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Spring

Spring configuration properties

Spring reads settings from application.yml, environment variables and command-line flags, and injects them into beans with @Value or @ConfigurationProperties, so behaviour changes per environment without rebuilding.

🔎 Unravel this examplePrerequisites (1)

Learn these first

Spring beans and dependency injection

At a glance

Mental model

Settings are the dials on the back of a machine.

What happens, in order
  1. 1@Value("${app.timeout:30}") injects one property, with 30 as the default if it is missing
  2. 2@ConfigurationProperties(prefix = "app") binds a whole group (app.timeout, app.retries) onto a typed class, which is cleaner and validates at startup
  3. 3Sources are layered: command line over environment variables over application-{profile}.yml over application.ymlSPRING_DATASOURCE_URL in the environment overrides spring.datasource.url.
  4. 4@Profile("prod") loads a bean only when that profile is active (spring.profiles.active=prod)
Remember
Watch out
Hard-coding environment-specific valuesCommitting secrets to application.ymlForgetting that environment variables override the file
Example (highlighted lines explain it)
@Componentpublic class RetryPolicy {    @Value("${app.retry.max-attempts:3}")    private int maxAttempts;    @Value("${app.retry.delay-ms}")    private long delayMs;}
  1. line 3 uses Value, the core of Spring configuration properties
  2. line 6 uses Value, the core of Spring configuration properties
Read the full explanation (about 1 min)

Spring reads settings from application.yml, environment variables and command-line flags, and injects them into beans with @Value or @ConfigurationProperties, so behaviour changes per environment without rebuilding.

A mental model

Settings are the dials on the back of a machine. The same machine is built once and the operator turns the dials differently in test and in production.

How it works

@Value("${app.timeout:30}") injects one property, with 30 as the default if it is missing.

@ConfigurationProperties(prefix = "app") binds a whole group (app.timeout, app.retries) onto a typed class, which is cleaner and validates at startup.

Sources are layered: command line over environment variables over application-{profile}.yml over application.yml. SPRING_DATASOURCE_URL in the environment overrides spring.datasource.url.

@Profile("prod") loads a bean only when that profile is active (spring.profiles.active=prod).

Going deeper

A missing property without a default fails the application at startup, which is good: you find out when deploying, not at 3 a.m. Keep secrets out of the repository and supply them through environment variables or a secret manager.

Typed properties classes can use bean validation (@Validated, @NotNull) so a bad value stops the boot with a clear message.

Under the hood

The Environment holds an ordered list of PropertySources. Relaxed binding maps app.max-retries, APP_MAXRETRIES and app.maxRetries to the same property, which is why environment variable names look different from YAML keys.

Common mistakes

  • Hard-coding environment-specific values.
  • Committing secrets to application.yml.
  • Forgetting that environment variables override the file.

How Unravel recognises it

When you paste code, Unravel looks for these signs of spring configuration properties. Spotting them yourself is a good habit when you read unfamiliar code:

Part of a learning path

Related concepts

Spring beans and dependency injectionSpring

Builds on this

Service discovery and central config (Spring Cloud)Spring

Frequently asked questions

What is Spring configuration properties?

Spring reads settings from application.yml, environment variables and command-line flags, and injects them into beans with @Value or @ConfigurationProperties, so behaviour changes per environment without rebuilding.

What are the most common mistakes with Spring configuration properties?

Hard-coding environment-specific values. Committing secrets to application.yml. Forgetting that environment variables override the file.

How can I see Spring configuration properties in real code?

Paste your own Java into Unravel and it will point out where spring configuration properties appears, explain how it works at the detail level you choose, and draw the flow of the code around it.