Spring Boot cheat sheet

A scannable Spring Boot reference: 19 short snippets across 13 topics, each linking back to the lesson it came from.

At a glance

Quick snippets

Project setup and your first controller

Generating the project

# download from start.spring.io, or use your IDE wizard
unzip demo.zip -d demo
cd demo

./mvnw spring-boot:run       # starts the app on http://localhost:8080
./mvnw test                  # runs the generated context test
./mvnw package               # builds target/demo-0.0.1-SNAPSHOT.jar
java -jar target/demo-0.0.1-SNAPSHOT.jar

Full lesson: Project setup and your first controller →

REST controllers and dependency injection

REST controllers

record CreateUser(@NotBlank String name, @Email String email) {}

record UserDto(long id, String name, String email) {
    static UserDto from(User u) {
        return new UserDto(u.getId(), u.getName(), u.getEmail());
    }
}

Full lesson: REST controllers and dependency injection →

Configuration, JPA data access and profiles

Configuration and profiles

# src/main/resources/application.properties
spring.application.name=demo
server.port=8080
spring.jpa.open-in-view=false
app.greeting=Hello

Configuration and profiles

# src/main/resources/application.yml
spring:
  datasource:
    url: jdbc:postgresql://localhost:5432/demo
    username: demo
    password: ${DB_PASSWORD}     # read from the environment, never committed
  jpa:
    hibernate:
      ddl-auto: validate

Configuration and profiles

@Component
@ConfigurationProperties(prefix = "app")
class AppProperties {
    private String greeting = "";
    // getters and setters
}

@Configuration
@Profile("prod")
class ProdCacheConfig {
    // only active when the prod profile is on
}

Full lesson: Configuration, JPA data access and profiles →

Request validation and global error handling

Validating the boundary

public record CreateBookRequest(
    @NotBlank String title,
    @NotBlank @Size(max = 64) String author,
    @ISBN String isbn,
    @PositiveOrZero Integer copies,
    @Email String contactEmail,
    @Valid AddressRequest address) {}

@PostMapping("/books")
public ResponseEntity<BookResponse> create(@Valid @RequestBody CreateBookRequest req) {
    return ResponseEntity.status(HttpStatus.CREATED).body(service.create(req));
}

Full lesson: Request validation and global error handling →

JPA relationships, queries and pagination

Pagination and sorting from the controller

@GetMapping("/books")
Page<BookSummary> list(@PageableDefault(size = 20, sort = "title") Pageable pageable) {
  return repo.findSummaries(pageable);
}

// GET /books?page=0&size=20&sort=publishedAt,desc

Full lesson: JPA relationships, queries and pagination →

Schema migrations with Flyway and seed data

Reference data and test seeding

-- R__seed_countries.sql  (repeatable: re-runs when the file changes)
insert into country (code, name)
values ('DE', 'Germany'), ('FR', 'France'), ('GB', 'United Kingdom')
on conflict (code) do update set name = excluded.name;

Full lesson: Schema migrations with Flyway and seed data →

Transactions, locking and concurrency

Optimistic and pessimistic locking

@Retryable(retryFor = OptimisticLockingFailureException.class,
           maxAttempts = 3, backoff = @Backoff(delay = 50, multiplier = 2))
@Transactional
public void adjust(Long id, BigDecimal delta) { /* re-read inside the retry */ }

Full lesson: Transactions, locking and concurrency →

Securing the API with Spring Security

Method security and ownership checks

@PreAuthorize("hasRole('ADMIN') or #ownerId == authentication.name")
public Order getOrder(String ownerId, Long orderId) { ... }

@PostAuthorize("returnObject.owner == authentication.name")
public Order load(Long id) { ... }

@PreAuthorize("@perm.canEdit(authentication, #id)")
public void update(Long id, UpdateDto dto) { ... }

Full lesson: Securing the API with Spring Security →

Testing Spring Boot applications

Integration tests against a real database

// transaction boundary check in a @SpringBootTest
@Test
void rollbackOnFailure() {
  assertThatThrownBy(() -> service.transferWithoutFunds(1L, 2L, HUNDRED))
      .isInstanceOf(IllegalStateException.class);
  assertThat(accounts.balanceOf(1L)).isEqualByComparingTo(TEN);
}

What to assert and what to mock

@TestConfiguration
class TestClock {
  @Bean Clock clock() { return Clock.fixed(Instant.parse("2026-01-01T00:00:00Z"), ZoneOffset.UTC); }
}

Full lesson: Testing Spring Boot applications →

Observability with Actuator, logging and metrics

Logging that survives production

logging:
  level:
    root: INFO
    com.example.orders: DEBUG
    org.hibernate.SQL: WARN
  pattern:
    console: '{"ts":"%d{yyyy-MM-dd''T''HH:mm:ss.SSSXXX}","level":"%p","logger":"%c{1.}","msg":"%m"}%n'

Full lesson: Observability with Actuator, logging and metrics →

Caching, async processing and scheduling

Async work with a real executor

@Async("mailExecutor")
public CompletableFuture<Void> sendReceipt(Order order) {
  mailer.send(order);
  return CompletableFuture.completedFuture(null);
}

Scheduling in a multi-instance world

@Scheduled(cron = "0 */15 * * * *", zone = "UTC")
@SchedulerLock(name = "refreshCatalog")   // ShedLock: one node at a time
public void refreshCatalog() { catalog.reload(); }

@Scheduled(fixedDelayString = "${jobs.reconcile.delay:PT5M}")
public void reconcile() { ... }

Full lesson: Caching, async processing and scheduling →

Packaging and deploying a Spring Boot service

Building the artefact

# layered jar: dependencies change rarely, code changes often
./gradlew bootJar
java -Djarmode=tools -jar build/libs/orders-0.0.1.jar extract --destination build/extracted

# inspect the layers
unzip -l build/libs/orders-0.0.1.jar | head -20

# run with an external config file and an explicit profile
java -jar orders.jar --spring.profiles.active=prod \
  --spring.config.additional-location=file:/etc/orders/

A production Dockerfile

FROM eclipse-temurin:21-jre-alpine AS runtime

RUN addgroup -S app && adduser -S app -G app
WORKDIR /app
COPY --chown=app:app build/libs/*.jar app.jar

USER app
EXPOSE 8080
ENV JAVA_TOOL_OPTIONS="-XX:MaxRAMPercentage=75.0 -XX:+ExitOnOutOfMemoryError"

ENTRYPOINT ["java","-jar","/app/app.jar"]

Startup, probes and rollout

# Kubernetes excerpt
livenessProbe:
  httpGet: { path: /internal/health/liveness, port: 8081 }
  initialDelaySeconds: 60
  failureThreshold: 3
readinessProbe:
  httpGet: { path: /internal/health/readiness, port: 8081 }
  periodSeconds: 5
lifecycle:
  preStop:
    exec: { command: ["sh","-c","sleep 5"] }   # let the LB deregister first

Full lesson: Packaging and deploying a Spring Boot service →

Next steps: WebFlux, messaging and native images

Native images

./gradlew nativeCompile          # via GraalVM
# or a container build
./gradlew bootBuildImage --imageName=orders-native -Pnative

# run it
./build/native/nativeCompile/orders
# typical result: ~100 MB RSS, 50-100 ms to first response

Full lesson: Next steps: WebFlux, messaging and native images →

FAQ

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Last refreshed 2026-09-27.