Spring IoC Container

Spring IoC Container with XML-based Application Context

This section explains how Spring initializes the IoC container based on the following setting in web.xml when Tomcat is on.

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
    <!-- Reference: https://my.oschina.net/u/4381731/blog/3673717 -->

    <!-- Other configurations -->

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:spring/applicationContext.xml</param-value>
    </context-param>

    <!-- Other configurations -->
</web-app>

The following sequence diagram shows the business logic of how Spring initializes an xml based ApplicationContext by letting Tomcat invoke the initWebApplicationContext(servletContext:javax.servlet.ServletContext) method contracted in the javax.servlet.ServletContextListener interface, and implemented in org.springframework.web.context.ContextLoaderListener.

The Sequence Diagram from The Starting of The Tomcat Container to The refresh Method of AbstractApplicationContext

As one can see, the core logic of obtaining the bean factory with the specified bean definitions is implemtned in the AbstractApplicationContext#refresh method. The following sequence diagram shows the most important logics of this method.

The Sequence Diagram from The refresh Method of AbstractApplicationContext to The loadBeanDefinitions Method (only for XML-based application context)

Notice that XML-based application context classes have a different implementation of the refreshBeanFactory(beanFactory:BeanFactory) method than the annotation-based application context classes. The former one will invoke the loadBeanDefinitions(beanFactory:DefaultListableBeanFactory) declared in AbstractRefreshableApplicationContext wheras the latter one loads bean definitions in different places (the register and scan method in AnnotationConfigApplicationContext).