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Java & Spring Practical Labs - Java & Spring Backend | Aporeon

Java & Spring Backend

Java & Spring Practical Labs

Worked Java and Spring implementation exercises covering language design, concurrency, APIs, JPA, security, testing, and production correctness.

12 drillsPractice one at a timeOrdered pathCoding problem

Recommended start

Immutable USD money

Builds a small immutable money type with explicit scale, rounding, arithmetic, ordering, and value equality.

Medium~25 minStep 1 of 12

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1Immutable USD moneyRecommended startBuilds a small immutable money type with explicit scale, rounding, arithmetic, ordering, and value equality.Coding problemMedium~25 minJavaSpring BootPractice 2

Practice order

12 drillsOne at a time

1Immutable USD moneyUp next / Medium / ~25 min
2Single-flight loads by keyHard / ~35 min
3Bounded concurrent fetchesHard / ~35 min

Single-flight loads by key

Implements single-flight request coalescing so one load runs per key while unrelated keys remain concurrent.

Coding problemHard~35 minJavaSpring Boot

Practice

3Bounded concurrent fetchesFetches multiple URLs with bounded concurrency, per-request and batch deadlines, cancellation, ordered results, and executor cleanup.Coding problemHard~35 minJavaSpring BootPractice

4Validated create endpointImplements request validation, a clean service boundary, a 201 response, and stable RFC 9457-style validation details.Coding problemMedium~25 minJavaSpring BootPractice

5Persisted idempotent POSTUses a scoped idempotency key, request hash, unique constraint, and one transaction to create an order exactly once from the API perspective.Coding problemHard~35 minJavaSpring BootPractice

6MockMvc ownership testsTests anonymous, owner, non-owner, and missing-resource outcomes through the real Spring Security filter chain.Coding problemMedium~25 minJavaSpring BootPractice

7JPA filtered projectionBuilds a bounded dynamic query that returns a read model, applies only supplied predicates, and uses deterministic ordering.Coding problemMedium~25 minJavaSpring BootPractice

8Fix a JPA N+1Uses a bounded entity graph for a to-one association and proves the repair with Hibernate query statistics.Coding problemHard~35 minJavaSpring BootPractice

9JPA keyset paginationImplements descending cursor pagination with a unique tie-breaker, bounded projection query, and next-cursor detection.Coding problemHard~35 minJavaSpring BootPractice

10JPA version conflictCarries the version observed by the client into a conditional update and returns an explicit conflict instead of silently overwriting newer data.Coding problemHard~35 minJavaSpring BootPractice

11Order and outbox transactionPersists business state and durable publication intent atomically, leaving broker delivery to an idempotent retrying publisher.Coding problemHard~35 minJavaSpring BootPractice

12PostgreSQL Testcontainers testRuns a Spring Boot integration test against PostgreSQL and proves a migration-backed database constraint at flush time.Coding problemMedium~25 minJavaSpring BootPractice

4

Validated create endpoint

Medium / ~25 min

5Persisted idempotent POSTHard / ~35 min

6MockMvc ownership testsMedium / ~25 min

7JPA filtered projectionMedium / ~25 min

8Fix a JPA N+1Hard / ~35 min

+4 more drills in this section