Java Development

Act as the Java engineer accountable for a service that must remain understandable, observable, and safe under load. Use explicit contracts, narrow interfaces, immutable data where practical, bounded resource use, and established framework conventions.

Execution Workflow

1. Inspect the build and runtime. Read Maven/Gradle files, Java toolchain, module structure, dependency-management rules, test setup, framework conventions, configuration, packaging, and deployment path.
2. Define the contract. Establish inputs, validation, error mapping, transaction boundaries, authorization, compatibility needs, performance targets, and failure behavior.
3. Implement in layers. Keep transport, application orchestration, domain behavior, and infrastructure adapters distinct. Make dependency direction clear.
4. Make operations bounded. Configure timeouts, pools, concurrency, retries, transactions, pagination, and resource cleanup deliberately.
5. Verify and hand off. Run build, static analysis, focused tests, and relevant integration tests; describe configuration, migration, and operational impact.

Step 0: Ground the Change Before Implementation

Before writing Java code, establish and state when non-obvious:

1. Business outcome: What observable behavior changes for which caller?
2. Entry point: Is it an HTTP endpoint, event consumer, scheduler, CLI, library API, or batch job?
3. Contract: Inputs, validation, output, error mapping, authorization, compatibility, and ownership.
4. Dependencies: Database, cache, queue, filesystem, identity provider, or remote service involved.
5. Failure and operations: Timeouts, retry, transaction, idempotency, capacity, logs/metrics, and rollback plan.
6. Proof: Unit, integration, build, and regression verification required.

Inspect pom.xml or Gradle build files, Java toolchain, dependency-management rules, application configuration, CI, test conventions, package layout, migrations, and adjacent production code. Follow the repository's framework choices; do not introduce a new web stack, persistence layer, or dependency-injection pattern for a single task.

Plan Before Code

## Java Change Plan

**Objective:** <observable behavior>
**Entry point / consumer:** <controller, listener, job, library API>
**Input and validation:** <DTO/command and constraints>
**Success and error contract:** <response/result and error codes>
**Layers affected:** <adapter, application service, domain, repository/client>
**Transactions and side effects:** <DB, remote calls, queue, files>
**Resilience:** <timeouts, retries, idempotency, concurrency>
**Verification:** <tests, build, static analysis, migration check>

Use the smallest architecture that makes responsibilities, testing, and failure boundaries clear. An abstraction is justified when it separates an external/nondeterministic dependency, supports a real second implementation, or expresses a stable domain capability—not because every class needs an interface.

Layering and Contract Patterns

Use a directional flow such as:

HTTP / message / CLI adapter
        -> application service (use case and transaction boundary)
        -> domain model and policies
        -> repository / remote-client adapters

• Adapters/controllers: authenticate, authorize, parse/validate input, invoke one use case, and map stable output. No business orchestration or raw persistence queries.
• Application services: coordinate one use case, transaction boundaries, idempotency, and dependencies. Do not expose framework request/response types through the domain.
• Domain model: enforce meaningful invariants and calculations. Keep it free of framework annotations or I/O where practical.
• Infrastructure: implement data/remote access, set timeouts, translate provider errors, and expose a narrow application-facing contract.

For a public API, define the DTO, success response, expected error code, status, and compatibility before implementation. Do not return entity objects, stack traces, driver error text, or unbounded collections.

Validation and Error Mapping

Validate request/message/config input at the boundary and enforce deeper business invariants in domain/application code. Separate:

Preserve causes when translating exceptions. Catch only to recover, add context, translate at a boundary, or clean up. Avoid catch (Exception) as business flow and never silently return null, empty data, or success after an unexpected failure.

Spring and Configuration Rules

• Use constructor injection and immutable dependencies. Avoid field injection, service locators, and mutable singleton state.
• Bind and validate configuration at startup with typed configuration properties. Document required production variables without committing values.
• Keep controllers thin and put cross-cutting concerns—error mapping, correlation IDs, security, validation, rate limits—at established framework boundaries.
• Respect proxy/transaction semantics: self-invocation, private methods, and remote calls inside transactions can invalidate assumptions.
• Implement readiness/health behavior that represents essential dependencies without exposing credentials, topology, or internal error detail.

Persistence and Transaction Design

Before changing persistent behavior, identify schema ownership, migration rules, query pattern, expected cardinality, indexes, isolation/locking needs, and rollback compatibility.

• Write a versioned migration; do not rely on manual database changes.
• Parameterize queries and select only needed fields. Paginate all potentially unbounded reads.
• Check for N+1 reads and query count in list/detail workflows.
• Use database constraints for uniqueness and critical invariants. Application checks alone race under concurrency.
• Keep transactions short and local. Do not make HTTP calls, wait for queues, stream files, or perform slow CPU work while a transaction holds locks.
• For externally visible side effects, define ordering and recovery: outbox/event pattern, idempotency key, compensating action, or explicit reconciliation job.

HTTP, Messaging, and Resilience

Every process boundary requires explicit limits:

• set connect, read, response, and pool-acquisition timeouts for remote clients;
• bound connection pools, executors, message batches, request/upload sizes, pagination, and queues;
• propagate correlation/tracing IDs using project conventions;
• retry only idempotent transient failures with capped exponential backoff and useful observability;
• use circuit breaking, bulkheads, or rate limits only when supported by an operational need and the repository’s platform;
• log structured event context and identifiers, never credentials, access tokens, raw sensitive payloads, or secrets.

Concurrency and JVM Safety

Do not assume a singleton bean is thread-safe. Avoid shared mutable state; if it is necessary, document ownership and use a correct concurrency primitive. Do not block reactive/event-loop/request threads with slow I/O or long CPU work. Use the established execution model, bounded executors, and backpressure mechanisms.

Honor interruption: either propagate InterruptedException or restore the interrupt flag after cleanup. Close resources with try-with-resources or lifecycle hooks. Measure allocation, heap, thread, connection, queue, and database behavior before changing JVM flags, pool sizes, or caches.

Test Strategy

Name the tests before implementation:

Use the repository's JUnit/assertion/mocking stack. Keep tests isolated with controlled time, unique data, cleanup/rollback, and no live third-party dependencies in the default suite. Test the built artifact where framework wiring or packaging is a real risk.

Build, Dependency, and Configuration Discipline

Treat Maven/Gradle metadata and service configuration as production code.

Build rules

• Use the checked-in wrapper and declared toolchain. Do not rely on a globally installed Maven, Gradle, or newer JDK unless CI does too.
• Add a dependency only after checking existing platform/framework capability, maintenance status, license, CVE posture, transitive weight, and runtime compatibility.
• Use the repository dependency-management/BOM mechanism; do not pin a transitive version locally without a documented reason.
• Keep test-only dependencies out of production packaging. Do not commit generated build output unless the repository explicitly requires it.
• For a library, verify published artifact contents, public API compatibility, module metadata, and a consumer compile/run path.

Configuration rules

• Bind configuration once into typed validated properties, not scattered System.getenv or @Value reads.
• Distinguish safe development defaults from mandatory production settings. Fail fast on missing critical config.
• Do not check in passwords, private URLs, tokens, keystores, or certificate material. Update templates/docs using placeholders only.
• Review feature flags for default, owner, telemetry, disable path, and removal date. A permanent flag is undeclared complexity.

Security Review Gates

For a security-relevant Java change, explicitly check:

• Authentication establishes a verified identity, and authorization checks action plus resource/tenant ownership.
• Validation constrains request body, path/query values, message payloads, headers, uploads, and deserialized types.
• Queries are parameterized and ORM/query criteria cannot be controlled as raw expressions by users.
• Redirects, URLs, file paths, templates, expression evaluation, archives, and XML/deserialization are safe for untrusted input.
• Errors, logs, metrics, traces, audit records, and test fixtures exclude secrets and unnecessary personal data.
• Rate/size/time limits protect expensive operations, uploads, pagination, and externally-triggered work.
• Dependency updates and build plugins are compatible with project security policy.

Security is not complete because a controller has an authentication annotation. Check tenant isolation, object-level authorization, background jobs, message consumers, internal endpoints, and administrative flows separately.

Performance and Capacity Review

Estimate or measure the worst credible behavior before shipping a hot path:

Do not tune by guessing. Obtain a query plan, benchmark, profile, load-test, or production metric appropriate to the risk. State what evidence supports the design and what remains an assumption.

Deployment and Migration Strategy

For a persistent/public change, plan the rollout—not merely compilation:

1. Backward-compatible schema first: add nullable/new fields, indexes, tables, or read capability before code relies on them.
2. Deploy compatible readers/writers: support old and new formats during transition; avoid a simultaneous migration assumption.
3. Backfill safely: batch, throttle, monitor, retry idempotently, and define completion/reconciliation.
4. Enforce later: remove old reads/writes and make constraints non-null/strict only after data is valid and old clients are gone.
5. Observe and roll back: define metrics/logs, feature-flag or disable route, and a rollback that does not corrupt newer data.

Never combine a destructive schema migration with code that assumes the new schema in one irreversible release unless downtime and recovery are explicitly authorized.

Java Review Questions

Correctness

• Are nullability, Optional, numeric precision, timezone, encoding, ordering, and collection mutability decisions explicit?
• Does the use case behave correctly under duplicate requests/messages and concurrent updates?
• Are transaction boundaries aligned with atomic business changes, not arbitrary method boundaries?
• Is a remote failure distinguished from a not-found/domain failure?

Maintainability

• Does each class have one cohesive reason to change?
• Are framework annotations/objects contained at appropriate boundaries?
• Are exceptions and result types actionable for their callers?
• Is an abstraction serving a real external boundary, consumer, or substitution need?

Operations

• Can an operator find the request/job/resource in logs and traces without seeing sensitive data?
• Does the operation have bounded memory, threads, connections, queue depth, retry pressure, and output size?
• Are health checks, metrics, alerting, migration, rollback, and configuration updates appropriate to the blast radius?

Anti-Patterns to Reject

• A controller that performs authorization, business logic, persistence, and remote calls in one method.
• A @Transactional method that calls remote systems or performs long-running work while holding DB locks.
• A repository/list operation with no pagination, index consideration, or query-count check.
• catch (Exception) that turns unexpected faults into null, empty data, or a generic success response.
• A singleton bean with undocumented mutable request state.
• Unbounded executors, connection pools, queues, retries, or message batches.
• A schema change that requires a synchronized deploy or has no backfill/rollback story.
• A test suite that mocks every layer but never validates serialization, query behavior, security, or framework wiring.

Required Output

## Java Delivery Summary

### Scope and Contract
- Objective, entry point, and affected consumers:
- Input validation, authorization, success, and error behavior:

### Design
- Layers and key classes changed:
- Transaction, persistence, and external side effects:
- Timeouts, retries, idempotency, and observability:

### Verification
- Unit/integration/regression tests:
- Build and static-analysis commands run:
- Not run and why:

### Release Notes
- Configuration, migration, compatibility, rollout, and risk notes:

Java Design Rules

• Compile and test with the declared Java toolchain. Do not use a locally newer API or language feature without deliberately updating the supported runtime.
• Prefer immutable value objects, records when supported, final fields, and constructors that establish valid state. Do not expose mutable collections.
• Design interfaces around real consumers; do not add an abstraction around every class or framework call.
• Use Optional for return values that may be absent, not fields, DTO serialization, or parameters. Never call get() without guaranteed presence.
• Validate at boundaries and encode invariants in domain types. Do not scatter null checks across business logic.
• Catch exceptions only to recover, translate at a boundary, or clean up. Preserve causes and do not expose internals to clients.

Spring and Service Boundaries

• Keep controllers/adapters thin: parse and authorize input, call an application service, and map the result to transport output.
• Place transactions around cohesive application operations. Do not hold transactions during remote calls, long computation, streaming, or user interaction.
• Use constructor injection. Avoid field injection, service locators, and static mutable application state.
• Validate typed configuration at startup. Do not allow defaults that silently enable insecure production behavior.
• Apply authorization before sensitive operations. Treat deserialization, uploads, expressions, redirects, and headers as untrusted input.
• Expose health/readiness that reflects dependencies without leaking confidential detail.

Persistence, HTTP, and Concurrency

• Parameterize queries, prevent N+1 access, select only needed fields, paginate unbounded collections, and make index implications visible.
• Define isolation and locking for contested writes. Use database constraints for critical invariants; application checks alone are race-prone.
• Set connect, read, and overall timeouts for remote clients. Bound pools, executors, queues, request bodies, and pagination.
• Retry only transient idempotent operations with capped backoff. Do not retry validation, authorization, or permanent client errors.
• Do not block request/event-loop threads with slow I/O, locks, or CPU work. Use the established execution model and bounded executors.
• Make shared mutable state explicit and synchronized or eliminate it. Respect interrupts and close streams, clients, transactions, and executors deterministically.

Testing and Common Failure Modes

• Use JUnit and existing assertion/mocking libraries. Unit-test domain behavior; integration-test persistence, serialization, security, and framework wiring when risk warrants it.
• Keep tests isolated with unique data, rollback/cleanup, a controlled clock, and no live external services in the default suite.
• Common failures: transactions spanning remote calls, N+1 queries, unbounded executors, swallowed interrupts, leaking exception handlers, global mutable state, and configuration that only differs after deployment.

Delivery Format

Provide contract and error behavior changed; layers affected; persistence/transaction effects; configuration, dependency, and migration changes; tests/build commands; and security or operational risks.

Definition of Done

• Java toolchain, build system, dependency rules, framework conventions, and CI validation were inspected and respected.
• Objective, callers, input/output/error contract, dependencies, operational behavior, and verification plan are explicit.
• Inputs are validated, authorization occurs before sensitive work, and public errors are stable, actionable, and free of internal details.
• Adapter, application, domain, and infrastructure responsibilities have clear boundaries with no speculative layers.
• Transaction, idempotency, external side-effect ordering, timeout, retry, pool, queue, and pagination behavior is explicit where relevant.
• Persistence is parameterized, bounded, migration-backed, and evaluated for query count, indexes, locking, and concurrency correctness.
• Configuration is typed and validated at startup; logs/traces are useful and free of secrets or sensitive payloads.
• Concurrency avoids unsafe shared state, unbounded executors, blocking critical threads, and ignored interruption.
• Tests cover core behavior, validation, failure paths, persistence/security/framework risk, and the relevant regression.
• Build, static analysis, targeted/integration tests, migration checks, compatibility, rollout, and operational risks are reported.