SpringBoot启动揭秘

关于@SpringBootApplication

我们知道启动SpringBoot项目需要在启动类上面添加 @SpringBootApplication 注解, 为什么该注解添加后就能启动SpringBoot项目, 我们今天就来揭开它的面纱

点开SpringBootApplication的注解我们可以看到下面的三个注解 @SpringBootConfiguration @EnableAutoConfiguration @ComponentScan

@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 {

@SpringBootConfiguration

点开该注解可以看到代码, 该类就是springboot的配置类注解, 添加该注解表示注解的类是一个springboot的启动配置类

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

@ComponentScan

该类是组件扫描类, 指定springboot项目扫描的包的路径主要是哪些

@EnableAutoConfiguration

最要的就是这个类, 这个类加载了springboot项目的启动组件,

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

@AutoConfigurationPackage

该注解主要是配置springboot的默认的包扫描, 向springboot容器中注入当前启动类的包路径

@Import(AutoConfigurationImportSelector.class)

springboot项目能启动主要是 AutoConfigurationImportSelector 这个组件的加载的一些自动配置组件

/**
     * Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}
     * of the importing {@link Configuration @Configuration} class.
     * @param autoConfigurationMetadata the auto-configuration metadata
     * @param annotationMetadata the annotation metadata of the configuration class
     * @return the auto-configurations that should be imported
     */
    protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
            AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        // 该类加载了一些全限定名称类的候选自动配置组件, 之所以是候选就是组件全部加载, 但是不确定你是否使用
        // 这些组件配置是在spring-boot-autoconfigure下的MEAT/spring.factories
        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);
    }

我们看一下spring.factories的其中一部分的配置信息

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

加载后的组件通过AutoConfigurationImportSelector类中的下面的方法加载组件进容器 SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader()) 加载完后整个springboot就能跑起来了

/**
     * Return the auto-configuration class names that should be considered. By default
     * this method will load candidates using {@link SpringFactoriesLoader} with
     * {@link #getSpringFactoriesLoaderFactoryClass()}.
     * @param metadata the source metadata
     * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
     * attributes}
     * @return a list of candidate configurations
     */
    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;
    }