Spring Boot启动-构造ApplicationContext

创建ApplicationContext实现代码如下：

1. 根据Web应用类型，获取对应的ApplicationContext子类
2. 实例化ApplicationContext子类

// 常量定义
public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework.boot."
            + "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";
public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework."
            + "boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";
public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
            + "annotation.AnnotationConfigApplicationContext";

    protected ConfigurableApplicationContext createApplicationContext() {
        // 1. 根据Web应用类型，获取对应的ApplicationContext子类
        Class<?> contextClass = this.applicationContextClass;
        if (contextClass == null) {
            try {
                switch (this.webApplicationType) {
                case SERVLET:
                    contextClass = Class.forName(DEFAULT_WEB_CONTEXT_CLASS);
                    break;
                case REACTIVE:
                    contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
                    break;
                default:
                    contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
                }
            }
            catch (ClassNotFoundException ex) {
                throw new IllegalStateException(
                        "Unable create a default ApplicationContext, "
                                + "please specify an ApplicationContextClass",
                        ex);
            }
        }
        // 2. 实例化子类
        return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
    }

BeanFactory也是在此过程中被创建

    public GenericApplicationContext() {
        this.beanFactory = new DefaultListableBeanFactory();
    }