Suppose you need to develop a CLI application, what are the first tools that come to mind? Perhaps you think of shell script or Python. But what about Spring? Yes, that Java framework. I was intrigued when I discovered that the project provides a resource specifically for creating shells, so I decided to test it.

First steps #

The initial setup doesn’t have many secrets, we can use the good old Spring Initializr to create a project. In the dependencies list, we find Spring Shell, which will be the main object of interest in this article.

After downloading, extracting, and opening it, we need to make sure that the interactive shell is enabled in application.yml or application.properties:

spring:
    shell:
        interactive:
            enabled: true

It might also be a good idea to disable the banner and logging, as they won’t be very useful:

spring:
    main:
        banner-mode: off

logging:
    level:
        root: off

Running #

When compiling with gradlew build and executing the generated jar, we enter directly into a shell:

$ java -jar build/libs/shell-0.0.1.jar
shell:>

We even have some default commands:

shell:>help
AVAILABLE COMMANDS

Built-In Commands
       help: Display help about available commands
       stacktrace: Display the full stacktrace of the last error.
       clear: Clear the shell screen.
       quit, exit: Exit the shell.
       history: Display or save the history of previously run commands
       version: Show version info
       script: Read and execute commands from a file.

Adding commands #

Adding commands is also simple, reflection does the magic:

package com.lukidev.shell;

import org.springframework.shell.standard.ShellComponent;
import org.springframework.shell.standard.ShellMethod;
import org.springframework.shell.standard.ShellOption;

@ShellComponent
public class Commands {

    @ShellMethod("Returns the square of a number")
    public int square(
        @ShellOption(value = "-x", help = "Target") int x
    ) {
        return x * x;
    }

}

When recompiling and testing the square command, it works as expected:

shell:>square 25
625

A help option is also automatically generated:

shell:>square --help
NAME
       square - Returns the square of a number

SYNOPSIS
       square [-x int] --help 

OPTIONS
       -x int
       Target
       [Mandatory]

       --help or -h 
       help for square
       [Optional]

Finishing #

From here, you just need to use your creativity to better take advantage of the functionality. It’s a rather simple and functional solution. Although I find it inadequate for simpler programs, I imagine it could be more useful in conjunction with other existing Spring Boot applications. In any case, it’s a suggestion for those who didn’t know about it.

Source code: https://github.com/1ukidev/shell

References:

• https://docs.spring.io/spring-shell/reference/getting-started.html