Debugging auto-configuration: --debug flag and the conditions evaluation report.

When a bean is configured (or mysteriously isn't), run the app with --debug to print the conditions evaluation report — the authoritative list of which auto-configurations matched, which didn't, and the exact condition that decided each. This is the single best debugging tool for "why is this bean here / missing?"

Turning on the report

java -jar app.jar --debug

or in configuration:

debug=true

This does not raise your application log level to DEBUG globally (that's --debug on the logging side vs Boot's debug property — the Boot property specifically enables the conditions report). On startup you get a CONDITIONS EVALUATION REPORT.

Reading the report

============================
CONDITIONS EVALUATION REPORT
============================

Positive matches:        ← auto-config that fired, and why
-----------------
   DataSourceAutoConfiguration matched:
      - @ConditionalOnClass found required class 'javax.sql.DataSource' (OnClassCondition)

Negative matches:        ← auto-config that was skipped, and why
-----------------
   RabbitAutoConfiguration:
      Did not match:
      - @ConditionalOnClass did not find required class
        'com.rabbitmq.client.Channel' (OnClassCondition)

Exclusions:              ← explicitly excluded via spring.autoconfigure.exclude
Unconditional classes:   ← always-on infrastructure config

• Positive matches — every condition passed; the config contributed beans. Use this to confirm why something you didn't expect got configured.
• Negative matches — the precise condition that failed. "My DataSource isn't there" usually shows up here as @ConditionalOnClass did not find ... (missing dependency) or @ConditionalOnProperty not matching.

The most common findings

• Missing jar → a @ConditionalOnClass negative match. Add the starter.
• Your bean overrode the default → the default appears as a negative match: @ConditionalOnMissingBean found bean of type .... That's expected, not a bug.
• Wrong property value → @ConditionalOnProperty negative match. Fix the property name/value (watch relaxed binding).

Complementary tools

• Actuator /actuator/conditions exposes the same report over HTTP at runtime — no restart-with-flag needed.
• Actuator /actuator/beans lists every bean actually registered.
• spring.autoconfigure.exclude (or @SpringBootApplication(exclude=...)) to deliberately drop an auto-config, after which it appears under Exclusions.