0 of 64 lessons done

+ 84 advanced lessons in S2 Pro

Chapter 1 of 13

Core Kotlin

From your first println to senior-level coroutines, Flow and design.

72 lessons · 0 done

Unit 1
Hello, Kotlin
Print something, store something, and meet the basic types.
Hello, worldmain() and println()
val and varRead-only and changeable variables
String templatesPut values inside text
Basic typesInt, Double, Char, String, Boolean
Checkpoint: Hello, KotlinRe-test the whole unit
Unit 2
Collections
Group values in lists, sets and maps.
ListsOrdered items, duplicates allowed
SetsUnique items only
MapsKeys and values
Checkpoint: CollectionsRe-test the whole unit
Unit 3
Control flow
Make decisions and repeat work.
if and whenChoose between paths
Ranges and forCount and loop
while loopsRepeat until a condition fails
Checkpoint: Control flowRe-test the whole unit
Unit 4
Functions
Package code you can call again and again.
Declaring functionsfun, parameters and return types
Named and default argumentsFriendlier function calls
LambdasFunctions you can pass around
Checkpoint: FunctionsRe-test the whole unit
Unit 5
Classes
Model things with properties and behavior.
Classes and propertiesBlueprints for objects
Data classesClasses that mainly hold data
Checkpoint: ClassesRe-test the whole unit
Unit 6
Null safety
Meet Kotlin's way of avoiding the billion-dollar mistake.
Nullable typesWhich values may be null
Safe call and Elvis?. and ?:
Smart casts and !!Check once, use safely
Checkpoint: Null safetyRe-test the whole unit
Unit 7
Beyond the basics
The features that make Kotlin feel like Kotlin.
Extension functionsAdd functions to existing types
Scope functionslet, apply, run, also, with
Interfaces and inheritanceopen, override, abstract
ObjectsSingletons and companion objects
Enums and sealed classesFixed sets of options
CoroutinesDo work without blocking
Checkpoint: Beyond the basicsRe-test the whole unit
Unit 8
Collections like a pro
Transform data fluently and know what it costs.
Collection pipelinesS2 Promap, filter, flatMap, fold, reduce
Grouping and associatingS2 ProgroupBy, associateBy, partition, eachCount
SequencesS2 ProLazy, element-by-element processing
Costs and pitfallsS2 ProMembership checks, views and copies
Checkpoint: Collections like a proS2 ProRe-test the whole unit
Unit 9
The type system
Generics, variance and zero-cost abstractions.
Generics and boundsS2 ProType parameters, where, erasure
Variance: in and outS2 ProWhy List is covariant and MutableList is not
inline and reifiedS2 ProZero-cost lambdas and real type arguments
Value classes and ADTsS2 ProMake illegal states unrepresentable
Checkpoint: The type systemS2 ProRe-test the whole unit
Unit 10
Properties and delegation
Let Kotlin write the boilerplate for you.
Properties in depthS2 ProGetters, setters, backing fields, lateinit
Delegated propertiesS2 Prolazy, observable, vetoable, map
Class delegationS2 ProComposition over inheritance with by
Operators and conventionsS2 Proplus, get, invoke, componentN, equals
Checkpoint: Properties and delegationS2 ProRe-test the whole unit
Unit 11
Writing robust code
Fail early, model failure, clean up after yourself.
Exceptions and preconditionsS2 Prorequire, check, error
Modelling failuresS2 ProSealed results and Result<T>
Closing resourcesS2 Prouse and try/finally
Checkpoint: Writing robust codeS2 ProRe-test the whole unit
Unit 12
Coroutines in depth
Write concurrent code that stays correct under failure and cancellation.
Scopes and structured concurrencyS2 ProWho owns a coroutine?
Dispatchers and contextS2 ProWhere does the code run?
Cancellation and timeoutsS2 ProCooperative cancellation done right
Exceptions in coroutinesS2 Prolaunch, async, handlers and supervisors
Shared mutable stateS2 ProRaces, atomics, confinement and Mutex
Checkpoint: Coroutines in depthS2 ProRe-test the whole unit
Unit 13
Flow and channels
Streams of values, done properly.
Flow basicsS2 ProCold streams and context
Operators and back-pressureS2 Probuffer, conflate, collectLatest, catch
StateFlow and SharedFlowS2 ProHot flows for state and events
ChannelsS2 ProCommunicate between coroutines
Checkpoint: Flow and channelsS2 ProRe-test the whole unit
Unit 14
DSLs and functional design
APIs that read like a sentence.
Lambdas with receiverS2 ProT.() -> Unit
Type-safe buildersS2 ProNested DSLs and @DslMarker
Higher-order functionsS2 ProFunction types, references, closures, fun interface
Checkpoint: DSLs and functional designS2 ProRe-test the whole unit
Unit 15
Production Kotlin
Interop, tests, tooling and the judgment seniors are hired for.
Java interopS2 ProPlatform types and the @Jvm annotations
Testing Kotlin codeS2 Prokotlin.test, runTest and testable design
Build and conventionsS2 ProGradle Kotlin DSL, explicit API, ktlint, detekt
Multiplatform overviewS2 ProShare logic across JVM, iOS, web
Capstone: code reviewS2 ProSpot the problems seniors catch
Checkpoint: Production KotlinS2 ProRe-test the whole unit
Chapter review: Core KotlinS2 ProA hard mixed review of the whole chapter

Chapter 2 of 13Coming soon

Spring Boot

Build and ship REST services with Spring Boot.

Chapter 3 of 13Coming soon

Database

SQL, transactions and data access from Kotlin.

Chapter 4 of 13Coming soon

Kafka

Events, topics and consumers with Apache Kafka.

Chapter 5 of 13

Spring Core

The container under everything: beans, injection, lifecycle, scopes and proxies. No Boot yet.

7 lessons · 0 done

IoC & dependency injectionThe container creates and wires your objects instead of you calling constructors.
Bean definitions: @Bean vs component scanTwo ways to register beans: your own annotated classes or factory methods for classes you don't own.
Bean lifecycle & BeanPostProcessorS2 ProWhat happens between a bean definition and a usable (possibly proxied) bean.
Bean scopes & injecting shorter-lived beansSingleton is the default; injecting a request-scoped bean into a singleton needs a scoped proxy.
AOP proxies: JDK vs CGLIB, all-open, self-invocationS2 ProWhy @Transactional silently does nothing on some calls, and what Kotlin must do for proxies.
Writing custom aspectsS2 ProCross-cutting concerns (timing, auditing) as @Aspect with pointcuts and advice.
Chapter review: Spring CoreS2 ProA hard review of the whole chapter

Chapter 6 of 13

What Boot Does

Starters, configuration, auto-configuration and the embedded server: the magic, explained.

9 lessons · 0 done

Starters & dependency managementOne dependency pulls a tested set of libraries with versions managed for you.
Externalized config, profiles, property precedenceSame artifact, different config per environment, with a strict override order.
@ConfigurationProperties with Kotlin data classesType-safe, validated, immutable config instead of scattered @Value strings.
Auto-configuration, @Conditional*, conditions reportS2 ProHow Boot decides which beans to create, and how to see why.
Building a custom starterS2 ProPackage shared auto-configuration so teams get a feature by adding one dependency.
Embedded server & swapping containersThe app is a plain executable that starts its own server; swap Tomcat for Jetty if needed.
Start-up lifecycle: SpringApplication.runS2 ProThe eight stages between main() and ready
Boot 4: modular starters and upgradeS2 ProWhat was renamed, removed and switched on
Chapter review: What Boot DoesS2 ProA hard review of the whole chapter

Chapter 7 of 13

Web Layer

Request lifecycle, controllers, validation, errors, JSON, HTTP clients and the thread model.

12 lessons · 0 done

Request lifecycle: Servlet → filters → DispatcherServletTrace one HTTP request from the socket to your controller and back.
Controllers, routing, content negotiationMap URLs and methods to Kotlin functions and negotiate JSON vs other media types.
Validation with Bean Validation in Kotlin (@field:)Why annotations silently do nothing on constructor properties unless you pick the right target.
Error handling with @ControllerAdvice & ProblemDetailOne place that turns exceptions into consistent RFC 9457 problem responses.
Jackson + jackson-module-kotlinSerialize data classes, nullability and defaults without no-arg hacks.
Filters vs HandlerInterceptorsTwo interception layers at different levels; choose by what you need to see.
Custom argument resolvers & message convertersS2 ProExtend MVC: inject your own types into controller parameters and add custom body formats.
RestClient & HTTP interface clientsCall other services with a fluent client or a declarative interface.
Thread model & virtual threadsS2 ProOne platform thread per request vs cheap virtual threads for blocking I/O.
API versioning in Spring MVCS2 Proversion = "1.2+" and the version resolver
Jackson 3 in Boot 4S2 Protools.jackson, JsonMapper and renamed pieces
Chapter review: Web LayerS2 ProA hard review of the whole chapter

Chapter 8 of 13

Data

PostgreSQL, Flyway, JPA in Kotlin, transactions, locking, and the alternatives.

13 lessons · 0 done

SQL & PostgreSQL essentials (indexes, EXPLAIN)Read a query plan and know when an index helps.
Schema migrations with FlywayVersion the schema in SQL files applied in order, with checksums.
Spring Data JPA repositories & queriesDeclare an interface, get CRUD, derived queries and @Query for free.
Entity mapping in KotlinWhy data classes are wrong for entities, and the plugins you need.
Entity lifecycle, persistence context, dirty checking, flushS2 ProWhy modifying a managed entity updates the DB without calling save().
Relationship mapping & owning sideWhere the foreign key lives decides which side writes it.
Fetch strategies, N+1, open-in-viewS2 ProDetect and fix the query explosion, and know what OSIV hides.
@Transactional basicsDeclarative transactions via proxy: where to put it and what it guarantees.
Propagation & rollback rules (Kotlin, checked exceptions)S2 ProREQUIRES_NEW vs REQUIRED, rollback-only marking, and the Kotlin checked-exception trap.
Isolation levels, optimistic & pessimistic lockingS2 ProConcurrent updates: prevent lost updates with @Version or row locks.
Alternatives: Spring Data JDBC & jOOQS2 ProWhen JPA's session magic is too much: aggregates or type-safe SQL.
Spring Data MongoDBDocument model with repositories, Kotlin data classes allowed.
Chapter review: DataS2 ProA hard review of the whole chapter

Chapter 9 of 13

Testing

Fast unit tests, slice tests, full-context tests with real databases, and keeping the suite quick.

7 lessons · 0 done

JUnit 5 + MockK fundamentalsUnit-test services with no Spring context at all.
Slice tests: @WebMvcTest, @DataJpaTestLoad only the web or only the JPA slice of the context.
@SpringBootTest & bean overriding in testsFull context tests, and replacing one bean with a fake or mock.
Testcontainers & @ServiceConnectionTest against a real PostgreSQL, wired automatically.
Test context caching & suite performanceS2 ProWhy some suites take minutes, and how to keep contexts shared.
Test annotations in Boot 4S2 Pro@MockitoBean, @AutoConfigureMockMvc and RestTestClient
Chapter review: TestingS2 ProA hard review of the whole chapter

Chapter 10 of 13

Security

The filter chain, Kotlin DSL configuration, authentication, authorization, JWT and browser protections.

8 lessons · 0 done

Security architecture: filter chain & SecurityContextSecurity is a chain of servlet filters storing who you are in a thread-bound context.
SecurityFilterChain with the Kotlin DSLDeclare matcher rules in a bean; first matching rule wins.
Authentication & password encodingHow credentials are verified and why passwords are hashed, not encrypted.
Authorization & method securityProtect service methods with expressions, not just URLs.
JWT & OAuth2 resource serverS2 ProValidate bearer tokens statelessly and map claims to authorities.
CORS & CSRFTwo browser protections people confuse: cross-origin reads vs forged writes.
JWT authorities and SSRF protection (Boot 4.1)S2 ProAuthorities from claims with SpEL, and InetAddressFilter
Chapter review: SecurityS2 ProA hard review of the whole chapter

Chapter 11 of 13

Production

Health, metrics, tracing, logs, containers, graceful shutdown and native images.

8 lessons · 0 done

Actuator, health, liveness/readinessOperational endpoints and the Kubernetes probes that must not be confused.
Metrics with Micrometer & PrometheusS2 ProCounters, timers and gauges, and why label cardinality can sink your monitoring.
Distributed tracing with OpenTelemetryS2 ProFollow one request across services with trace and span ids.
Structured loggingEmit JSON logs machines can query, with correlation ids.
Containerization & graceful shutdownBuild an image and finish in-flight requests before the pod dies.
Native image & AOTS2 ProFaster startup and lower memory with GraalVM, in exchange for build-time rigidity.
The OpenTelemetry starterS2 ProOne starter for traces and metrics over OTLP
Chapter review: ProductionS2 ProA hard review of the whole chapter

Chapter 12 of 13S2 Pro

Distributed Systems

Modular monoliths, gateways, discovery, resilient clients, messaging, outbox and sagas.

9 lessons · 0 done

Modular monolith with Spring ModulithS2 ProEnforce module boundaries inside one deployable and communicate via events.
API gateway with Spring Cloud GatewayS2 ProOne entry point that routes, filters and rate-limits to internal services.
Service discovery & centralized configS2 ProFind instances by name and share configuration, or let Kubernetes do both.
Inter-service HTTP clientsS2 ProTimeouts, retries and error mapping when calling another service.
Resilience with Resilience4jS2 ProCircuit breaker, retry, rate limiter and bulkhead, and their composition order.
Messaging (Kafka/RabbitMQ) & idempotent consumersS2 ProAt-least-once delivery means duplicates: consumers must tolerate them.
Transactional outbox & sagasS2 ProReliable events across a DB and a broker, and multi-service workflows without 2PC.
Resilience in Spring Framework 7S2 Pro@Retryable, @ConcurrencyLimit and @EnableResilientMethods
Chapter review: Distributed SystemsS2 ProA hard review of the whole chapter

Chapter 13 of 13S2 Pro

Senior Review

Scenario drills that combine every phase: find the bug, name the cause, pick the fix.

3 lessons · 0 done

Review: proxies, transactions and persistence bugsS2 ProDiagnose bugs that come from proxies, transaction boundaries and the persistence context.
Review: production and security incidentsS2 ProReason about health probes, shutdown, metrics, tokens and browser security under pressure.
Review: designing a reliable distributed flowS2 ProCombine outbox, idempotency, retries, breakers and sagas into a correct order-processing design.

0 of 64 lessons done

+ 84 advanced lessons in S2 Pro

Chapter 1 of 13

Core Kotlin

From your first println to senior-level coroutines, Flow and design.

72 lessons · 0 done

Unit 1
Hello, Kotlin
Print something, store something, and meet the basic types.
Hello, worldmain() and println()
val and varRead-only and changeable variables
String templatesPut values inside text
Basic typesInt, Double, Char, String, Boolean
Checkpoint: Hello, KotlinRe-test the whole unit
Unit 2
Collections
Group values in lists, sets and maps.
ListsOrdered items, duplicates allowed
SetsUnique items only
MapsKeys and values
Checkpoint: CollectionsRe-test the whole unit
Unit 3
Control flow
Make decisions and repeat work.
if and whenChoose between paths
Ranges and forCount and loop
while loopsRepeat until a condition fails
Checkpoint: Control flowRe-test the whole unit
Unit 4
Functions
Package code you can call again and again.
Declaring functionsfun, parameters and return types
Named and default argumentsFriendlier function calls
LambdasFunctions you can pass around
Checkpoint: FunctionsRe-test the whole unit
Unit 5
Classes
Model things with properties and behavior.
Classes and propertiesBlueprints for objects
Data classesClasses that mainly hold data
Checkpoint: ClassesRe-test the whole unit
Unit 6
Null safety
Meet Kotlin's way of avoiding the billion-dollar mistake.
Nullable typesWhich values may be null
Safe call and Elvis?. and ?:
Smart casts and !!Check once, use safely
Checkpoint: Null safetyRe-test the whole unit
Unit 7
Beyond the basics
The features that make Kotlin feel like Kotlin.
Extension functionsAdd functions to existing types
Scope functionslet, apply, run, also, with
Interfaces and inheritanceopen, override, abstract
ObjectsSingletons and companion objects
Enums and sealed classesFixed sets of options
CoroutinesDo work without blocking
Checkpoint: Beyond the basicsRe-test the whole unit
Unit 8
Collections like a pro
Transform data fluently and know what it costs.
Collection pipelinesS2 Promap, filter, flatMap, fold, reduce
Grouping and associatingS2 ProgroupBy, associateBy, partition, eachCount
SequencesS2 ProLazy, element-by-element processing
Costs and pitfallsS2 ProMembership checks, views and copies
Checkpoint: Collections like a proS2 ProRe-test the whole unit
Unit 9
The type system
Generics, variance and zero-cost abstractions.
Generics and boundsS2 ProType parameters, where, erasure
Variance: in and outS2 ProWhy List is covariant and MutableList is not
inline and reifiedS2 ProZero-cost lambdas and real type arguments
Value classes and ADTsS2 ProMake illegal states unrepresentable
Checkpoint: The type systemS2 ProRe-test the whole unit
Unit 10
Properties and delegation
Let Kotlin write the boilerplate for you.
Properties in depthS2 ProGetters, setters, backing fields, lateinit
Delegated propertiesS2 Prolazy, observable, vetoable, map
Class delegationS2 ProComposition over inheritance with by
Operators and conventionsS2 Proplus, get, invoke, componentN, equals
Checkpoint: Properties and delegationS2 ProRe-test the whole unit
Unit 11
Writing robust code
Fail early, model failure, clean up after yourself.
Exceptions and preconditionsS2 Prorequire, check, error
Modelling failuresS2 ProSealed results and Result<T>
Closing resourcesS2 Prouse and try/finally
Checkpoint: Writing robust codeS2 ProRe-test the whole unit
Unit 12
Coroutines in depth
Write concurrent code that stays correct under failure and cancellation.
Scopes and structured concurrencyS2 ProWho owns a coroutine?
Dispatchers and contextS2 ProWhere does the code run?
Cancellation and timeoutsS2 ProCooperative cancellation done right
Exceptions in coroutinesS2 Prolaunch, async, handlers and supervisors
Shared mutable stateS2 ProRaces, atomics, confinement and Mutex
Checkpoint: Coroutines in depthS2 ProRe-test the whole unit
Unit 13
Flow and channels
Streams of values, done properly.
Flow basicsS2 ProCold streams and context
Operators and back-pressureS2 Probuffer, conflate, collectLatest, catch
StateFlow and SharedFlowS2 ProHot flows for state and events
ChannelsS2 ProCommunicate between coroutines
Checkpoint: Flow and channelsS2 ProRe-test the whole unit
Unit 14
DSLs and functional design
APIs that read like a sentence.
Lambdas with receiverS2 ProT.() -> Unit
Type-safe buildersS2 ProNested DSLs and @DslMarker
Higher-order functionsS2 ProFunction types, references, closures, fun interface
Checkpoint: DSLs and functional designS2 ProRe-test the whole unit
Unit 15
Production Kotlin
Interop, tests, tooling and the judgment seniors are hired for.
Java interopS2 ProPlatform types and the @Jvm annotations
Testing Kotlin codeS2 Prokotlin.test, runTest and testable design
Build and conventionsS2 ProGradle Kotlin DSL, explicit API, ktlint, detekt
Multiplatform overviewS2 ProShare logic across JVM, iOS, web
Capstone: code reviewS2 ProSpot the problems seniors catch
Checkpoint: Production KotlinS2 ProRe-test the whole unit
Chapter review: Core KotlinS2 ProA hard mixed review of the whole chapter

Chapter 2 of 13Coming soon

Spring Boot

Build and ship REST services with Spring Boot.

Chapter 3 of 13Coming soon

Database

SQL, transactions and data access from Kotlin.

Chapter 4 of 13Coming soon

Kafka

Events, topics and consumers with Apache Kafka.

Chapter 5 of 13

Spring Core

The container under everything: beans, injection, lifecycle, scopes and proxies. No Boot yet.

7 lessons · 0 done

IoC & dependency injectionThe container creates and wires your objects instead of you calling constructors.
Bean definitions: @Bean vs component scanTwo ways to register beans: your own annotated classes or factory methods for classes you don't own.
Bean lifecycle & BeanPostProcessorS2 ProWhat happens between a bean definition and a usable (possibly proxied) bean.
Bean scopes & injecting shorter-lived beansSingleton is the default; injecting a request-scoped bean into a singleton needs a scoped proxy.
AOP proxies: JDK vs CGLIB, all-open, self-invocationS2 ProWhy @Transactional silently does nothing on some calls, and what Kotlin must do for proxies.
Writing custom aspectsS2 ProCross-cutting concerns (timing, auditing) as @Aspect with pointcuts and advice.
Chapter review: Spring CoreS2 ProA hard review of the whole chapter

Chapter 6 of 13

What Boot Does

Starters, configuration, auto-configuration and the embedded server: the magic, explained.

9 lessons · 0 done

Starters & dependency managementOne dependency pulls a tested set of libraries with versions managed for you.
Externalized config, profiles, property precedenceSame artifact, different config per environment, with a strict override order.
@ConfigurationProperties with Kotlin data classesType-safe, validated, immutable config instead of scattered @Value strings.
Auto-configuration, @Conditional*, conditions reportS2 ProHow Boot decides which beans to create, and how to see why.
Building a custom starterS2 ProPackage shared auto-configuration so teams get a feature by adding one dependency.
Embedded server & swapping containersThe app is a plain executable that starts its own server; swap Tomcat for Jetty if needed.
Start-up lifecycle: SpringApplication.runS2 ProThe eight stages between main() and ready
Boot 4: modular starters and upgradeS2 ProWhat was renamed, removed and switched on
Chapter review: What Boot DoesS2 ProA hard review of the whole chapter

Chapter 7 of 13

Web Layer

Request lifecycle, controllers, validation, errors, JSON, HTTP clients and the thread model.

12 lessons · 0 done

Request lifecycle: Servlet → filters → DispatcherServletTrace one HTTP request from the socket to your controller and back.
Controllers, routing, content negotiationMap URLs and methods to Kotlin functions and negotiate JSON vs other media types.
Validation with Bean Validation in Kotlin (@field:)Why annotations silently do nothing on constructor properties unless you pick the right target.
Error handling with @ControllerAdvice & ProblemDetailOne place that turns exceptions into consistent RFC 9457 problem responses.
Jackson + jackson-module-kotlinSerialize data classes, nullability and defaults without no-arg hacks.
Filters vs HandlerInterceptorsTwo interception layers at different levels; choose by what you need to see.
Custom argument resolvers & message convertersS2 ProExtend MVC: inject your own types into controller parameters and add custom body formats.
RestClient & HTTP interface clientsCall other services with a fluent client or a declarative interface.
Thread model & virtual threadsS2 ProOne platform thread per request vs cheap virtual threads for blocking I/O.
API versioning in Spring MVCS2 Proversion = "1.2+" and the version resolver
Jackson 3 in Boot 4S2 Protools.jackson, JsonMapper and renamed pieces
Chapter review: Web LayerS2 ProA hard review of the whole chapter

Chapter 8 of 13

Data

PostgreSQL, Flyway, JPA in Kotlin, transactions, locking, and the alternatives.

13 lessons · 0 done

SQL & PostgreSQL essentials (indexes, EXPLAIN)Read a query plan and know when an index helps.
Schema migrations with FlywayVersion the schema in SQL files applied in order, with checksums.
Spring Data JPA repositories & queriesDeclare an interface, get CRUD, derived queries and @Query for free.
Entity mapping in KotlinWhy data classes are wrong for entities, and the plugins you need.
Entity lifecycle, persistence context, dirty checking, flushS2 ProWhy modifying a managed entity updates the DB without calling save().
Relationship mapping & owning sideWhere the foreign key lives decides which side writes it.
Fetch strategies, N+1, open-in-viewS2 ProDetect and fix the query explosion, and know what OSIV hides.
@Transactional basicsDeclarative transactions via proxy: where to put it and what it guarantees.
Propagation & rollback rules (Kotlin, checked exceptions)S2 ProREQUIRES_NEW vs REQUIRED, rollback-only marking, and the Kotlin checked-exception trap.
Isolation levels, optimistic & pessimistic lockingS2 ProConcurrent updates: prevent lost updates with @Version or row locks.
Alternatives: Spring Data JDBC & jOOQS2 ProWhen JPA's session magic is too much: aggregates or type-safe SQL.
Spring Data MongoDBDocument model with repositories, Kotlin data classes allowed.
Chapter review: DataS2 ProA hard review of the whole chapter

Chapter 9 of 13

Testing

Fast unit tests, slice tests, full-context tests with real databases, and keeping the suite quick.

7 lessons · 0 done

JUnit 5 + MockK fundamentalsUnit-test services with no Spring context at all.
Slice tests: @WebMvcTest, @DataJpaTestLoad only the web or only the JPA slice of the context.
@SpringBootTest & bean overriding in testsFull context tests, and replacing one bean with a fake or mock.
Testcontainers & @ServiceConnectionTest against a real PostgreSQL, wired automatically.
Test context caching & suite performanceS2 ProWhy some suites take minutes, and how to keep contexts shared.
Test annotations in Boot 4S2 Pro@MockitoBean, @AutoConfigureMockMvc and RestTestClient
Chapter review: TestingS2 ProA hard review of the whole chapter

Chapter 10 of 13

Security

The filter chain, Kotlin DSL configuration, authentication, authorization, JWT and browser protections.

8 lessons · 0 done

Security architecture: filter chain & SecurityContextSecurity is a chain of servlet filters storing who you are in a thread-bound context.
SecurityFilterChain with the Kotlin DSLDeclare matcher rules in a bean; first matching rule wins.
Authentication & password encodingHow credentials are verified and why passwords are hashed, not encrypted.
Authorization & method securityProtect service methods with expressions, not just URLs.
JWT & OAuth2 resource serverS2 ProValidate bearer tokens statelessly and map claims to authorities.
CORS & CSRFTwo browser protections people confuse: cross-origin reads vs forged writes.
JWT authorities and SSRF protection (Boot 4.1)S2 ProAuthorities from claims with SpEL, and InetAddressFilter
Chapter review: SecurityS2 ProA hard review of the whole chapter

Chapter 11 of 13

Production

Health, metrics, tracing, logs, containers, graceful shutdown and native images.

8 lessons · 0 done

Actuator, health, liveness/readinessOperational endpoints and the Kubernetes probes that must not be confused.
Metrics with Micrometer & PrometheusS2 ProCounters, timers and gauges, and why label cardinality can sink your monitoring.
Distributed tracing with OpenTelemetryS2 ProFollow one request across services with trace and span ids.
Structured loggingEmit JSON logs machines can query, with correlation ids.
Containerization & graceful shutdownBuild an image and finish in-flight requests before the pod dies.
Native image & AOTS2 ProFaster startup and lower memory with GraalVM, in exchange for build-time rigidity.
The OpenTelemetry starterS2 ProOne starter for traces and metrics over OTLP
Chapter review: ProductionS2 ProA hard review of the whole chapter

Chapter 12 of 13S2 Pro

Distributed Systems

Modular monoliths, gateways, discovery, resilient clients, messaging, outbox and sagas.

9 lessons · 0 done

Modular monolith with Spring ModulithS2 ProEnforce module boundaries inside one deployable and communicate via events.
API gateway with Spring Cloud GatewayS2 ProOne entry point that routes, filters and rate-limits to internal services.
Service discovery & centralized configS2 ProFind instances by name and share configuration, or let Kubernetes do both.
Inter-service HTTP clientsS2 ProTimeouts, retries and error mapping when calling another service.
Resilience with Resilience4jS2 ProCircuit breaker, retry, rate limiter and bulkhead, and their composition order.
Messaging (Kafka/RabbitMQ) & idempotent consumersS2 ProAt-least-once delivery means duplicates: consumers must tolerate them.
Transactional outbox & sagasS2 ProReliable events across a DB and a broker, and multi-service workflows without 2PC.
Resilience in Spring Framework 7S2 Pro@Retryable, @ConcurrencyLimit and @EnableResilientMethods
Chapter review: Distributed SystemsS2 ProA hard review of the whole chapter

Chapter 13 of 13S2 Pro

Senior Review

Scenario drills that combine every phase: find the bug, name the cause, pick the fix.

3 lessons · 0 done

Review: proxies, transactions and persistence bugsS2 ProDiagnose bugs that come from proxies, transaction boundaries and the persistence context.
Review: production and security incidentsS2 ProReason about health probes, shutdown, metrics, tokens and browser security under pressure.
Review: designing a reliable distributed flowS2 ProCombine outbox, idempotency, retries, breakers and sagas into a correct order-processing design.

Backend Kotlin course

Hello, Kotlin

Collections

Control flow

Functions

Classes

Null safety

Beyond the basics

Collections like a pro

The type system

Properties and delegation

Writing robust code

Coroutines in depth

Flow and channels

DSLs and functional design

Production Kotlin

Backend Kotlin course

Hello, Kotlin

Collections

Control flow

Functions

Classes

Null safety

Beyond the basics

Collections like a pro

The type system

Properties and delegation

Writing robust code

Coroutines in depth

Flow and channels

DSLs and functional design

Production Kotlin