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This article introduces the knowledge of "how to use the @ Component annotations of Spring". Many people will encounter this dilemma in the operation of actual cases, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

1. Preface

Implement a @ Controller-like annotation (or inherit a unified interface) to dynamically inject some common Bean, such as unified injection of timing tasks or unified injection of Websocket processors.

/ imitate Controller @ XBean (description = "ETL JOB") public class JobShedule {@ Caller (cron = "* * 0Tax 5 * *?") Public void exec () {/ / job}}

The pseudocode above is a scheduled task Bean that mimics Controller.

two。 Design ideas

I have summed up the detailed development and design ideas, as long as the students can achieve this function step by step.

2.1 define scan comments

Define a custom ImportBeanDefinitionRegistrar similar to @ MappScan for import, and specify the scope of the scan package.

Documented@Inherited@Retention (RetentionPolicy.RUNTIME) @ Target ({ElementType.TYPE}) @ Import (XBeanDefinitionRegistrar.class) public @ interface XBeanScan {String [] basePackages ();}

We customized a scan annotation @ XBeanScan. It has two functions:

Specify the scope of the scan packet through basePackages.

Import our custom ImportBeanDefinitionRegistrar implementation XBeanDefinitionRegistrar.

2.2 Common tags that define the target Bean

Usually we can choose an identification interface, all of its implementation classes will be injected into Spring IoC; or with more convenient annotations, and all classes marked with that annotation will be injected into Spring IoC. Here we implement an @ XBean tag annotation using a more flexible and convenient annotation:

@ Documented@Inherited@Retention (RetentionPolicy.RUNTIME) @ Target ({ElementType.TYPE}) public @ interface XBean {String description () default "";} 2.3 implement the scanner

The Spring framework provides us with a scanner to register the tagged Bean, which is the ClassPathBeanDefinitionScanner mentioned in the previous section, which we inherit and modify slightly:

Public class XBeanDefinitionScanner extends ClassPathBeanDefinitionScanner {public XBeanDefinitionScanner (BeanDefinitionRegistry registry, boolean useDefaultFilters) {super (registry, useDefaultFilters); super.addIncludeFilter (new AnnotationTypeFilter (XBean.class));}}

Instead of using the default filter here, we specify that the target of the scanner scan is the Bean marked with @ XBean.

2.4Implementation of Bean register machine

The big deal is that we need to assemble the components defined in 2. 1 to 2. 3 in the implementation of ImportBeanDefinitionRegistrar.

/ * The type X bean definition registrar. * * @ author felord.cn * @ since 2020 / 9 ax 18 22:59 * / public class XBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {private ResourceLoader resourceLoader; @ Override public void registerBeanDefinitions (AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {/ / do not use the default filter XBeanDefinitionScanner xBeanDefinitionScanner = new XBeanDefinitionScanner (registry, false); xBeanDefinitionScanner.setResourceLoader (resourceLoader); / / scan the package xBeanDefinitionScanner.scan specified in the XBeanScan note (getBasePackagesToScan (importingClassMetadata)) } @ Override public void setResourceLoader (ResourceLoader resourceLoader) {this.resourceLoader = resourceLoader;} / * * get the scan packet path declared in {@ link XBeanScan} * @ param metadata the meta * @ return package path array * / private String [] getBasePackagesToScan (AnnotationMetadata metadata) {String name = XBeanScan.class.getName (); AnnotationAttributes attributes = AnnotationAttributes.fromMap (metadata.getAnnotationAttributes (name, true)) Assert.notNull (attributes, ()-> "No auto-configuration attributes found. Is "+ metadata.getClassName () +" annotated with "+ ClassUtils.getShortName (name) +"?); return attributes.getStringArray ("basePackages");}}

Parse the scan path basePackages and other metadata we need from the annotated metadata importingClassMetadata, and then let the scanner scan in that path.

2.5 use

Isn't it easy to use @ XBeanScan on a class with a @ Configuration tag or a Main class of Spring Boot?

In fact, @ ComponentScan provides similar functionality.

This is the end of "how to use Spring's @ Component annotations". Thank you for reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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