Spring Boot Externalized Configuration and Profiles Questions and Answers

Question 1

A Spring Boot application defines the property `server.port=8080` in its `application.properties` file. An OS environment variable is set as `SERVER_PORT=7070`. The application is then launched with the command: `java -jar app.jar --server.port=9090`. On which port will the application start?

Accepted Answer

9090

Answer

Spring Boot uses a specific order of precedence for externalized configuration. Command-line arguments have a higher priority than both OS environment variables and properties defined in `application.properties`. Therefore, the value `9090` from the command-line argument will override the other values.

Question 2

A developer needs to create a configuration bean that is only active when the `production` profile is NOT active. Which annotation should be used on the bean's definition to achieve this?

Accepted Answer

@Profile("!production")

Answer

The `@Profile` annotation supports a NOT operator, represented by the exclamation mark `!`. Placing `@Profile("!production")` on a bean ensures it will be registered in the application context only when the 'production' profile is not active.

Question 3

A developer wants to bind a group of related properties under the prefix `app.credentials` to a type-safe POJO class. Which annotation is the most idiomatic and robust way to achieve this in Spring Boot?

Accepted Answer

Using `@ConfigurationProperties(prefix = "app.credentials")` on the POJO.

Answer

`@ConfigurationProperties` is specifically designed for binding structured, hierarchical properties to a dedicated object in a type-safe manner. It is superior to using multiple `@Value` annotations for related properties as it provides better encapsulation, validation, and is less error-prone.

Question 4

An application has an `application-dev.properties` file with `logging.level.root=DEBUG` and an `application-prod.properties` file with `logging.level.root=WARN`. If the application is launched with `spring.profiles.active=dev,prod`, what will be the effective root logging level?

Accepted Answer

WARN

Answer

When multiple profiles are active, properties from profiles declared later in the list take precedence. Since `prod` is the last profile in the `dev,prod` list, its `logging.level.root` value of `WARN` will override the value from the `dev` profile. This is known as the "last-one-wins" strategy.

Question 5

Which of the following `application.yml` snippets correctly configures a list of strings for the property `myapp.api-keys`?

Accepted Answer

myapp:
  api-keys:
    - key1
    - key2
    - key3

Answer

YAML uses a hyphen (`-`) followed by a space to denote items in a list. Each item is typically on a new line and indented under the parent key.

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Spring Boot Externalized Configuration and Profiles Questions and Answers