What is Spring Boot DevTools and how does it improve development productivity?

<!-- Add DevTools (automatically excluded from production builds): -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <scope>runtime</scope>
    <optional>true</optional>  <!-- excluded from fat JAR / image -->
</dependency>

# What DevTools provides:
# 1. Automatic restart: detects classpath changes and restarts the app
#    (much faster than cold start -- uses two ClassLoaders)
# 2. LiveReload: refreshes browser on static resource changes
# 3. Property overrides for development:
#    - Disables caching (Thymeleaf, Jackson)
#    - Enables H2 console
#    - Sets DEBUG logging for web and SQL

# application.yml (DevTools default overrides in dev):
# spring.thymeleaf.cache=false         (disabled by DevTools)
# spring.jackson.serialization.indent_output=true
# logging.level.web=DEBUG
# spring.h2.console.enabled=true

# DevTools remote restart (for Docker/remote dev):
spring:
  devtools:
    remote:
      secret: ${DEVTOOLS_SECRET}
      # Enable in IDE to push class changes to running remote container

# Disable specific DevTools features:
spring:
  devtools:
    restart:
      enabled: false    # disable auto-restart
    livereload:
      enabled: false    # disable LiveReload server
    additional-exclude: "static/**,public/**"  # dont restart on these changes