Java code explainers

java

@Service
@RequiredArgsConstructor
public class ExchangeRateService {

Request-scoped rate caching in Spring

request-scope caching memoization

Advanced 6 steps

java

public final class CircuitBreaker {
 
    private enum State { CLOSED, OPEN, HALF_OPEN }

How a circuit breaker guards failing calls

state-machine resilience concurrency

Advanced 7 steps

java

public Map<Long, CustomerDto> indexByCustomerId(List<CustomerDto> customers) {
    return customers.stream()
            .collect(Collectors.toMap(
                    CustomerDto::getId,

Building maps from lists with Collectors.toMap

streams collectors maps

Intermediate 5 steps

java

@Service
public class InventoryService {
 
    private final RestClient warehouseClient;

Bulkhead-protected HTTP calls in Spring

bulkhead resilience fallback

Intermediate 7 steps

java

@Configuration
public class DataSourceLoggingConfig {
 
    @Bean

Wrapping a Spring DataSource for query tracing

proxy pattern observability data source

Intermediate 7 steps

java

@RestController
@RequestMapping("/api/products")
@Validated
public class ProductSearchController {

Validating query params in a Spring controller

validation pagination rest-api

Intermediate 8 steps

java

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = PasswordsMatchValidator.class)
public @interface PasswordsMatch {

Custom cross-field validation in Spring

bean-validation custom-constraints cross-field-validation

Intermediate 8 steps

java

public final class LogRedactor {
 
    private static final Pattern SECRET = Pattern.compile(
        "(?i)(password|token|api[_-]?key|secret|authorization)\\s*[=:]\\s*\\S+");

Streaming log redaction in Java

regex streaming-io try-with-resources

Intermediate 9 steps

java

@Component
public class HeaderMergeFilter {
 
    private static final String PER_REQUEST_HEADERS = HeaderMergeFilter.class.getName() + ".headers";

Merging default and per-request headers in Spring

webclient filters http-headers

Intermediate 8 steps

java

import java.util.HashSet;
import java.util.Set;
 
public final class CollectionDiff<T> {

Diffing two collections with set operations

set-operations immutability generics

Intermediate 8 steps

java

public record AppConfig(
        String host,
        int port,
        String databaseUrl,

Loading typed config from env vars in Java

records configuration environment-variables

Intermediate 6 steps

java

public class RequestThrottler {
 
    private final Semaphore permits;
    private final long acquireTimeoutMillis;

Bounding concurrency with a Semaphore in Java

concurrency semaphore rate-limiting

Intermediate 6 steps

java

public List<DailySample> backfillMissingDates(List<DailySample> samples, double fillValue) {
    if (samples.isEmpty()) {
        return List.of();
    }

Backfilling gaps in a daily time series in Java

streams time-series treemap

Intermediate 5 steps

java

public final class Ulid {
 
    private static final char[] ENCODING = "0123456789ABCDEFGHJKMNPQRSTVWXYZ".toCharArray();
    private static final SecureRandom RANDOM = new SecureRandom();

How to generate a ULID in Java

base32-encoding bit-manipulation identifiers

Intermediate 8 steps

java

@RestController
@RequestMapping("/api/reports")
public class ReportController {

Header-driven endpoints in a Spring controller

rest api request headers dependency injection

Intermediate 7 steps

java

public final class CaseConverter {
 
    private static final Pattern CAMEL_BOUNDARY =
            Pattern.compile("([a-z0-9])([A-Z])|([A-Z]+)([A-Z][a-z])");

Converting camelCase to snake_case in Java

regex string-manipulation utility-class

Intermediate 7 steps

java

@Component
public class RegionCacheWarmer implements SmartInitializingSingleton {
 
    private static final Logger log = LoggerFactory.getLogger(RegionCacheWarmer.class);

Warming a Spring cache at startup

caching startup-hook dependency-injection

Intermediate 7 steps

java

public final class EmailNormalizer {
 
    private static final Pattern EMAIL_PATTERN = Pattern.compile(
        "^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$"

Normalizing email addresses in Java

validation regex normalization

Intermediate 8 steps