Multiple ApplicationRunners on classpath, how to make SpringApplication.run() only run one - spring

Question

Context: I have a project with some utilities to do things like data fixing. Each utility is a Java application, i.e. class with main() method. I want to define them as Spring Boot applications so I can use the ApplicationRunner and ApplicationArguments facility. The Spring configuration is defined via annotations in a shared configuration class. I've put a minimal example of this setup below.
Expectation: if I call SpringApplication.run(SomeClass.class, args) where SomeClass is an ApplicationRunner, it runs the run() on that class and not on any other classes that may be in the app context.
What actually happens: it calls all ApplicationRunners that it has in the context.
Why? I understood SpringApplication.run(Class, String[]) to mean, "run this class" whereas it appears to mean "load an app context from this class and run anything you can find in it". How should I fix it to run only 1 class? I don't mind if my other application class isn't in the app context, because all the configuration I need is in the shared config class. But I don't want to have to edit code (e.g. add or remove annotations) according to which class I need to run.
Minimal example:
A Spring config class (shared):
package com.stackoverflow.example;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
#Configuration
public class ExampleSpringConfig {
/** Some bean - just here to check that beans from this config are injected */
#Bean public FooService fooService () {
return new FooService();
}
}
Two application classes
package com.stackoverflow.example;
import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.ApplicationRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import javax.annotation.Resource;
#SpringBootApplication
public class SomethingJob implements ApplicationRunner {
#Resource private FooService fooService;
public void run(ApplicationArguments args) throws Exception {
System.out.println("Doing something"); // do things with FooService here
}
public static void main(String[] args) {
SpringApplication.run(SomethingJob.class, args);
}
}
and another that is identical except that it prints "Doing something else".
Output:
[Spring Boot startup logs...]
Doing something else
Doing something
[Spring Boot shutdown logs...]

Answer 1

Firstly, only one class should be annotated with #SpringBootApplication. As you've noticed in your answer, this defines the external "main" entry point. I would recommend this is a different class to your ApplicationRunner classes for clarity and conceptual separation.
To only have some but not all runners run, I've done this by parsing the arguments, and quickly exiting from the runner which should not be called. e.g.
package com.stackoverflow.example;
import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.ApplicationRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import javax.annotation.Resource;
#Component
public class SomethingJob implements ApplicationRunner {
#Resource private FooService fooService;
public void run(ApplicationArguments args) throws Exception {
if (!args.containsOption("something")) return
System.out.println("Doing something"); // do things with FooService here
}
}
That way you can do java -jar myjar.jar --something or java -jar myjar.jar --something-else depending which one you want to be run.

Answer 2

I found a workaround while experimenting with my minimal example.
#SpringBootApplication is just an alias for #ComponentScan, #EnableAutoConfiguration and #Configuration. By applying them separately, I discovered that it's the #Configuration annotation that causes this behaviour. If I only apply the other 2, I don't get the issue.
I guess this is because #Configuration means "I'm a configuration class, and any beans I define should be pulled into the context during component scan" and although this class doesn't define an ApplicationRunner, it is one, which has the same effect. Therefore if you have 2 such beans on the classpath, they both get pulled into the app context.
Without #Configuration, the bean you want to run still gets registered since it's referenced by the call to run(), but other ApplicationRunners on the classpath don't.
This fixes my immediate problem by making sure I only have one ApplicationRunner in my app context. But it doesn't answer the wider question, "If I do have several ApplicationRunners, how do I tell Spring Boot which one to run?" So I'd still appreciate any more complete answer or suggestions for a different approach.