springIOC原理加载过程

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关于spring ,我以前学过很多次,也看过很多的书.以及博客,但是总是不得要领,这次我再学习一遍,在这里做点记录,如果不对的地方还请大家指正

Ioc: inverse of controller 控制反转 . 对象的创建权利由程序反转给spring

什么是IOC容器呢?

所谓的IOC容器是指的spring bean 工厂里面MAP存储结构(存储了bean的实例)

spring框架中的工厂有哪些?

beanFactory 是spring早期创建bean对象工厂接口

applicationContext()接口实现了beanFactory接口

实现applicationContext接口的工厂,可以获取到容器中具体bean对象

实现了beanFactory接口的工厂也可以获取到bean对象

l ApplicationContext和BeanFactory的区别？

beanFactory采取的延迟加载,第一次getBean时才会初始化Bean

applicationContext是加载完applicationContext.xml 就创建了具体的bean实例(只对BeanDefition中描述是单例的bean,才进行恶汉模式创建)

这里我们看到顶层都是继承与beanfactory 我们常用的就是applicationContext

applicationContext接口常用实现类

classpathXmlApplicationContext : 它是从类的跟路劲下加载配置文件,推荐使用这种
FileSystemXmlApplicationContext: 它是从磁盘上加载配置文件,配置文件可以在磁盘的任意位置
AnnotationConfigApplicationContext : 当我们使用注解配置容器对象时，需要使用此类来创建 spring 容器。它用来读取注解。

IOC容器的创建方式

java创建IOC容器

ApplicationContext context = new ClassPathXmlApplicationContext(xml路径);

web 创建IOC 容器:(重点要知道)

web服务器（tomcat）启动会加载web.xml（启动ContextLoaderListener监听器）:
public class ContextLoaderListener extends ContextLoader implements ServletContextListener { public ContextLoaderListener(WebApplicationContext context) {
super(context);
} /**
serveltContext域对象创建的时候调用这个方法
* Initialize the root web application context.
*/
@Override
public void contextInitialized(ServletContextEvent event) {
//初始化web环境中的spring容器
initWebApplicationContext(event.getServletContext());
} /**
* Close the root web application context.
*/
@Override
public void contextDestroyed(ServletContextEvent event) {
closeWebApplicationContext(event.getServletContext());
ContextCleanupListener.cleanupAttributes(event.getServletContext());
}}

当我们点击initWebApplicationContext方法中的时候会发现以下源码

这个方法在contextLoader之中也就是Contextloader创建spring容器并初始化spring bean实例

try {
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
if (this.context == null) {
//这一步就是创建spring容器
this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent ->
// determine parent for root web application context, if any.
ApplicationContext parent = loadParentContext(servletContext);
cwac.setParent(parent);
}
//spring 容器初始化单例bean的实例
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}

configureAndRefreshWebApplicationContext 方法中调用最终初始化Bean的refresh方法

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
// The application context id is still set to its original default value
// -> assign a more useful id based on available information
String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
if (idParam != null) {
wac.setId(idParam);
}
else {
// Generate default id...
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(sc.getContextPath()));
}
} wac.setServletContext(sc);
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
} // The wac environment's #initPropertySources will be called in any case when the context
// is refreshed; do it eagerly here to ensure servlet property sources are in place for
// use in any post-processing or initialization that occurs below prior to #refresh
ConfigurableEnvironment env = wac.getEnvironment();
if (env instanceof ConfigurableWebEnvironment) {
((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
} customizeContext(sc, wac);
//不管是web还是java环境都是会调用这个方法实例化bean
wac.refresh();
}

如果web.xml中没有contextClass的元素,那么spring会找到自己的contextLoad.properties文件来加载

# Default WebApplicationContext implementation class for ContextLoader.
# Used as fallback when no explicit context implementation has been specified as context-param.
# Not meant to be customized by application developers.org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

IOC容器如何加载bean对象呢?

跟着上面的refresh()方法我们来到他的源码看看

源码来自AbstractApplicationContext

@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
//1. 创建真正的bean容器 ConfiurabaleBeanFactroy

//2.加载beandefiniton（描述要初始化的Bean的信息）

//3.将beandefiniton注册到BeanDefitionRegistry

ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); // Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory); try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);

//执行实现了BeanFactoryPostProcessor接口的Bean

//比如PropertyPlaceHolderConfigurer（context:property-placeholer）就是此处被调用的，替换掉BeanDefition中的占位符（${}）中的内容

// Invoke factory processors registered as beans in the context.
//针对beandifition实例进行操作
invokeBeanFactoryPostProcessors(beanFactory); // Register bean processors that intercept bean creation.
//主要针对bean实例进行操作的的,
//比如容器自动装载了一个AutowiredAnnotationBeanPostProcessor后置处理器（实现@Autowired注解功能）
registerBeanPostProcessors(beanFactory); // Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster(); // Initialize other special beans in specific context subclasses.
onRefresh(); // Check for listener beans and register them.
registerListeners(); // Instantiate all remaining (non-lazy-init) singletons.

//初始化非懒加载方式的单例bean实例自定的的java类 compent 之类的
这里的单例可以用过 scope作用域来进行自定义
finishBeanFactoryInitialization(beanFactory); // Last step: publish corresponding event.
finishRefresh();
} catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
} // Destroy already created singletons to avoid dangling resources.
destroyBeans(); // Reset 'active' flag.
cancelRefresh(ex); // Propagate exception to caller.
throw ex;
} finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}