1. Command-Line Arguments

• To take argument from command line or passing values to main method and access them using
  • CommandLineRunner
  • ApplicationRunner
  • SpringApplication.getRunArguments()
  • or even with @Value injection
• Example main(String[]args) : passing values to main method via command line arguments.

2. CommandLineRunner and ApplicationRunner

• executed immediately after SpringApplication.run() completes.
• Having abstract method.

Example:

@SpringBootApplication
public static void main(String[] args) {
    SpringApplication.run(MyApplication.class, args);
    // Runners execute AFTER this
}

3. Runner class

A component class that implements CommandLineRunner or ApplicationRunner and is registered as a Spring bean is called a Spring Boot runner class.

Why is it called a Spring Boot runner class?

beacuse such a class :

• Runs automatically at startup.
• Is a Spring-managed bean.
• Implements CommandLineRunner or ApplicationRunner to define startup logic

Key Points

• impementing abstract method .
• Executed only once and run automatically.
• Executed logic which write inside implemented of Runner interface .
• used to define logic for loading configuration values, properties.

Spring Boot application startup lifecycle

1. Spring Boot App
2. Start
3. Run method
   • IoC Container
   • Bean Object created
   • Runners
   • Started (Application is ready to use. perform operation)

How many Runners can we add in Spring Boot Application?

Answer: Can add unlimited runner classes.

Multiple Runner class Execution Order

1. Execute randomly.
2. Control execution order :using @Order

   least number : higher priority

Example Code

@Order(1) //executed  second
@Component
public class FirstRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("FirstRunner ");
    }
}

@Order(-5) //executed  first
@Component
public class SecondRunner implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) throws Exception {
        System.out.println("SecondRunner");
    }
}

@Order(2) //executed  third
@Component
public class ThirdRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("ThirdRunner ");
    }
}

4. Difference between CommandLineRunner and ApplicationRunner?

1. CommandLineRunner

   • Method : run(String... args)
   • Argument Type : String of Array
   • alamgirPassasArguemts -> main(String[] args)-> SpringApplication.run(MyApplication.class, args)->run(String... args)
   • passing arguemt share the all Runners.

2. ApplicationRunner

   • Method : run(ApplicationArguments args)
   • Argument Type : Object (ApplicationArguments)
   • alamgirPassasArguemts -> main(String[] args)-> SpringApplication.run(MyApplication.class, args)-> run(ApplicationArguments args)
     

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Github Code : Spring Runner : Spring Boot