ApplicationRunner is a Spring Boot interface that allows developers to execute custom logic immediately after the Spring application context has been fully initialized and the application has started successfully. It is commonly used for startup tasks such as loading initial data, validating configurations, warming caches, or performing setup operations.
Key Points: • ApplicationRunner executes automatically after the Spring Boot application starts. • It is useful for initialization and startup-related tasks. • It provides access to application startup arguments through ApplicationArguments. • Multiple ApplicationRunner implementations can be executed in a specific order using @Order. • It is managed as a Spring Bean and runs only once during application startup.
Why Do We Need ApplicationRunner?
Sometimes applications need to perform tasks immediately after startup.
Examples:
• Load default data into a database • Validate external service connections • Initialize cache data • Read startup parameters • Trigger startup jobs
ApplicationRunner provides a clean and structured way to perform such operations.
How Does It Work?
Application Startup | Spring Context Initialization | Bean Creation | Application Ready | ApplicationRunner Executes | Application Starts Serving Requests
The run() method is called automatically once the application is fully initialized.
Code Example:
import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.ApplicationRunner;
import org.springframework.stereotype.Component;
@Component
public class StartupRunner
implements ApplicationRunner {
@Override
public void run(ApplicationArguments args)
throws Exception {
System.out.println(
"Application Started Successfully");
}
}Output:
Application Started Successfully
In this example:
• Spring creates the bean. • The run() method executes automatically after startup.
Accessing Startup Arguments
ApplicationRunner provides access to command-line arguments through the ApplicationArguments object.
Code Example:
@Component
public class StartupRunner
implements ApplicationRunner {
@Override
public void run(
ApplicationArguments args) {
System.out.println(
args.getOptionNames());
}
}Application Start:
java -jar app.jar --env=prod
The argument can be accessed inside the run() method.
Common Use Cases
• Database initialization • Loading reference data • Cache warming • Startup validation • External system health checks • Scheduling startup tasks
Example: Suppose an e-commerce application requires product categories to be loaded into memory.
At startup:
• ApplicationRunner loads categories from the database. • Stores them in cache. • Makes them readily available for incoming requests.
ApplicationRunner vs CommandLineRunner
Spring Boot provides two startup interfaces.
ApplicationRunner
public void run(ApplicationArguments args)
Benefits:
• Structured argument handling • Easier access to named parameters
CommandLineRunner
public void run(String... args)
Benefits:
• Simpler implementation • Direct access to raw arguments
Example:
CommandLineRunner:
public void run( String... args)
ApplicationRunner:
public void run( ApplicationArguments args)
Most modern Spring Boot applications prefer ApplicationRunner because of its richer argument API.
Executing Multiple Runners
If multiple runners exist, execution order can be controlled.
Code Example:
@Component
@Order(1)
public class FirstRunner
implements ApplicationRunner {
}
@Component
@Order(2)
public class SecondRunner
implements ApplicationRunner {
}Execution Order:
FirstRunner
↓
SecondRunner
Real-World Example
Consider a banking application.
After startup:
• Verify database connectivity. • Load currency exchange rates. • Initialize cache. • Validate external payment services.
ApplicationRunner is an ideal place for these startup tasks.
Best Practices
• Keep startup logic lightweight. • Avoid long-running operations. • Use separate services for complex initialization. • Handle exceptions carefully during startup. • Use @Order when multiple runners exist.
Interview Tip: A concise interview answer is:
"ApplicationRunner is a Spring Boot interface used to execute custom code immediately after the application starts and the Spring context is fully initialized. It is commonly used for startup tasks such as loading initial data, validating configurations, warming caches, and processing startup arguments through the ApplicationArguments object."