why this is
a skill and not `rules/` — it only costs context when invoked.

**Entry rule: no pattern without an observed symptom in the code.** Applying a pattern
preemptively is overengineering — a Factory for two lines, a Strategy for a single `if`,
an interface with one implementation that will never have a second. If you can't point
to the file and line that hurts, the answer is "apply no pattern at all".

## Procedure

1. Name the symptom in the actual code (`file:line`), not in the abstract.
2. Look up the symptom in the table. No matching row, stop — write simple code.
3. Confirm the pattern doesn't collide with the active blueprint's `packages.map`.
4. Generate from the exemplar in `templates/`, with the names from
   `@.claude/rules/naming.md`. Size limits: `@.claude/rules/code-quality.md`.
5. Record it in an ADR if the pattern changes the shape of an entire module.

## Catalog

| Symptom in code | Pattern | When **not** |
|---|---|---|
| `switch`/`if-else` over a type or enum, repeated in ≥ 2 places, growing with each new rule | Strategy | Two stable branches that have never grown |
| Construction with invariants, many steps, or possible invalid combinations | Static factory (`of`, `from`) | A record with validation in the compact constructor is already enough |
| ≥ 4 optional constructor parameters | Builder | Record + `with*`; Lombok `@Builder` on a domain object hides invariants |
| Cross-cutting behavior (cache, retry, audit) repeated across implementations | Decorator | A Spring aspect already exists (`@Transactional`, `@Cacheable`) that does the same thing |
| Business predicate duplicated between service and query | Specification | Predicate used only once |
| Output adapter with multiple technologies (JPA + external REST) behind the same port | Adapter (already the blueprint) | A single technology — the port is enough |
| Aggregate that emits ≥ 2 events consumed by several modules | Domain Events | A single consumer in the same module — direct call |

This table is the enforceable subset: seven rows, each with a symptom threshold and a
"when not" column. For a symptom that doesn't match any row above,
`references/pattern-catalog.md` covers the full GoF catalog — every pattern on
<https://refactoring.guru/design-patterns/java>, each with a shape reference in
`templates/` and this repo's verdict on the ones that don't (forbidden, or already
covered by the language/framework). It's triage, not a second menu: the entry rule below
still gates every row in it the same way it gates the seven here — an exemplar existing
is not permission to apply it without a real symptom.

## Forbidden in this repository

- **Singleton** — the Spring container already gives singleton scope; the manual version
  with `static` prevents testing and hides global state.
- **Service Locator / `ApplicationContext.getBean`** — violates the composition set in
  `@.claude/rules/architecture-ddd.md`.
- **`Impl` suffix**, always — `@.claude/rules/naming.md`: the name says the role, not
  that it's an implementation of something.
- **Pattern name standing in for a role name that exists** (`OrderStrategy` instead of
  `DiscountPolicy`, `OrderDecorator` instead of naming what it actually adds) — same
  rule, and the two curated exemplars (`Strategy.java.example`, `Decorator.java.example`)
  deliberately avoid the pattern name for exactly this reason. Not forbidden when the
  pattern name **is** the established Java role vocabulary with no better alternative —
  `Adapter`, `Repository`, `Builder`, `Factory` are accepted the same way
  `@.claude/rules/naming.md`'s own convention table already accepts
  `<Resource>RepositoryAdapter`. The test: is there a business-meaningful name this
  class could have instead? If yes, use it. If the honest answer is "no, this is just
  the JDK-idiomatic word for this role," the pattern name stays.

## Contract

**Reads** `.claude/rules/code-quality.md`, `.claude/rules/naming.md`,
`.claude/rules/architecture-ddd.md`, the active blueprint's `packages.map`, and
`references/pattern-catalog.md` when the symptom doesn't match any row of the curated
table above.

**Writes** domain and application code in the generated project. Doesn't touch the
inbound REST adapter — the package the blueprint's `packages.map` assigns to that role,
whether `adapter.in.rest` or `infrastructure.rest` — which is `rest-api-architect`'s
territory.

**Two invocation modes, one writer per mode.** Standalone — a human runs
`/java-patterns` against real code with an observed symptom; this skill is the sole
writer for that turn, as above. Preloaded — `java-spring-boot-developer` carries this
skill's catalog in full via its `skills:` frontmatter field and applies a matching row
directly while generating a new feature, never as a separate invocation
(`disable-model-invocation` wouldn't allow that anyway). In that mode this skill writes
nothing itself: the executor remains the sole owner of `src/**` (invariant 2), and the
catalog is reference material, the same role `templates/*.example` plays for the other
four pipeline skills. `@.claude/decisions/0025-java-patterns-preloaded-in-executor.md`.

**Doesn't collide with `domain-modeling`**, which also deals with domain and
application. The split is by moment, not by folder: that one produces the
`10-dominio.md` spec **before** code exists; this one refactors code that already
exists and already hurts, from a concrete `file:line`. Without a symptom in the code,
this isn't the right skill — it's the other one, or none.

**Does not** create rules. If a new limit has to always hold, it goes to
`rules/code-quality.md`, not here.