Springboot启动过程

元注解

元标签有 @Retention、@Documented、@Target、@Inherited、@Repeatable 5 种。

FactoryBean getObject

1. 静态注入

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2. 动态引入

基于ImportSelector接口实现

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3. 基于ImportBeanDefinition

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4. spring.factories

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@conditional 加载到容器的条件

第一个决定要加载的类型, 第二个决定是否加载?

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扩展下. 其实springCloud组件 很多用到@Import @Conditional , 还有些select等, 顺藤摸瓜 我简答列列举几个:

1. SpringBoot 启动注解

@SpringBootApplication-->@EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {}

@Import(AutoConfigurationImportSelector.class)

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

重点说下Selector:

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {}

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最顶层继承了ImportSelector 和 Aware接口:

• ImportSelector---控制哪些类可以被加载到Spring容器进行管理

public interface ImportSelector {

/**
 * Select and return the names of which class(es) should be imported based on
 * the {@link AnnotationMetadata} of the importing @{@link Configuration} class.
 */
String[] selectImports(AnnotationMetadata importingClassMetadata);

}

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;
}

protected Class<?> getSpringFactoriesLoaderFactoryClass() {
return EnableAutoConfiguration.class;
}

其实最终是寻找EnableAutoConfiguration对应的类组合, EnableAutoConfiguration来自package org.springframework.boot.autoconfigure;

所以最终初始化时, 是加载了spring-boot-autoconfigure里面的spring.factories的内容, 简单列举出现:

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说明下:

SpringFactoriesLoader属于Spring框架私有的一种扩

展方案，其主要功能就是从指定的配置文件META-INF/spring.factories加载配置。

public final class SpringFactoriesLoader {

/**
 * The location to look for factories.
 * <p>Can be present in multiple JAR files.
 */
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);

private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>();

}