spring初始化随笔

spring的初始化入口，是一个名叫ContextLoaderListener的监听器类，实现了ServletContextListener接口，继承ContextLoader类

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    public ContextLoaderListener() {
    }
     ...

    public void contextInitialized(ServletContextEvent event) {
        this.initWebApplicationContext(event.getServletContext());
    }
    ....
}

其中，初始化的方法就是contextInitialize，此方法实现自ServletContextListener。ServletContextListener中有两个接口，一个用于初始化容器使用，一个用于容器关闭时使用：

public interface ServletContextListener extends EventListener {
    default void contextInitialized(ServletContextEvent sce) {
    }

    default void contextDestroyed(ServletContextEvent sce) {
    }
}

接着看具体的实现方法initWebApplicationContext方法，此处调用的是父类ContextLoader中的方法：

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
    // 判断容器ServletContext中如果已经创建了WebApplicationContext对象，如果有，就抛出异常
    if(servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
        throw new IllegalStateException("Cannot initialize context because there is already a root application context present - check whether you have multiple ContextLoader* definitions in your web.xml!");
    } else {
       ....

        try {

            if(this.context == null) {
                // 创建WebApplicationContext对象，即初始化
                this.context = this.createWebApplicationContext(servletContext);
            }

            if(this.context instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext)this.context;
                if(!cwac.isActive()) {
                    if(cwac.getParent() == null) {
                        ApplicationContext parent = this.loadParentContext(servletContext);
                        cwac.setParent(parent);
                    }

                    this.configureAndRefreshWebApplicationContext(cwac, servletContext);
                }
            }

           // 将WebApplicationContext以ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE名字存储到servletContext中
           // String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT"; 

           servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
           ...

            return this.context;
        } catch (Error | RuntimeException var8) {
            logger.error("Context initialization failed", var8);
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, var8);
            throw var8;
        }
    }
}

可以在跟踪创建webApplicationContext的方法，如下

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
    Class contextClass = this.determineContextClass(sc);
    if(!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException("Custom context class [" + contextClass.getName() + "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
    } else {
        return (ConfigurableWebApplicationContext)BeanUtils.instantiateClass(contextClass);
    }
}

protected Class determineContextClass(ServletContext servletContext) {
    String contextClassName = servletContext.getInitParameter("contextClass");
    if(contextClassName != null) {
       ....
    } else {
        contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());

       ...
        }
    }
}

defaultStrategies就是读取文件的流对象

static {
    try {
        ClassPathResource resource = new ClassPathResource("ContextLoader.properties", ContextLoader.class);
        defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
    } catch (IOException var1) {
        throw new IllegalStateException("Could not load 'ContextLoader.properties': " + var1.getMessage());
    }

    currentContextPerThread = new ConcurrentHashMap(1);
}

其中，ContextLoader.properties如下内容org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

通过子类XmlWebApplicationContext进行反射初始化WebApplicationContext