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Spring Boot not working with dependent application.properties

I'm working with the Spring Boot 2.2.9.RELEASE. I have the main app and some plain starter (which just uses spring-actuator functionality) with some properties in its some-starter/src/main/resources/application.properties file:
management.server.port=9000
management.server.ssl.enabled=false
management.endpoints.enabled-by-default=false
management.endpoint.health.enabled=true
management.endpoints.web.base-path=/
management.endpoints.web.path-mapping.health=health
I've imported the starter into my main app and I believe that the healthcheck endpoint should work with the port 9000 and with the path /health (smth like that localhost:9000/health).
But it doesn't. However, it works in case of the same properties in my main app main-app/src/main/resources/application.properties.
Is it problem with the property overriding in Spring Boot? Should i configure my resources via something like maven-resources-plugin?

When application.properties is loaded from the classpath, the first one on the classpath is loaded and any others on the classpath are ignored. In your case, the file in main-app/src/main/resources/application.properties will appear on the classpath before the application.properties in the jar of some-starter.
As its name suggests, application.properties is intended for configuring your application and it shouldn't be used in a starter. You should either configure all of the properties in your application, or you could update your starter to include an EnvironmentPostProcessor that is registered via spring.factories and adds some default properties to the Environment.

spring boot autoconfiguration based on env variables

In my understanding in a spring-boot application when having spring-boot-starter-data-jpa on your classpath and also mysql-connector
spring-boot will still try to use in memory h2 (as per default), if no db related properties are defined.
But if you have properties like the following:
spring.jpa.hibernate.ddl-auto=update
spring.datasource.url=jdbc:mysql://${MYSQL_HOST:localhost}:3306/db_example
spring.datasource.username=springuser
spring.datasource.password=ThePassword
It will use mysql database.
My question is:
what happens if you have those properties only declared as environmental variables ?
Is this behaviour still valid ?
thanks!

I tried using the following dependencies together: "web", "mysql", "jpa". A simple run without any configuration shows the following error:
***************************
APPLICATION FAILED TO START
***************************
Description:
Failed to configure a DataSource: 'url' attribute is not specified and no embedded datasource could be configured.
Reason: Failed to determine a suitable driver class
Action:
Consider the following:
If you want an embedded database (H2, HSQL or Derby), please put it on the classpath.
If you have database settings to be loaded from a particular profile you may need to activate it (no profiles are currently active).
And if you use the environment route instead of "application.properties" as long as you set them correctly it should act the same as a fully configured "application.properties" file.

How to externalize application.properties files in Spring Boot to e.g. external dependent JAR?

I have a simple Maven module (not a Spring Boot application) in which I have placed my application.properties file.
I have 6-7 Spring Boot applications and I don't want to have an application.properties file in each and every application directory. I prefer it, if it is at one single place (external Maven module).
I am adding the maven module as a dependency in each of those Spring Boot application poms.
But, when I run those applications, it is not able to auto-detect the application.properties file because it is coming from a dependent jar not present physically in each of their application directories.
Is there any way to make this possible? I would like to avoid having properties files in 6-7 different locations, because that becomes tough to manage and handle.
Thank you in advance!

Consider using Spring Cloud Config that provides server and client-side support for externalized configuration in a distributed system. It requires some small effort, but it is very useful in long term. Config server manages configuration files (.properties or .yml), you can still use different config per profile (e.g. application-test.properties, application-prod.properties etc.). Your application has a higher priority, so you can always override properties coming from config server if needed. Another cool feature is that config server can utilize Git repository, so you can easily version all your configuration files. It also supports encryption - any fragile data can be encrypted so only your application knows how to decrypt it.
Config server
Config server is nothing else than a simple Spring Boot application that can be implemented as:
#SpringBootApplication
#EnableConfigServer
public class ConfigServer {
public static void main(String[] args) {
SpringApplication.run(ConfigServer.class, args);
}
}
with simple application.properties file included:
server.port: 8888
spring.cloud.config.server.git.uri: file://${user.home}/config-repo
with dependency in pom.xml
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-config-server</artifactId>
</dependency>
Config client
On client side you add a dependency to your pom.xml (or its equivalent in build.gradle if you use Gradle):
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-config-client</artifactId>
</dependency>
and all you have to do is add a URL to config server to your application.properties (or application.yml if you use YAML insted):
spring.cloud.config.uri: http://myconfigserver.com
Config files structure
Now let's say you have set up Git repository for your configuration files. Let's assume that your applications are named like horus, venus, mercury etc. and you have 3 different profiles: dev, test and prod. You also have some configuration that is common for all applications. In this case your configuration files structure would look like this (I will use properties files here but it applies to YAML as well):
application.properties - common config for all apps no matter what profile they use
application-dev.properties - common config for all apps running with dev profile
application-test.properties - common config for all apps running with test profile
application-prod.properties - common config for all apps running with prod profile
horus.properties - horus app config for, common for all profiles
horus-dev.properties - horus app config for dev profile only
horus-test.properties - horus app config for test profile only
horus-prod.properties - horus app config for prod profile only
etc.
There are some additional options that can be set (like encryption, connection strategy (fail fast or ignore) etc.), everything is well described and documented in official documentation https://cloud.spring.io/spring-cloud-config/ Hope it helps you making a good choice for managing your configuration in distributed application environment. Config server is a solution that was invented to solve this problem.

While Szymon Stepniak's answer certainly is a "by-the-book" of Spring Boot answer, I understand your situation, and even tried to do what you try to do by myself. Indeed, you can't define application.properties in other "external modules".
Here is how I've solved it:
Create a configuration in the "common" module
Create a property file in src/main/resources. It shouldn't be named application properties, It's better to provide a unique name for it (at least this is how I've done it, so let's assume that the file is called application-common.properties)
Use #PropertySources annotation to define a property file and load it with configuration.
Here is an example:
package com.myapp.common;
#Configuration
#PropertySources({
#PropertySource("classpath:application-common.properties")
})
public class MyConfiguration {
// you don't really have to define beans
}
Now if you want this configuration to load automatically only because the dependency is defined in your spring boot module of your build system, I've found the best to utilize spring factories:
Create the file src/main/resources/META-INF/spring.factories
Place the following into this file:
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.myapp.common.MyConfiguration

Spring Cloud Config in web application

I have legacy Java web application (with Spring MVC and web.xml) which are deployed in Tomcat. I want to swtich configuration to Spring Cloud Config.
Structure of this application (just simplify it for example) - jar file with Spring Controller and services, which I want to do Spring Cloud Config compatible. And another module with web.xml, which add jar as dependency.
I added bootstrap.yml with application name to module, which packaged to jar in "resources" folder and add "EnableAutoConfiguration" and "SpringBootApplication" annotations, but got exception:
java.lang.IllegalArgumentException: Could not resolve placeholder 'foo' in string value "${foo}"
at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:174)
at org.springframework.util.PropertyPlaceholderHelper.replacePlaceholders(PropertyPlaceholderHelper.java:126)
at org.springframework.core.env.AbstractPropertyResolver.doResolvePlaceholders(AbstractPropertyResolver.java:219)
at org.springframework.core.env.AbstractPropertyResolver.resolveRequiredPlaceholders(AbstractPropertyResolver.java:193)
Config server is run. I wrote simple client, which I run via main method and it works, but when I deploy legacy app in Tomcat - it doesn't work.
Can someone help with it?

Using Spring boot/cloud with Amazon AWS lambda does not inject values

I have an AWS lambda RequestHandler class which is invoked directly by AWS. Eventually I need to get it working with Spring Boot because I need it to be able to retrieve data from Spring Cloud configuration server.
The problem is that the code works if I run it locally from my own dev environment but fails to inject config values when deployed on AWS.
#Configuration
#EnableAutoConfiguration
#ComponentScan("my.package")
public class MyClass implements com.amazonaws.services.lambda.runtime.RequestHandler<I, O> {
public O handleRequest(I input, Context context) {
ApplicationContext applicationContext = new SpringApplicationBuilder()
.main(getClass())
.showBanner(false)
.web(false)
.sources(getClass())
.addCommandLineProperties(false)
.build()
.run();
log.info(applicationContext.getBean(SomeConfigClass.class).foo);
// prints cloud-injected value when running from local dev env
//
// prints "${path.to.value}" literal when running from AWS
// even though Spring Boot starts successfully without errors
}
}
#Configuration
public class SomeConfigClass {
#Value("${path.to.value}")
public String foo;
}
src/main/resources/bootstrap.yml:
spring:
application:
name: my_service
cloud:
config:
uri: http://my.server
failFast: true
profile: localdev
What have I tried:
using regular Spring MVC, but this doesn't have integration with #Value injection/Spring cloud.
using #PropertySource - but found out it doesn't support .yml files
verified to ensure the config server is serving requests to any IP address (there's no IP address filtering)
running curl to ensure the value is brought back
verified to ensure that .jar actually contains bootstrap.yml at jar root
verified to ensure that .jar actually contains Spring Boot classes. FWIW I'm using Maven shade plugin which packages the project into a fat .jar with all dependencies.
Note: AWS Lambda does not support environment variables and therefore I can not set anything like spring.application.name (neither as environment variable nor as -D parameter). Nor I can control the underlying classes which actually launch MyClass - this is completely transparent to the end user. I just package the jar and provide the entry point (class name), rest is taken care of.
Is there anything I could have missed? Any way I could debug this better?

After a bit of debugging I have determined that the issue is with using the Maven Shade plugin. Spring Boot looks in its autoconfigure jar for a META-INF/spring.factories jar see here for some information on this. In order to package a Spring Boot jar correctly you need to use the Spring Boot Maven Plugin and set it up to run during the maven repackage phase. The reason it works in your local IDE is because you are not running the Shade packaged jar. They do some special magic in their plugin to get things in the right spot that the Shade plugin is unaware of.
I was able to create some sample code that initially was not injecting values but works now that I used the correct plugin. See this GitHub repo to check out what I have done.
I did not connect it with Spring Cloud but now that the rest of the Spring Boot injection is working I think it should be straightforward.
As I mentioned in the comments you may want to consider just a simple REST call to get the cloud configuration and inject it yourself to save on the overhead of loading a Spring application with every request.
UPDATE: For Spring Boot 1.4.x you must provide this configuration in the Spring Boot plugin:
<configuration>
<layout>MODULE</layout>
</configuration>
If you do not then by default the new behavior of the plugin is to put all of your jars under BOOT-INF as the intent is for the jar to be executable and have the bootstrap process load it. I found this out while addressing adding a warning for the situation that was encountered here. See https://github.com/spring-projects/spring-boot/issues/5465 for reference.