What is the order of precedence when there are multiple Spring's environment profiles as set by spring.profiles.active

Question 1

The order of the profiles in the spring.profiles.active system property doesn't matter. "Precedence" is defined by the declaration order of the beans, including beans specific to a profile, and the last bean definition wins.

Using your example, if -Dspring.profiles.active="default,dev" is used, the props bean in the default profile would be used here, simply because it's the last active definition of that bean:

<beans profile="dev">
    <bean id="props" class="org.springframework.context.support.PropertySourcesPlaceholderConfigurer">
        <property name="location" value="classpath:META-INF/dev.properties"/>
    </bean>
</beans>
<beans profile="default">
    <bean id="props" class="org.springframework.context.support.PropertySourcesPlaceholderConfigurer">
        <property name="location" value="classpath:META-INF/default.properties"/>
    </bean>
</beans>

Invert the order of the beans, and then the dev version would be used, regardless of how the profiles are ordered in spring.profiles.active.

Notice that I did not use <context:property-placeholder/> because it does not allow you to explicitly specify a bean id, and so I'm not sure what behavior it would exhibit if more than one is used. I imagine that the properties would be merged, so that properties defined by both would use the last definition, but properties specific to each file would remain intact.

Otherwise, in my experience, you would typically define beans in this order:

"Default" bean definitions, not specific to a profile
Overriding bean definitions in an environment-specific profile
Overriding bean definitions in a test-specific profile

This way, test profile beans would win if used in combination with other profiles; else you would either use environment-specific beans or default beans based on the profile.

Question 2

The last definition wins. I keep it in mind but:

It is very important to remember that if you have some default content of application.properties inside jar resources, then this resource content will overwrite entries from external content of less important profiles (other profiles defined earlier in spring.profiles.active).

Example profiles: spring.profiles.active=p1,p2,p3

Files in Jar resources: application-p1.properties and application-p3.properties

External files: application-p1.properties and application-p2.properties

Final order will be (last wins):

resource application.properties
external application.properties
resource application-p1.properties
external application-p1.properties
external application-p2.properties
resource application-p3.properties - HERE IS THE TRICK! this will overwrite properties defined in external files for p1 and p2 with values from resource version of p3
external application-p3.properties

So keep in mind that last wins but also that resource goes just before external

Question 3

I had to experiment to convince myself.

Create the most simple spring boot application from Spring Initializr

Then added 3 properties files to resources dir (first one is already there but empty)

# application.properties
foo=foo in application.properties
bar=bar in application.properties
baz=baz in application.properties

# application-foobar.properties
foo=foo in foobar override properties
bar=bar in foobar override properties

# application-barbaz.properties
bar=bar in barbaz override properties
baz=bar in barbaz override properties

Then I added this @Config class to run at startup:

package com.example.profilesexperiment;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.CommandLineRunner;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.env.Environment;

@Configuration
class StartupConfig {

    @Autowired
    private Environment environment;

    @Value("${foo}")
    private String foo;

    @Value("${bar}")
    private String bar;

    @Value("${baz}")
    private String baz;

    @Bean
    CommandLineRunner startup() {
        return args -> {
            System.err.println("Active profiles: " + String.join(", ", environment.getActiveProfiles()));
            System.err.println("Foo = " + foo);
            System.err.println("Bar = " + bar);
            System.err.println("Baz = " + baz);
        };
    }
}

Then I ran it with different combinations of profiles. You can try that yourself, but here are the outputs of some:

just foobar

java -Dspring.profiles.active=foobar -jar target/profiles-experiment-0.0.1-SNAPSHOT.jar   
Active profiles: foobar
Foo = foo in foobar override properties
Bar = bar in foobar override properties
Baz = baz in application.properties

foobar then barbaz

java -Dspring.profiles.active=foobar,barbaz -jar ...
Active profiles: foobar, barbaz
Foo = foo in foobar override properties
Bar = bar in barbaz override properties
Baz = bar in barbaz override properties

barbaz then foobar

java -Dspring.profiles.active=barbaz,foobar -jar ...
Active profiles: barbaz, foobar
Foo = foo in foobar override properties
Bar = bar in foobar override properties
Baz = bar in barbaz override properties

Verdict: clearly, last one wins!

Oh, and also: non-overridden properties merge to one big happy property set (which is the reason I came searching here)

Question 4

superEB is right the order of the profiles doesn't matter for beans, the declaration order is more important there, but keep in mind that the order is important if you use profile based configuration files!