Type-Safe Configuration with @ConfigurationProperties

shop.name=JavaBrahman Books
shop.currencies=USD,EUR,INR
shop.contact.email=orders@example.com
shop.contact.phone=+44 20 7946 0000
shop.delivery-days.GB=2
shop.delivery-days.IN=5
shop.session-timeout=30m
shop.max-upload-size=10MB

//ShopProperties.java
package com.javabrahman.boot.props;

import java.time.Duration;
import java.util.List;
import java.util.Map;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.boot.context.properties.bind.DefaultValue;
import org.springframework.util.unit.DataSize;

/**
 * Settings of the online shop.
 *
 * @param name name of the shop, shown on every page
 * @param pageSize number of books listed per page
 * @param currencies currencies the shop accepts
 * @param contact contact details shown on the help page
 * @param deliveryDays expected delivery time in days, per country code
 * @param sessionTimeout how long an idle shopping session is kept
 * @param maxUploadSize largest file a customer may upload
 */
@ConfigurationProperties("shop")
public record ShopProperties(
    String name,
    @DefaultValue("20") int pageSize,
    List<String> currencies,
    Contact contact,
    Map<String, Integer> deliveryDays,
    Duration sessionTimeout,
    DataSize maxUploadSize) {

  /**
   * Contact details.
   *
   * @param email address for orders
   * @param phone phone number for orders
   */
  public record Contact(String email, String phone) {
  }
}

//ShopPropertiesApp.java
package com.javabrahman.boot.props;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.context.properties.ConfigurationPropertiesScan;
import org.springframework.stereotype.Service;

@Service
class StorefrontService {
  private final ShopProperties shop;

  StorefrontService(ShopProperties shop) {
    this.shop = shop;
  }

  String banner() {
    return shop.name() + " - " + shop.pageSize() + " books per page, prices in " + String.join("/", shop.currencies())
        + ", delivery to India in " + shop.deliveryDays().get("IN") + " days";
  }
}

@SpringBootApplication
@ConfigurationPropertiesScan   // finds @ConfigurationProperties classes in this package and below
public class ShopPropertiesApp {
  public static void main(String[] args) {
    try (var context = SpringApplication.run(ShopPropertiesApp.class, args)) {
      ShopProperties shop = context.getBean(ShopProperties.class);
      System.out.println(context.getBean(StorefrontService.class).banner());
      System.out.println("contact        = " + shop.contact());
      System.out.println("sessionTimeout = " + shop.sessionTimeout() + " (" + shop.sessionTimeout().toMinutes() + " minutes)");
      System.out.println("maxUploadSize  = " + shop.maxUploadSize() + " (" + shop.maxUploadSize().toKilobytes() + " KB)");
    }
  }
}

//MailProperties.java
package com.javabrahman.boot.validated;

import jakarta.validation.constraints.Email;
import jakarta.validation.constraints.Max;
import jakarta.validation.constraints.Min;
import jakarta.validation.constraints.NotBlank;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.validation.annotation.Validated;

@Validated
@ConfigurationProperties("mail")
public record MailProperties(
    @NotBlank String host,
    @Min(1) @Max(65535) int port,
    @Email String from) {
}

//MailApp.java
package com.javabrahman.boot.validated;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.context.properties.EnableConfigurationProperties;

@SpringBootApplication
@EnableConfigurationProperties(MailProperties.class)
public class MailApp {
  public static void main(String[] args) {
    try (var context = SpringApplication.run(MailApp.class, args)) {
      System.out.println("Started with " + context.getBean(MailProperties.class));
    } catch (Exception e) {
      System.out.println("Startup failed: " + e.getClass().getSimpleName());
    }
  }
}

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <executions>
    <execution>
      <id>default-compile</id>
      <phase>compile</phase>
      <goals>
        <goal>compile</goal>
      </goals>
      <configuration>
        <annotationProcessorPaths>
          <path>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-configuration-processor</artifactId>
          </path>
        </annotationProcessorPaths>
      </configuration>
    </execution>
  </executions>
</plugin>

@Bean
@ConfigurationProperties("reports.http")   // reports.http.connect-timeout=2s calls setConnectTimeout(...)
ReportClientSettings reportClientSettings() {
  return new ReportClientSettings();       // a JavaBean from a library, with getters and setters
}