public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {

/**

void initialize(C applicationContext);

}

- 上下文刷新即refersh方法前调用；

- 用来编码设置一些属性变量，通常用在web环境中；

- 可以通过order接口进行排序；

<img src="https://tva1.sinaimg.cn/large/008i3skNly1gt2l2yqsddj30zg0u077q.jpg" alt="image-20210802174243992" align="left" width="600" />

- 步骤一：在刷新上下文前，调用prepareContext#()准备上下文的方法

public ConfigurableApplicationContext run(String... args) {

StopWatch stopWatch = new StopWatch();

stopWatch.start();

ConfigurableApplicationContext context = null;

Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();

configureHeadlessProperty();

SpringApplicationRunListeners listeners = getRunListeners(args);

listeners.starting();

try {

ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);

ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);

configureIgnoreBeanInfo(environment);

Banner printedBanner = printBanner(environment);

context = createApplicationContext();

exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,

new Class[] { ConfigurableApplicationContext.class }, context);

// 准备上下文

prepareContext(context, environment, listeners, applicationArguments, printedBanner);

// 刷新上下文

refreshContext(context);

afterRefresh(context, applicationArguments);

stopWatch.stop();

if (this.logStartupInfo) {

new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);

}

listeners.started(context);

callRunners(context, applicationArguments);

}

catch (Throwable ex) {

handleRunFailure(context, ex, exceptionReporters, listeners);

throw new IllegalStateException(ex);

}

try {

listeners.running(context);

}

catch (Throwable ex) {

handleRunFailure(context, ex, exceptionReporters, null);

throw new IllegalStateException(ex);

}

return context;

}

- 步骤二：调用初始化器 applyInitializers(context) 方法

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,

SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {

context.setEnvironment(environment);

postProcessApplicationContext(context);

// 调用初始化器方法

applyInitializers(context);

listeners.contextPrepared(context);

if (this.logStartupInfo) {

logStartupInfo(context.getParent() == null);

logStartupProfileInfo(context);

}

// Add boot specific singleton beans

ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();

beanFactory.registerSingleton("springApplicationArguments", applicationArguments);

if (printedBanner != null) {

beanFactory.registerSingleton("springBootBanner", printedBanner);

}

if (beanFactory instanceof DefaultListableBeanFactory) {

((DefaultListableBeanFactory) beanFactory)

.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);

}

// Load the sources

Set<Object> sources = getAllSources();

Assert.notEmpty(sources, "Sources must not be empty");

load(context, sources.toArray(new Object[0]));

listeners.contextLoaded(context);

}

- 步骤三：系统初始化器的具体实现，调用 ApplicationContextInitializer.initialize() 方法

@SuppressWarnings({ "rawtypes", "unchecked" })

protected void applyInitializers(ConfigurableApplicationContext context) {

// getInitializers() 方法，从this.initializers 成员中，获取初始化器的实现

for (ApplicationContextInitializer initializer : getInitializers()) {

// 判断初始化器的实现类，是否实现了ApplicationContextInitializer接口类

Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(initializer.getClass(),

ApplicationContextInitializer.class);

Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");

// 调用初始化器实现类中的 initialize#() 方法，进行初始化

initializer.initialize(context);

}

}

- 第一种：定义在spring.factories文件中，通过SpringFactoriesLoader发现与注册

```java

SpringBoot框架中从类路径jar包中读取特定的文件（META-INF/spring.factories）实现扩展类的载入

最终调用SpringApplication.addInitializers#()方法添加到this.initializers成员中

```

- 第二种：通过SpringApplication.addInitializers#() 方法手动添加到系统初始化器中

```java

public static void main(String[] args) {

// SpringApplication.run(SpringbootSourceCodeAnalysisApplication.class, args);

SpringApplication springApplication = new SpringApplication(SpringbootSourceCodeAnalysisApplication.class);

// 手动添加一个框架的初始化器

springApplication.addInitializers(new SecondInitializer());

springApplication.run();

}

```

- 第三种：在application.properties 配置文件中定义环境变量，通过DelegatingApplicationContextInitializer 发现与注册

- 步骤一：DelegatingApplicationContextInitializer 在Spring框架中，会被SpringFactoriesLoader 自动加载，且优先级最高 order = 0

```java

# 在spring.factories配置文件中

# Application Context Initializers

org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\

```

- 步骤二：通过DelegatingApplicationContextInitializer 中的 initialize#() 方法，获取ApplicationContextInitializer.class的实现

```java

private static final String PROPERTY_NAME = "context.initializer.classes";

private int order = 0;

@Override

public void initialize(ConfigurableApplicationContext context) {

// 获取application.properties配置中的 context.initializer.classes 属性

ConfigurableEnvironment environment = context.getEnvironment();

// 获取所有 ApplicationContextInitializer.class 接口的实现类

List<Class<?>> initializerClasses = getInitializerClasses(environment);

if (!initializerClasses.isEmpty()) {

// 初始化系统初始化器

applyInitializerClasses(context, initializerClasses);

}

}

```

#### 4、ApplicationContextInitializer.class 的调用流程总结

<img src="https://tva1.sinaimg.cn/large/008i3skNly1gt2mlwhgguj315m0u077s.jpg" alt="image-20210802183526156" align="left" width="500" />