Make it easier to use Spring Shell without Spring Boot

[fmbenhassine]

As of v3.4.x, there is no documentation on how to use Spring Shell without Spring Boot. Moreover, the configuration of shell runners is quite laborious:

import org.jline.reader.LineReader;

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.shell.core.Shell;
import org.springframework.shell.core.command.annotation.Command;
import org.springframework.shell.core.command.annotation.EnableCommand;
import org.springframework.shell.core.command.annotation.Option;
import org.springframework.shell.core.context.ShellContext;
import org.springframework.shell.core.jline.InteractiveShellRunner;
import org.springframework.shell.core.jline.PromptProvider;

@EnableCommand(SpringShellApplication.class)
public class SpringShellApplication {

	public static void main(String[] args) throws Exception {
		ApplicationContext context = new AnnotationConfigApplicationContext(SpringShellApplication.class);
		InteractiveShellRunner shellRunner = context.getBean(InteractiveShellRunner.class);
		shellRunner.run(args);
	}

	@Command
	public String hello(@Option(defaultValue = "World") String name) {
		return "Hello " + name + "!";
	}

	// TODO define collaborators as beans to be injected in InteractiveShellRunner: not so easy..

	@Bean
	public InteractiveShellRunner interactiveApplicationRunner(LineReader lineReader, PromptProvider promptProvider,
															   Shell shell, ShellContext shellContext) {
		return new InteractiveShellRunner(lineReader, promptProvider, shell, shellContext);
	}

}

What would be even better, is that the user is not required to do that, even without Spring Boot:

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.shell.core.command.annotation.Command;
import org.springframework.shell.core.command.annotation.EnableCommand;
import org.springframework.shell.core.command.annotation.Option;
import org.springframework.shell.core.jline.InteractiveShellRunner;

@EnableCommand(SpringShellApplication.class)
public class SpringShellApplication {

	public static void main(String[] args) throws Exception {
		ApplicationContext context = new AnnotationConfigApplicationContext(SpringShellApplication.class);
        // choose which shell runner to use (interactive, non interactive or script) and launch it
        // runners (and their collaborators) should be pre-configured by EnableCommand and ready to be used here
		InteractiveShellRunner shellRunner = context.getBean(InteractiveShellRunner.class);
		shellRunner.run(args);
	}

	@Command
	public String hello(@Option(defaultValue = "World") String name) {
		return "Hello " + name + "!";
	}

}

In this case (similar to other projects like Batch, Integration, etc), EnableCommand would import a ImportBeanDefinitionRegistrar that defines shell runners and their collaborators as beans in the context ready to be used (and possibly customized) by Spring Shell users.