Springboot源码解析是一件大工程，逐行逐句的去研究代码，会很枯燥，也不容易坚持下去。
我们不追求大而全，而是试着每次去研究一个小知识点，最终聚沙成塔，这就是我们的springboot源码管中窥豹系列。
上一节我们介绍了Runner,它是在项目加载完成之后执行的
有后就有前，有没有在项目加载之前执行的呢？
我们今天介绍的ApplicationContextInitializer就是在spring的bean加载之前执行的
public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> { void initialize(C applicationContext); }使用很简单，实现ApplicationContextInitializer接口就可以了
它是先于普通bean加载的，所以不能用@Component的方式
具体怎么被springboot加载，往下看，我们分析源码的时候说
三、源码解析如果对springboot源码一点都不了解的，推荐先看第一节：整体架构
1、获取ApplicationContextInitializer我们直接先看SpringApplication的构造方法
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) { ... setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class)); ... }我们先看setInitializers方法，再看里面的getSpringFactoriesInstances方法
public void setInitializers(Collection<? extends ApplicationContextInitializer<?>> initializers) { this.initializers = new ArrayList<>(initializers); }很简单，把查询的initializers集合赋值到本地变量上
接着看getSpringFactoriesInstances方法，这个initializers集合怎么查的
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) { return getSpringFactoriesInstances(type, new Class<?>[] {}); } private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) { ClassLoader classLoader = getClassLoader(); // Use names and ensure unique to protect against duplicates Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader)); List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names); AnnotationAwareOrderComparator.sort(instances); return instances; }(1) 获取ApplicationContextInitializer实现类的名称集合
(2) 加载成实例instances
(3) 排序，返回
我们先研究下SpringFactoriesLoader.loadFactoryNames(type, classLoader)这个方法：
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) { String factoryTypeName = factoryType.getName(); return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList()); } private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) { MultiValueMap<String, String> result = cache.get(classLoader); if (result != null) { return result; } try { Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) : ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION)); result = new LinkedMultiValueMap<>(); while (urls.hasMoreElements()) { URL url = urls.nextElement(); UrlResource resource = new UrlResource(url); Properties properties = PropertiesLoaderUtils.loadProperties(resource); for (Map.Entry<?, ?> entry : properties.entrySet()) { String factoryTypeName = ((String) entry.getKey()).trim(); for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) { result.add(factoryTypeName, factoryImplementationName.trim()); } } } cache.put(classLoader, result); return result; } catch (IOException ex) { throw new IllegalArgumentException("Unable to load factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex); } }注意：factoryType就是我们传入的参数 ApplicationContextInitializer.class
这里有个新的map结构：MultiValueMap<String, String>，它和Map<String, List>是一样的
我们先看一下这里：classLoader.getResources(FACTORIES_RESOURCE_LOCATION)
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";加载所有的META-INF/spring.factories，按接口名称放入MultiValueMap<String, String>，并cache
注意，这类文件不止一个，它们分布在不同的jar包里面
这么说你可能不懂，我们看一下这类文件长什么样，我们看一个系统自带的
# PropertySource Loaders org.springframework.boot.env.PropertySourceLoader=\ org.springframework.boot.env.PropertiesPropertySourceLoader,\ org.springframework.boot.env.YamlPropertySourceLoader # Run Listeners org.springframework.boot.SpringApplicationRunListener=\ org.springframework.boot.context.event.EventPublishingRunListener # Error Reporters org.springframework.boot.SpringBootExceptionReporter=\ org.springframework.boot.diagnostics.FailureAnalyzers # Application Context Initializers org.springframework.context.ApplicationContextInitializer=\ org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\ org.springframework.boot.context.ContextIdApplicationContextInitializer,\ org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\ org.springframework.boot.rsocket.context.RSocketPortInfoApplicationContextInitializer,\ org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer # Application Listeners org.springframework.context.ApplicationListener=\ org.springframework.boot.ClearCachesApplicationListener,\ org.springframework.boot.builder.ParentContextCloserApplicationListener,\ org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\ org.springframework.boot.context.FileEncodingApplicationListener,\ org.springframework.boot.context.config.AnsiOutputApplicationListener,\ org.springframework.boot.context.config.ConfigFileApplicationListener,\ org.springframework.boot.context.config.DelegatingApplicationListener,\ org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\ org.springframework.boot.context.logging.LoggingApplicationListener,\ org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener # Environment Post Processors org.springframework.boot.env.EnvironmentPostProcessor=\ org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\ org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\ org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor,\ org.springframework.boot.reactor.DebugAgentEnvironmentPostProcessor # Failure Analyzers org.springframework.boot.diagnostics.FailureAnalyzer=\ org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.BeanDefinitionOverrideFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.BindValidationFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.UnboundConfigurationPropertyFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.NoSuchMethodFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyNameFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyValueFailureAnalyzer # FailureAnalysisReporters org.springframework.boot.diagnostics.FailureAnalysisReporter=\ org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter接口 = 实现类1，实现类2，实现类3