Runners in Spring Boot

Runners in Spring Boot | Runners in Spring Boot are used to execute any logic only once when the application is getting started. They are used for setup/test purposes/batch processing. Usually, they are used in dev and testing environments.

There are two types of Runners in Spring Boot:-

• CommandLineRunner(I)
• ApplicationRunner(I)

Spring boot starter class (main class) will find all runner classes and execute/call them.

CommandLineRunner Interface In Spring Boot

It is a marker interface and contains only one abstract method:- run(). It does not contain any other static or default methods.

To demonstrate it create a Spring boot project. We have to define a class that implements the CommandLineRunner interface overriding the run() method and add @Component.

MyRunner class:- Add @Component to the class, and implement the CommandLineRunner marker interface, which will include the run() method. Write the required logic in the run() method.

package com.knowprogram.demo;

import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

@Component
public class MyRunner implements CommandLineRunner {

    @Override
    public void run(String... args) throws Exception {
        System.out.println("MyRunner.run()");
    }

}

Output In Console:-

MyRunner.run()

Example-2:- Instead of creating a separate class, we can also implement CommandLineRuneer in the starter class as follows:-

package com.knowprogram.demo;

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class CommandLineRunnerDemoApplication implements CommandLineRunner {

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

    @Override
    public void run(String... args) throws Exception {
        System.out.println("CommandLineRunnerDemoApplication.run()");
    }

}

Example-3:- Reading Properties file values and displaying them through CommandLineRunner.

In application.properties file:-

know.app.id = 5
know.app.code = knowprogram
know.app.cost = 5000

package com.knowprogram.demo;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

@Component
public class DataRunner implements CommandLineRunner {

    @Value("${know.app.id}")
    private Integer id;
    
    @Value("${know.app.code}")
    private String code;
    
    @Value("${know.app.cost}")
    private Double cost;

    @Override
    public void run(String... args) throws Exception {
        System.out.println(this);
    }

    @Override
    public String toString() {
        return "DataRunner [id=" + id + ", code=" + code + ", cost=" + cost + "]";
    }

}

In console:-

DataRunner [id=5, code=knowprogram, cost=5000.0]

Working with Multiple Runners in Spring Boot

We can even define multiple runners in the application. They are executed in naming order (A-Z naming rule).

Consider these runners:- SecurityRunner, MvcRunner, and JdbcRunner. They are executed in Naming order:-

1. JdbcRunner
2. MvcRunner
3. SecurityRunner

We can provide our own execution order using annotation @Order(“number”). The lowest number of orders executed first. Example:-

• ARunner – @Order(30)
• BRunner – @Order(12)
• CRunner – @Order(55)

Execution order is: BRunner, ARunner, CRunner

Case-1. If we provide -ve numbers even the same concept. The lowest number is the highest priority. Example:-

• ARunner – @Order(-51)
• BRunner – @Order(36)
• CRunner – @Order(0)
• DRunner – @Order(12)
• ERunner – @Order(-25)

Execution order is: ARunner, ERunner, CRunner, DRunner, BRunner.

Case-2. In case of multiple runners, if any runner is not assigned with order then executed last in order.

Case-3. If we give @Order but no value or no annotation even, then the value is: 2147483647 i.e. unordered Runners are executed at last (with naming rule applied). Example:-

1. A – @Order(5)
2. B – @Order // value: 2147483647
3. C – @Order(-9)
4. D – no annotation even // value: 2147483647
5. E – @Order // value: 2147483647

Execution flow:-

• First consider @Order with value: C, A
• Next, Consider @Order without value / No annotation (Naming Order): B,
• D, E

Case-4. If runners are given the same order, then again naming rule is applied:-

• Z – @Order(5)
• M – @Order(5)
• A – @Order(5)

Execution flow:- A, M, Z

Always Execution order is A-Z or -ve -> zero -> +ve
The reverse is not possible.

Example of Multiple Runners

We need to execute a few sets of runners in order, like
4 Runners – First
3 Runners – Next
6 Runners – Next
2 Runners – Last

In this case, define one CustomOrder interface and use the same for:-

public interface CustomOrder {
    int FIRST = -9999;
    int NEXT = 1111;
    int MID = 5555;
    int LAST = 9999;
}

@Component
@Order(CustomOrder.MID)
public class AdminRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("Admin.run()");
    }
}

@Component
@Order(CustomOrder.MID)
public class AppTestRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("AppTestRunner.run()");
    }
}

@Component
@Order(CustomOrder.FIRST)
public class DataSourceRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("DataReadRunner.run()");
    }
}

@Component
@Order(CustomOrder.FIRST)
public class DBPoolRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("DBPoolRunner.run()");
    }
}

@Component
@Order(CustomOrder.NEXT)
public class JDBCRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("JDBCRunner.run()");
    }
}

@Component
@Order(CustomOrder.LAST)
public class MVCRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("MVCRunner.run()");
    }
}

@Component
@Order(CustomOrder.LAST)
public class SecurityRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("SecurityRunner.run()");
    }
}

Execution Order:-

DataReadRunner.run()
DBPoolRunner.run()
JDBCRunner.run()
Admin.run()
AppTestRunner.run()
SecurityRunner.run()
MVCRunner.run()

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