一、前言

Springboot源码解析是一件大工程，逐行逐句的去钻研代码，会很干燥，也不容易坚持下去。
咱们不谋求大而全，而是试着每次去钻研一个小知识点，最终聚沙成塔，这就是咱们的springboot源码管中窥豹系列。

二、Initializer

上一节咱们介绍了Runner,它是在我的项目加载实现之后执行的
有后就有前，有没有在我的项目加载之前执行的呢？

咱们明天介绍的ApplicationContextInitializer就是在spring的bean加载之前执行的

public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {    void initialize(C applicationContext);}

应用很简略，实现ApplicationContextInitializer接口就能够了
它是先于一般bean加载的，所以不能用@Component的形式
具体怎么被springboot加载，往下看，咱们剖析源码的时候说

三、源码解析

如果对springboot源码一点都不理解的，举荐先看第一节：整体架构

1、获取ApplicationContextInitializer

咱们间接先看SpringApplication的构造方法

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {        ...    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));        ...}

咱们先看setInitializers办法，再看外面的getSpringFactoriesInstances办法

public void setInitializers(Collection<? extends ApplicationContextInitializer<?>> initializers) {    this.initializers = new ArrayList<>(initializers);}

很简略，把查问的initializers汇合赋值到本地变量上
接着看getSpringFactoriesInstances办法，这个initializers汇合怎么查的

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实现类的名称汇合
(2) 加载成实例instances
(3) 排序，返回

咱们先钻研下SpringFactoriesLoader.loadFactoryNames(type, classLoader)这个办法：

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);    }}

留神：factoryType就是咱们传入的参数 ApplicationContextInitializer.class
这里有个新的map构造：MultiValueMap<String, String>，它和Map<String, List>是一样的
咱们先看一下这里：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
留神，这类文件不止一个，它们散布在不同的jar包外面
这么说你可能不懂，咱们看一下这类文件长什么样，咱们看一个零碎自带的

# PropertySource Loadersorg.springframework.boot.env.PropertySourceLoader=\org.springframework.boot.env.PropertiesPropertySourceLoader,\org.springframework.boot.env.YamlPropertySourceLoader# Run Listenersorg.springframework.boot.SpringApplicationRunListener=\org.springframework.boot.context.event.EventPublishingRunListener# Error Reportersorg.springframework.boot.SpringBootExceptionReporter=\org.springframework.boot.diagnostics.FailureAnalyzers# Application Context Initializersorg.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 Listenersorg.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 Processorsorg.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 Analyzersorg.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# FailureAnalysisReportersorg.springframework.boot.diagnostics.FailureAnalysisReporter=\org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter

接口 = 实现类1，实现类2，实现类3
咱们如果有了本人的ApplicationContextInitializer实现类，咱们在resource上面新建/META-INF/spring.factories文件，按下面的格局写上就能够被加载了

org.springframework.context.ApplicationContextInitializer=\org.my.zb.MyApplicationContextInitializer

咱们把思维拉回去，讲完了怎么取的实现类名称汇合
回去看 createSpringFactoriesInstances();

private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes,        ClassLoader classLoader, Object[] args, Set<String> names) {    List<T> instances = new ArrayList<>(names.size());    for (String name : names) {        try {            Class<?> instanceClass = ClassUtils.forName(name, classLoader);            Assert.isAssignable(type, instanceClass);            Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes);            T instance = (T) BeanUtils.instantiateClass(constructor, args);            instances.add(instance);        }        catch (Throwable ex) {            throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);        }    }    return instances;}

(1) 获取Class
(2) 获取构造函数
(3) 利用反射新建instance对象
(4) 退出汇合

至此，咱们就失去了：List instances

2、执行ApplicationContextInitializer

咱们看SpringApplication的run办法：

public ConfigurableApplicationContext run(String... args) {        ...    try {                ...        prepareContext(context, environment, listeners, applicationArguments, printedBanner);                ...    }    catch (Throwable ex) {        ...    }        ...    return context;}

进入到prepareContext办法：

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,        SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {        ...    applyInitializers(context);        ...}

定位到了applyInitializers（）：

protected void applyInitializers(ConfigurableApplicationContext context) {    for (ApplicationContextInitializer initializer : getInitializers()) {        Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(initializer.getClass(),                ApplicationContextInitializer.class);        Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");        initializer.initialize(context);    }}

前两行判断类型
最初一行回调执行

欢送关注公众号：丰极，更多技术学习分享。