What is use of Spring Boot CommandLineRunner Interaface?

CommandLineRunner Interface is used to execute specific lines of code/task just before the Spring Boot Application starts-up.

How CommandLineRunner works?

We need to create spring bean using CommandLineRunner interface and spring boot will automatically detect them. CommandLineRunner Interface have run() method that needs to be overridden in implementing class and make the class as bean by using spring stereotype such as @Component.

The arguments which we pass to main() method while starting spring boot, can be accessed in the run() method of CommandLineRunner. run() method accepts array of String as an argument.

We can create more than one bean of CommandLineRunner implementing classes. To execute them in an order, we use spring @Order annotation or Ordered interface.

 CommandLineRunner and ApplicationRunner serve the same purpose – If you need access to ApplicationArguments instead of the raw String array consider using ApplicationRunner.

Steps for implementing CommandLineRunner

  1. Create a class CommandLineRunnerBean
  2. Implement CommandLineRunner interface for CommandLineRunnerBean class
  3. Override its run() method
  4. When the SpringApplication.run() starts the spring boot application then just before finishing startup, CommandLineRunner.run() will be executed. The run() method of CommandLineRunner accepts the arguments that are passed while starting server.

CommandLineRunnerBean.java

@Component
public class CommandLineRunnerBean implements CommandLineRunner{

  private static Logger LOG = LoggerFactory.getLogger(CommandLineRunnerBean.class);
  
  @Override
  public void run(String... args) throws Exception {
    LOG.info("EXECUTING : command line runner");
      
        for (int i = 0; i < args.length; ++i) {
            LOG.info("args[{}]: {}", i, args[i]);
        }
  }
}

Main class of Spring Boot Application

SpringConsoleApplication.java

@SpringBootApplication
public class SpringConsoleApplication{

  private static Logger LOG = LoggerFactory.getLogger(SpringConsoleApplication.class);
  
  public static void main(String[] args) {
    LOG.info("STARTING THE APPLICATION");
    SpringApplication.run(SpringConsoleApplication.class, args);
    LOG.info("APPLICATION FINISHED");
  }
}

POM.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.1.6.RELEASE</version>
    <relativePath/> <!-- lookup parent from repository -->
  </parent>
  <groupId>com.talksinfo</groupId>
  <artifactId>SpringConsoleApplication</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <name>SpringConsoleApplication</name>
  <description>Demo project for Spring Console Application</description>

  <properties>
    <java.version>1.8</java.version>
  </properties>

  <dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter</artifactId>
    </dependency>

    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-devtools</artifactId>
      <scope>runtime</scope>
      <optional>true</optional>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-test</artifactId>
      <scope>test</scope>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
      </plugin>
    </plugins>
  </build>

</project>

Application.properties

spring.main.web-application-type=NONE

Now execute the JAR file of application as below

java -jar SpringConsoleApplication0.0.1-SNAPSHOT.jar HELLO WORLD FROM TALKSINFO

RESULT

2019-08-11 19:39:27.094  INFO 41828 --- [  restartedMain] com.talksinfo.SpringConsoleApplication   : Started SpringConsoleApplication in 0.572 seconds (JVM running for 1.206)
2019-08-11 19:39:27.094  INFO 41828 --- [  restartedMain] com.talksinfo.CommandLineRunnerBean      : EXECUTING : command line runner
2019-08-11 19:39:27.095  INFO 41828 --- [  restartedMain] com.talksinfo.CommandLineRunnerBean      : args[0]: Hello
2019-08-11 19:39:27.095  INFO 41828 --- [  restartedMain] com.talksinfo.CommandLineRunnerBean      : args[1]: World
2019-08-11 19:39:27.095  INFO 41828 --- [  restartedMain] com.talksinfo.CommandLineRunnerBean      : args[2]: from
2019-08-11 19:39:27.095  INFO 41828 --- [  restartedMain] com.talksinfo.CommandLineRunnerBean      : args[3]: TalksInfo
2019-08-11 19:39:27.096  INFO 41828 --- [  restartedMain] com.talksinfo.SpringConsoleApplication   : APPLICATION FINISHED

Notice that the run method is called after application context is loaded but before the execution of the main method is complete.

Most console applications will only have a single class that implements CommandLineRunner. If your application has multiple classes that implement CommandLineRunner, the order of execution can be specified using spring @Order annotation or Ordered interface.

Example using @Order annotation:

  1. Create a class CommandLineRunnerOne and CommandLineRunnerTwo – Project remains same except 2 classes compared with above example
  2. Implement CommandLineRunner interface for CommandLineRunnerOne and CommandLineRunnerTwo class
  3. Override its run() method
  4. When the SpringApplication.run() starts the spring boot application then just before finishing startup, CommandLineRunnerOne.run() and CommandLineRunnerTwo.run() will be executed one after one. The run() method of CommandLineRunner accepts the arguments that are passed while starting server.
  5. Attached CommandLineRunnerOneand CommandLineRunnerTwoclass

CommandLineRunnerOne.java

@Component
@Order(1)
public class CommandLineRunnerOne implements CommandLineRunner{

  private static Logger LOG = LoggerFactory.getLogger(CommandLineRunnerTwo.class);
  
  @Override
  public void run(String... args) throws Exception {
    LOG.info("EXECUTING : command line runner 1");
      
        for (int i = 0; i < args.length; ++i) {
            LOG.info("args[{}]: {}", i, args[i]);
        }
  }
}

CommandLineRunnerTwo.java

@Component
@Order(2)
public class CommandLineRunnerTwo implements CommandLineRunner{

  private static Logger LOG = LoggerFactory.getLogger(CommandLineRunnerTwo.class);
  
  @Override
  public void run(String... args) throws Exception {
    LOG.info("EXECUTING : command line runner 2");
      
        for (int i = 0; i < args.length; ++i) {
            LOG.info("args[{}]: {}", i, args[i]);
        }
  }
}

Console

2019-08-11 20:34:58.169  INFO 72916 --- [  restartedMain] com.talksinfo.SpringConsoleApplication   : Started SpringConsoleApplication in 0.572 seconds (JVM running for 1.206)
2019-08-11 20:34:58.170  INFO 72916 --- [  restartedMain] com.talksinfo.CommandLineRunnerTwo       : EXECUTING : command line runner 1
2019-08-11 20:34:58.170  INFO 72916 --- [  restartedMain] com.talksinfo.CommandLineRunnerTwo       : args[0]: Hello
2019-08-11 20:34:58.171  INFO 72916 --- [  restartedMain] com.talksinfo.CommandLineRunnerTwo       : args[1]: World
2019-08-11 20:34:58.171  INFO 72916 --- [  restartedMain] com.talksinfo.CommandLineRunnerTwo       : args[2]: from
2019-08-11 20:34:58.171  INFO 72916 --- [  restartedMain] com.talksinfo.CommandLineRunnerTwo       : args[3]: TalksInfo
2019-08-11 20:34:58.171  INFO 72916 --- [  restartedMain] com.talksinfo.CommandLineRunnerTwo       : EXECUTING : command line runner 2
2019-08-11 20:34:58.171  INFO 72916 --- [  restartedMain] com.talksinfo.CommandLineRunnerTwo       : args[0]: Hello
2019-08-11 20:34:58.171  INFO 72916 --- [  restartedMain] com.talksinfo.CommandLineRunnerTwo       : args[1]: World
2019-08-11 20:34:58.171  INFO 72916 --- [  restartedMain] com.talksinfo.CommandLineRunnerTwo       : args[2]: from
2019-08-11 20:34:58.171  INFO 72916 --- [  restartedMain] com.talksinfo.CommandLineRunnerTwo       : args[3]: TalksInfo
2019-08-11 20:34:58.172  INFO 72916 --- [  restartedMain] com.talksinfo.SpringConsoleApplication   : APPLICATION FINISHED