以ClassPathXmlApplicationContext为例：

构造方法：

    public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, @Nullable ApplicationContext parent) throws BeansException {
        super(parent);
        this.setConfigLocations(configLocations);
        if (refresh) {
            this.refresh();
        }

    }

refresh()方法

主要有四步：

①启动，记录时间和检验xml文件

②解析XML文件，将bean信息放入beanDefinitionMap中

③设置Bean类加载器和加载特殊的bean

④完成初始化bean，根据bean的作用域来创建对象

public void refresh() throws BeansException, IllegalStateException {
        synchronized(this.startupShutdownMonitor) {
            //①启动，记录时间和检验xml文件
            this.prepareRefresh();
            //②解析XML文件，将bean信息放入beanDefinitionMap中
            ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
            //③设置Bean类加载器和加载特殊的bean
            this.prepareBeanFactory(beanFactory);

            try {
                this.postProcessBeanFactory(beanFactory);
                this.invokeBeanFactoryPostProcessors(beanFactory);
                this.registerBeanPostProcessors(beanFactory);
                this.initMessageSource();
                this.initApplicationEventMulticaster();
                this.onRefresh();
                this.registerListeners();
                //④完成初始化bean，根据bean的作用域来创建对象
                this.finishBeanFactoryInitialization(beanFactory);
                this.finishRefresh();
            } catch (BeansException var9) {
                if (this.logger.isWarnEnabled()) {
                    this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
                }

                this.destroyBeans();
                this.cancelRefresh(var9);
                throw var9;
            } finally {
                this.resetCommonCaches();
            }

        }
    }