How does Spring Boot decide which configuration value wins?

Explains ordered property sources, profile documents, environment overrides, command-line values, and evidence-driven diagnosis.

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TL;DR

Spring Boot builds an Environment from ordered property sources, and a higher-priority source overrides a lower-priority value for the same key. In the common deployment path, command-line arguments can override system properties and environment variables, which can override configuration data; external config files override packaged files, and profile-specific files override non-profile files at the same location. The full order contains additional sources, so I diagnose the effective value and its origin rather than guessing from one file.

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External configuration precedence, deployment overrides, effective-value diagnosis, and avoidance of guessed source order.

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Core answer
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Overview
Spring Boot combines ordered property sources; a later higher-priority source can override an earlier value.
Spring Boot builds an Environment from ordered property sources, and a higher-priority source overrides a lower-priority value for the same key. In the common deployment path, command-line arguments can override system properties and environment variables, which can override configuration data; external config files override packaged files, and profile-specific files override non-profile files at the same location. The full order contains additional sources, so I diagnose the effective value and its origin rather than guessing from one file.
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Stronger answer
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A packaged application.yml can provide safe defaults while deployment configuration overrides the values that vary by environment. Profile-specific documents participate in the same ordering rules, and files outside the jar can override packaged files. Environment-variable name conversion and duplicate YAML keys can make a value look missing. For a failure, I compare active profiles, loaded config locations, sanitized environment evidence, and the bound properties object.

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Memorizing an incomplete two-source precedence rule.
Assuming the value in the repository is the runtime value.
Logging secrets while diagnosing property origins.

How does Spring Boot decide which configuration value wins?

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