Spring Boot ApplicationRunner Example

1. Introduction

Spring Boot provides various ways to run logic on startup, one of which is the ApplicationRunner interface. It offers a typed interface to application arguments and is particularly useful when you need access to the raw String[] arguments of the public static void main(String[] args) method. In this blog post, we’ll demonstrate how to implement and use the ApplicationRunner in a Spring Boot application.

Key Points:

1. ApplicationRunner is a simple Spring Boot interface with a run method to execute startup logic.

2. It provides access to application arguments as an ApplicationArguments object, which offers convenience methods for argument parsing.

3. ApplicationRunner beans are executed before the application is fully started, making them suitable for initializing tasks.

4. Multiple ApplicationRunner beans can be defined and ordered using the @Order or @Priority annotations.

5. It is often used for tasks such as preloading data, checking system resources, or interacting with external services.

2. Implementation Steps

1. Define a Spring Boot application with the necessary setup.

2. Implement the ApplicationRunner interface in a component bean.

3. Use the ApplicationArguments object to retrieve and process the application arguments.

4. Add logging or other logic within the run method to confirm execution.

5. Start the Spring Boot application with custom arguments to test the ApplicationRunner.

3. Implementation Example

// Step 1: Implement the ApplicationRunner interface in a component class
@Component
public class AppStartupRunner implements ApplicationRunner {

    private static final Logger logger = LoggerFactory.getLogger(AppStartupRunner.class);

    // Step 2: Override the run method to execute the custom logic
    @Override
    public void run(ApplicationArguments args) throws Exception {
        // Step 3: Process the application arguments if necessary
        if (args.containsOption("appArg")) {
            logger.info("Found appArg in ApplicationArguments");
        }

        // Step 4: Log the non-option arguments and option values
        logger.info("Non-option args: {}", args.getNonOptionArgs());
        logger.info("Option Names: {}", args.getOptionNames());

        // Additional logic here
    }
}

// Step 5: Define the main Spring Boot application class
@SpringBootApplication
public class ApplicationRunnerExample {

    public static void main(String[] args) {
        SpringApplication.run(ApplicationRunnerExample.class, args);
    }
}

// When you start the Spring Boot application, AppStartupRunner will automatically be called with the arguments passed.

Output:

// Assuming you started the Spring Boot application with the argument --appArg=exampleArg, the output would be:
INFO Found appArg in ApplicationArguments
INFO Non-option args: []
INFO Option Names: [appArg]

Explanation:

1. AppStartupRunner: This component class implements ApplicationRunner to provide custom startup logic.

2. @Component: Marks the class as a candidate for component scanning, so it's detected by Spring Boot.

3. @Override: Indicates that the run method is implementing the method from the ApplicationRunner interface.

4. ApplicationArguments: The object provided by Spring Boot that wraps the command-line arguments passed to main.

5. logger: Used to log messages to the console or a file.

6. ApplicationRunnerExample: This class contains the main method and is annotated with @SpringBootApplication, which triggers auto-configuration and component scanning.

7. SpringApplication.run(): Boots up the Spring Boot application and ensures that AppStartupRunner is executed.