Making it easier to replace beans that are auto-configured by SpringBootBatchConfiguration

[commonquail]

This is #1655 again, with the catch that BatchConfigurer (BasicBatchConfigurer?) is not available in Spring Boot 3. Therefore, the Spring Boot autoconfigured Spring Batch JobLauncher's TaskExecutor cannot be directly customized at all. Users can only provide an alternative JobLauncher bean, as demonstrated in that issue, or forgo Spring Boot's autoconfiguration entirely.

The practical impact of this does not appear significant. It is evident from inspecting the autoconfiguration that it is aimed at command line runners, where synchronous execution seems reasonable to me. In a web context synchronous execution is possibly an acceptable default (debatable, but all right) but the inability to switch to asynchronous execution is not acceptable, however, the autoconfiguration does not help a web context much at all so opting out of it is a minor issue.

Although I think the executor should be possible to configure I might well argue that the bigger issue is that the Spring Boot reference documentation could be clearer about where the autoconfiguration is useful and where it is preferable to skip it.

[wilkinsona]

I think we could achieve this by having SpringBootBatchConfiguration override the @Bean methods that are declared in DefaultBatchConfiguration and marking them as @ConditionalOnMissingBean. This would be similar to what we've done for some Spring MVC-related beans:

spring-boot/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration.java

Lines 444 to 470 in 7c4ce13

	@Override
	@Bean
	@ConditionalOnMissingBean(name = DispatcherServlet.LOCALE_RESOLVER_BEAN_NAME)
	public LocaleResolver localeResolver() {
	if (this.webProperties.getLocaleResolver() == WebProperties.LocaleResolver.FIXED) {
	return new FixedLocaleResolver(this.webProperties.getLocale());
	}
	AcceptHeaderLocaleResolver localeResolver = new AcceptHeaderLocaleResolver();
	localeResolver.setDefaultLocale(this.webProperties.getLocale());
	return localeResolver;
	}
	@Override
	@Bean
	@ConditionalOnMissingBean(name = DispatcherServlet.THEME_RESOLVER_BEAN_NAME)
	@Deprecated(since = "3.0.0", forRemoval = false)
	@SuppressWarnings("deprecation")
	public org.springframework.web.servlet.ThemeResolver themeResolver() {
	return super.themeResolver();
	}
	@Override
	@Bean
	@ConditionalOnMissingBean(name = DispatcherServlet.FLASH_MAP_MANAGER_BEAN_NAME)
	public FlashMapManager flashMapManager() {
	return super.flashMapManager();
	}