SpringBoot自動裝配

前言
@SpringBootConfiguration
@EnableAutoConfiguration（重要）
- AutoConfigurationImportSelector.class
- - getAutoConfigurationEntry()
  - - getCandidateConfigurations()
- META-INF/spring.factories
- - EmbeddedWebServerFactoryCustomizerAutoConfiguration
  - WebMvcAutoConfiguration
  - ServletWebServerFactoryAutoConfiguration
  - ThymeleafAutoConfiguration
  - TransactionAutoConfiguration

前言

本文基于SpringBoot2.1.xx版本

提示：以下是本篇文章正文内容，可供參考

@SpringBootConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {

}

@Target(ElementType.TYPE)：這是一個接口或者是類或者是枚舉

@Retention(RetentionPolicy.RUNTIME) ：會在class位元組碼檔案中存在，在運作時可以通過反射擷取到

@Document：該注解将被包含在javadoc中

@Configuration：表示加載IOC容器使這個類成為了一個IOC容器配置類

可以再點進去看一下:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
	@AliasFor(annotation = Component.class)
	String value() default "";
}

@Component：納入Spring容器中

@EnableAutoConfiguration（重要）

這個注解的意思就是開啟自動配置的功能。我們點進來看

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

	/**
	 * Exclude specific auto-configuration classes such that they will never be applied.
	 * @return the classes to exclude
	 */
	Class<?>[] exclude() default {};

	/**
	 * Exclude specific auto-configuration class names such that they will never be
	 * applied.
	 * @return the class names to exclude
	 * @since 1.3.0
	 */
	String[] excludeName() default {};

}

@Inherited：子類可以繼承父類中的該注解

@AutoConfigurationPackage：将主配置類所在的包作為自動配置包進行管理

@Import：自動裝配導入的選擇器

AutoConfigurationImportSelector.class

這裡點開AutoConfigurationImportSelector：

@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
				.loadMetadata(this.beanClassLoader);
		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
				annotationMetadata);
		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}

我們主要關注selectImports()這個方法，這個方法就是我們選擇導入的類。

//擷取自動裝配的類
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,annotationMetadata);

getAutoConfigurationEntry()

點開getAutoConfigurationEntry()：

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
			AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return EMPTY_ENTRY;
		}
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
		List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
		configurations = removeDuplicates(configurations);
		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
		configurations = filter(configurations, autoConfigurationMetadata);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		return new AutoConfigurationEntry(configurations, exclusions);
}

List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);

我們主要就是關注這行代碼，這行代碼就是擷取到我們的需要自動裝配的類然後以List< String >的形式進行一個存儲

getCandidateConfigurations()

點開getCandidateConfigurations()：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
				getBeanClassLoader());
		Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
				+ "are using a custom packaging, make sure that file is correct.");
		return configurations;
}

Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
				+ "are using a custom packaging, make sure that file is correct.");

Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
				+ "are using a custom packaging, make sure that file is correct.");

這個斷言的意思是如果發生了某些錯誤比如為空的話，就會輸出這個異常資訊，意思就是在META-INF/spring.factories下找不到這些自動裝配的類(factories也就是工廠)。spring.factories裡面都是一些自動裝配的類，然後這些類都是一個以String形式存儲的。