What is the difference between putting a property on application.yml or bootstrap.yml in spring boot?

I have just asked the Spring Cloud guys and thought I should share the info I have here.

bootstrap.yml is loaded before application.yml.

It is typically used for the following:

• when using Spring Cloud Config Server, you should specify spring.application.name and spring.cloud.config.server.git.uri inside bootstrap.yml
• some encryption/decryption information

Technically, bootstrap.yml is loaded by a parent Spring ApplicationContext. That parent ApplicationContext is loaded before the one that uses application.yml.

bootstrap.yml or bootstrap.properties

It's only used/needed if you're using Spring Cloud and your application's configuration is stored on a remote configuration server (e.g. Spring Cloud Config Server).

From the documentation:

A Spring Cloud application operates by creating a "bootstrap" context, which is a parent context for the main application. Out of the box it is responsible for loading configuration properties from the external sources, and also decrypting properties in the local external configuration files.

Note that the bootstrap.yml or bootstrap.properties can contain additional configuration (e.g. defaults) but generally you only need to put bootstrap config here.

Typically it contains two properties:

• location of the configuration server (spring.cloud.config.uri)
• name of the application (spring.application.name)

Upon startup, Spring Cloud makes an HTTP call to the config server with the name of the application and retrieves back that application's configuration.

application.yml or application.properties

Contains standard application configuration - typically default configuration since any configuration retrieved during the bootstrap process will override configuration defined here.

This answer has been very beautifully explained in book "Microservices Interview Questions, For Java Developers (Spring Boot, Spring Cloud, Cloud Native Applications) by Munish Chandel, Version 1.30, 25.03.2018.

The following content has been taken from this book, and total credit for this answer goes to the Author of the book i.e. Munish Chandel

application.yml

application.yml/application.properties file is specific to Spring Boot applications. Unless you change the location of external properties of an application, spring boot will always load application.yml from the following location:

/src/main/resources/application.yml

You can store all the external properties for your application in this file. Common properties that are available in any Spring Boot project can be found at: https://docs.spring.io/spring-boot/docs/current/reference/html/common-application-properties.html You can customize these properties as per your application needs. Sample file is shown below:

spring:
    application:
        name: foobar
    datasource:
        driverClassName: com.mysql.jdbc.Driver
        url: jdbc:mysql://localhost/test
server:
    port: 9000

bootstrap.yml

bootstrap.yml on the other hand is specific to spring-cloud-config and is loaded before the application.yml

bootstrap.yml is only needed if you are using Spring Cloud and your microservice configuration is stored on a remote Spring Cloud Config Server.

Important points about bootstrap.yml

1. When used with Spring Cloud Config server, you shall specify the application-name and config git location using below properties.

spring.application.name: "application-name"
spring.cloud.config.server.git.uri: "git-uri-config"

2. When used with microservices (other than cloud config server), we need to specify the application name and location of config server using below properties

spring.application.name: 
spring.cloud.config.uri: 

3. This properties file can contain other configuration relevant to Spring Cloud environment for e.g. eureka server location, encryption/decryption related properties.

Upon startup, Spring Cloud makes an HTTP(S) call to the Spring Cloud Config Server with the name of the application and retrieves back that application’s configuration.

application.yml contains the default configuration for the microservice and any configuration retrieved (from cloud config server) during the bootstrap process will override configuration defined in application.yml