BeanFactory创建过程（基于Servlet）

1. 概述
- 1.2 那么问题就来了
- - 1.2.1 谁负责AnnotationConfigServletWebServerApplicationContext实例呢？
  - 1.2.2 ApplicationContextFactory初始化过程又是怎么样的？
- 1.3 总结
2. 最后

1. 概述

AnnotationConfigServletWebServerApplicationContext的类图。
在这里插入图片描述

在GenericApplicationContext类有一个构造函数，直接实例化了DefaultListableBeanFactory，它就是BeanFactory。

public class GenericApplicationContext extends AbstractApplicationContext implements BeanDefinitionRegistry {/*** Create a new GenericApplicationContext.* @see #registerBeanDefinition* @see #refresh*/public GenericApplicationContext() {this.beanFactory = new DefaultListableBeanFactory();}
}

从上面的类图可以看到AnnotationConfigServletWebServerApplicationContext间接的继承了GenericApplicationContext，因为类的继承规则，所以只要创建AnnotationConfigServletWebServerApplicationContext对象，GenericApplicationContext也会被实例化，自然的DefaultListableBeanFactory也会被实例化。

1.2 那么问题就来了

1.2.1 谁负责AnnotationConfigServletWebServerApplicationContext实例呢？

在这里插入图片描述

有一个ApplicationContextFactory工厂类，从上图可以发现它有3个实现类，我们主要看基于注解的实现类AnnotationConfigServletWebServerApplicationContext。可以从下面的实现代码发现，create调用了AnnotationConfigReactiveWebServerApplicationContext无参的构造函数。这个时候问题又来了，ApplicationContextFactory又是怎么初始化的？

static class Factory implements ApplicationContextFactory {@Overridepublic Class<? extends ConfigurableEnvironment> getEnvironmentType(WebApplicationType webApplicationType) {return (webApplicationType != WebApplicationType.REACTIVE) ? null : ApplicationReactiveWebEnvironment.class;}@Overridepublic ConfigurableEnvironment createEnvironment(WebApplicationType webApplicationType) {return (webApplicationType != WebApplicationType.REACTIVE) ? null : new ApplicationReactiveWebEnvironment();}@Overridepublic ConfigurableApplicationContext create(WebApplicationType webApplicationType) {return (webApplicationType != WebApplicationType.REACTIVE) ? null: new AnnotationConfigReactiveWebServerApplicationContext();}}

1.2.2 ApplicationContextFactory初始化过程又是怎么样的？

在这里插入图片描述

在spring-boot-2.7.16的jar包中可以发现spring.factories中有下面两行代码，可以使用SpringFactoriesLoader.loadFactories(ApplicationContextFactory.class, getClass().getClassLoader()) 去获取到下面两个对象。

# Application Context Factories
org.springframework.boot.ApplicationContextFactory=\
org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext.Factory,\
org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext.Factory

现在清楚是什么情况了，那就把整个创建的过程走一下。

SpringApplication#run(String…args)
下面创建的BeanFactory其实就是ConfigurableApplicationContext，其实在ApplicationContextFactory中创建的是AnnotationConfigServletWebServerApplicationContext，从概述的类图中可以看到AnnotationConfigServletWebServerApplicationContext间接的继承了ConfigurableApplicationContext。

public ConfigurableApplicationContext run(String... args) {long startTime = System.nanoTime();DefaultBootstrapContext bootstrapContext = createBootstrapContext();ConfigurableApplicationContext context = null;configureHeadlessProperty();SpringApplicationRunListeners listeners = getRunListeners(args);listeners.starting(bootstrapContext, this.mainApplicationClass);try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);configureIgnoreBeanInfo(environment);Banner printedBanner = printBanner(environment);// 这里就是创建BeanFactory的方法context = createApplicationContext();context.setApplicationStartup(this.applicationStartup);prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);refreshContext(context);afterRefresh(context, applicationArguments);Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);if (this.logStartupInfo) {new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);}listeners.started(context, timeTakenToStartup);callRunners(context, applicationArguments);}catch (Throwable ex) {handleRunFailure(context, ex, listeners);throw new IllegalStateException(ex);}try {Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime);listeners.ready(context, timeTakenToReady);}catch (Throwable ex) {handleRunFailure(context, ex, null);throw new IllegalStateException(ex);}return context;
}

SpringApplication#createApplicationContext()
DefaultApplicationContextFactory#create() & getFromSpringFactories()
SpringApplication#createApplicationContext()方法最终调用了DefaultApplicationContextFactory类的create方法，又调用getFromSpringFactories方法，这个方法就是从spring.factory文件找ApplicationContextFactory的实现类。

public ConfigurableApplicationContext create(WebApplicationType webApplicationType) {try {return getFromSpringFactories(webApplicationType, ApplicationContextFactory::create,AnnotationConfigApplicationContext::new);}catch (Exception ex) {throw new IllegalStateException("Unable create a default ApplicationContext instance, "+ "you may need a custom ApplicationContextFactory", ex);}
}
// 从spring.factory中获取ApplicationContextFactory的实现类
private <T> T getFromSpringFactories(WebApplicationType webApplicationType,BiFunction<ApplicationContextFactory, WebApplicationType, T> action, Supplier<T> defaultResult) {for (ApplicationContextFactory candidate : SpringFactoriesLoader.loadFactories(ApplicationContextFactory.class,getClass().getClassLoader())) {// Java8 的写法这里的action就是ApplicationContextFactory的create方法// 可以理解为ApplicationContextFactory::create// 也可以理解为ApplicationContextFactory.create()T result = action.apply(candidate, webApplicationType);if (result != null) {return result;}}return (defaultResult != null) ? defaultResult.get() : null;
}

1.3 总结

这里来一个小总结，方便记忆。下面是整个过程：

SpringApplication#run -> SpringApplication#createAppliction -> DefaultApplicationContextFactory#create -> DefaultApplicationContextFactory#getFromSpringFactory -> SpringFactoriesLoader#loadFactories

可以知道实例化BeanFactory也就是AnnotationConfigServletWebServerApplicationContext是DefaultApplicationContextFactory的实现类去做的，而实例化DefaultApplicationContextFactory是抛给了spring.factory。