Auto-configure a bootstrapExecutor bean to be used by Framework's background bean initialization

[ballista01]

Summary:
This pull request introduces the ability to auto-configure a bootstrapExecutor bean for background bean initialization in the Spring Framework. It provides an @BootstrapExecutor annotation to configure the properties of the ThreadPoolTaskExecutor used for background initialization.

Related Issues:
spring-projects/spring-boot#39791
#13410 (comment)

Changes:

• Added @BootstrapExecutor annotation in org.springframework.context.annotation package to configure the bootstrap executor.
• Modified ConfigurationClassPostProcessor to detect the presence of @BootstrapExecutor annotation on configuration classes and register a ThreadPoolTaskExecutor bean with the configured properties.
• Added BootstrapExecutorBeanDefinitionParser to parse the <context:bootstrap-executor> XML element and register the ThreadPoolTaskExecutor bean. (Note: This is currently not functional and the logic is temporarily handled in ConfigurationClassPostProcessor.)
• Updated ContextNamespaceHandler to register the BootstrapExecutorBeanDefinitionParser.
• Added test cases in ConfigurationClassPostProcessorTests to verify the parallel bean initialization using the @BootstrapExecutor annotation.

Usage:
To use the bootstrapExecutor for background bean initialization:

1. Add the @BootstrapExecutor annotation to one of your @Configuration classes.
2. Configure the desired properties of the ThreadPoolTaskExecutor using the annotation attributes.
3. Use @Bean(bootstrap = BACKGROUND) on the beans that should be initialized in the background.

Example:

@BootstrapExecutor(threadNamePrefix = "bootstrap-", corePoolSize = 2)
@Configuration
public class AppConfig {
    @Bean(bootstrap = BACKGROUND)
    public SlowInitBean slowBean() {
        return new SlowInitBean();
    }
}

Known Issues:

• The BootstrapExecutorBeanDefinitionParser is currently not functional due to a parsing issue. The logic is temporarily handled in the processConfigBeanDefinitions method of ConfigurationClassPostProcessor.

Next Steps:

• Investigate and resolve the issue with BootstrapExecutorBeanDefinitionParser to properly parse the <context:bootstrap-executor> XML element.
• Consider automatically configuring a bootstrapExecutor based on the number of @Bean(bootstrap=BACKGROUND) annotations if @BootstrapExecutor is not found.

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[ballista01]

@wilkinsona Thanks for notifying me the Springboot upgrade progress in spring-projects/spring-boot#39791 (comment). I found that I could try to solve the issue spring-projects/spring-boot#39791 in Spring Framework first. The @BootstrapExecutor() annotation is working, but we could add other improvements. Do you think that I'm on the right track?

[snicoll]

The change you're referring to must happen on the Spring Boot side of things. Andy was telling you we can't really work on the feature until Spring Boot switches to Spring Framework 6.2. He also gave you the timeline.