spring-019: command-line-arguments-overview

In Spring Boot, command-line arguments offer a flexible way to configure applications at runtime, allowing you to override default settings or pass custom parameters without altering the codebase. Here's a structured overview:

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1. Passing Command-Line Arguments

You can supply command-line arguments to a Spring Boot application in various ways, depending on how you run the application:

• Executable JAR: When running the packaged JAR file, append arguments after the JAR name:

  java -jar myapp.jar --server.port=8081 --custom.argument=value

• Maven Plugin: If using the Spring Boot Maven plugin, pass arguments using the -Dspring-boot.run.arguments property:

  mvn spring-boot:run -Dspring-boot.run.arguments="--server.port=8081,--custom.argument=value"

• Gradle Plugin: Configure the bootRun task in your build.gradle to accept arguments:

  bootRun {
      if (project.hasProperty('args')) {
          args project.args.split(',')
      }
  }

Then run:

  ./gradlew bootRun --args="--server.port=8081,--custom.argument=value"

These methods allow you to override default configurations or provide custom parameters at runtime. (baeldung.com)

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2. Accessing Command-Line Arguments in Application Code

Spring Boot provides interfaces to access command-line arguments within your application:

• Implementing CommandLineRunner: This interface allows you to execute code with access to the command-line arguments after the application context is loaded:

  @SpringBootApplication
  public class MyApp implements CommandLineRunner {
      public static void main(String[] args) {
          SpringApplication.run(MyApp.class, args);
      }

      @Override
      public void run(String... args) {
          for (String arg : args) {
              System.out.println(arg);
          }
      }
  }

• Implementing ApplicationRunner: Similar to CommandLineRunner but provides the ApplicationArguments interface for more structured access:

  @SpringBootApplication
  public class MyApp implements ApplicationRunner {
      public static void main(String[] args) {
          SpringApplication.run(MyApp.class, args);
      }

      @Override
      public void run(ApplicationArguments args) {
          if (args.containsOption("custom.argument")) {
              System.out.println("Custom Argument: " + args.getOptionValues("custom.argument"));
          }
      }
  }

These approaches enable you to handle both option arguments (e.g., --custom.argument=value) and non-option arguments. (baeldung.com)

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3. Overriding Application Properties

Command-line arguments can override properties defined in application.properties or application.yml. For instance, to change the server port:

• Command-Line:

  java -jar myapp.jar --server.port=8081

• Maven:

  mvn spring-boot:run -Dspring-boot.run.arguments="--server.port=8081"

In the application's property hierarchy, command-line arguments have the highest precedence, ensuring they override other configurations. (docs.spring.io)

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4. Disabling Command-Line Property Conversion

By default, Spring Boot converts command-line arguments to properties and adds them to the Spring Environment. To disable this behavior:

@SpringBootApplication
public class MyApp {
    public static void main(String[] args) {
        SpringApplication app = new SpringApplication(MyApp.class);
        app.setAddCommandLineProperties(false);
        app.run(args);
    }
}

This prevents command-line arguments from being added to the Environment, which can be useful in certain scenarios. (docs.spring.io)

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For a visual demonstration, you might find this video helpful:

Pass and Read Command-line Arguments in Spring Boot