转载地址:http://www.cnblogs.com/bobzeng/articles/1877140.html
一般的Web项目都会在web.xml中加入Spring监听器,内容如下:
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<
listener
>
<
listener-class
>org.springframework.web.context.ContextLoaderListener</
listener-class
>
</
listener
>
<
context-param
>
<
param-name
>contextConfigLocation</
param-name
>
<
param-value
>classpath*:applicationContext-struts.xml,classpath*:spring/applicationContext.xml</
param-value
>
</
context-param
>
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我们的问题是,Spring是何时以及如何加载我们的配置文件来初始化Bean工厂的,带着这些问题,我们展开研究:
我们先来看看web.xml中配置的监听器的类,来回答我们的问题,Spring是何时来加载我们的配置文件的:
org.springframework.web.context.ContextLoaderListener
它继承了javax.servlet.ServletContextListener接口。
ServletContextListener是J2EE Servlet API中的一个标准接口,
它能够监听ServletContext对象的生命周期,实际上就是监听Web应用的生命周期。
当Servlet容器启动或终止Web应用时,会触发ServletContextEvent事件,该事件由ServletContextListener来处理。
这里面有两个方法我们比较感兴趣:
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/**
* Create the ContextLoader to use. Can be overridden in subclasses.
* @return the new ContextLoader
*/
protected
ContextLoader createContextLoader() {
return
new
ContextLoader();
}
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这个方法构造一个默认的ContextLoader,ContextLoader可以理解为Spring上下文的加载器。之所以这样去定义这样一个类,是为了开发人员进行重写此方法来使用一个自定义的Spring上下文的加载器。
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/**
* Initialize the root web application context.
*/
public
void
contextInitialized(ServletContextEvent event) {
this
.contextLoader = createContextLoader();
this
.contextLoader.initWebApplicationContext(event.getServletContext());
}
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这个方法很简单,仅仅只是调用了createContextLoader()构造了ContextLoader,并调用其初始化方法。
由此,我们可以得出结论,Spring是在Web项目启动时,通过ServletContextListener机制,来加载以及初始化Spring上下文的。
下面,我们好好研究一下Spring是如何加载其上下文的:
我们先定位ContextLoader类。
看看此类的initWebApplicationContext()方法(省略了不重要的语句)
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/**
* Initialize Spring's web application context for the given servlet context,
* according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and
* "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params.
* @param servletContext current servlet context
* @return the new WebApplicationContext
* @throws IllegalStateException if there is already a root application context present
* @throws BeansException if the context failed to initialize
* @see #CONTEXT_CLASS_PARAM
* @see #CONFIG_LOCATION_PARAM
*/
public
WebApplicationContext initWebApplicationContext(ServletContext servletContext)
throws
IllegalStateException, BeansException {
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!"
);
}
try
{
// Determine parent for root web application context, if any.
ApplicationContext parent = loadParentContext(servletContext);
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
this
.context = createWebApplicationContext(servletContext, parent);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE,
this
.context);
currentContextPerThread.put(Thread.currentThread().getContextClassLoader(),
this
.context);
return
this
.context;
}
catch
(RuntimeException ex) {
logger.error(
"Context initialization failed"
, ex);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
throw
ex;
}
catch
(Error err) {
logger.error(
"Context initialization failed"
, err);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
throw
err;
}
}
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其中的有两句比较重要,我们来看看:
ApplicationContext parent = loadParentContext(servletContext);
这个方法的用途主要是用来解决Spring共享环境的,即,如果我们有多个WAR包部署在同一个服务器上,而且这些WAR都共享某一套业务逻辑层。如何共享一套业务逻辑包配置而不要每个WAR都单独配置,这时我们就可能需要Spring的共享环境了。
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protected
ApplicationContext loadParentContext(ServletContext servletContext)
throws
BeansException {
ApplicationContext parentContext =
null
;
// 从web.xml中读取父工厂的配置文件,默认为:"classpath*:beanRefContext.xml"
String locatorFactorySelector = servletContext.getInitParameter(LOCATOR_FACTORY_SELECTOR_PARAM);
// 从web.xml中读取父类工厂的名称
String parentContextKey = servletContext.getInitParameter(LOCATOR_FACTORY_KEY_PARAM);
if
(parentContextKey !=
null
) {
// locatorFactorySelector may be null, indicating the default "classpath*:beanRefContext.xml"
BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
this
.parentContextRef = locator.useBeanFactory(parentContextKey);
parentContext = (ApplicationContext)
this
.parentContextRef.getFactory();
}
return
parentContext;
}
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现在我们引入BeanFactoryLocator,它是Spring配置文件的一个定位器,Spring官方给它的定义是用来查找,使用和释放一个BeanFactory或其子类的接口。下面我们看看此图:
ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
是根据参数locatorFactorySelector去一个单例工厂中去拿一个对应的BeanFactoryLocator,也即,如果工厂中没有对 应于locatorFactorySelector的BeanFactoryLocator对象,那就返回一个新的BeanFactoryLocator 实例(这里是ContextSingletonBeanFactoryLocator的实例),否则,就从工厂里取现有的 BeanFactoryLocator对象。
ContextSingletonBeanFactoryLocator里维护了一个静态的Map对象instances,每次需要新增 BeanFactoryLocator实例时都会更新这个Map对象,这个Map对象是以配置文件名为KEY,BeanFactoryLocator对象 为值。原因很简单,就是希望同一个配置文件只被初始化一次。
如果没有在web.xml中定义locatorFactorySelector这个参数,父环境的配置文件默认使用:"classpath*:beanRefContext.xml"
this.parentContextRef = locator.useBeanFactory(parentContextKey);
此方法定义在SingletonBeanFactoryLocator类中,同样是一个单例工厂模式,判断传入的参数parentContextKey对 应的BeanFactory是否有被初始化,经过上面的 ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector)指定 Spring父环境配置文件,这个方法判断指定的父环境是否被初始化,如果有则返回,没有就进行初始化。看看此方法的实现:
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public
BeanFactoryReference useBeanFactory(String factoryKey)
throws
BeansException {
synchronized
(
this
.bfgInstancesByKey) {
BeanFactoryGroup bfg = (BeanFactoryGroup)
this
.bfgInstancesByKey.get(
this
.resourceLocation);
if
(bfg !=
null
) {
bfg.refCount++;
}
else
{
// Create the BeanFactory but don't initialize it.
BeanFactory groupContext = createDefinition(
this
.resourceLocation, factoryKey);
// Record its existence now, before instantiating any singletons.
bfg =
new
BeanFactoryGroup();
bfg.definition = groupContext;
bfg.refCount =
1
;
this
.bfgInstancesByKey.put(
this
.resourceLocation, bfg);
this
.bfgInstancesByObj.put(groupContext, bfg);
// Now initialize the BeanFactory. This may cause a re-entrant invocation
// of this method, but since we've already added the BeanFactory to our
// mappings, the next time it will be found and simply have its
// reference count incremented.
try
{
initializeDefinition(groupContext);
}
catch
(BeansException ex) {
this
.bfgInstancesByKey.remove(
this
.resourceLocation);
this
.bfgInstancesByObj.remove(groupContext);
throw
new
BootstrapException(
"Unable to initialize group definition. "
+
"Group resource name ["
+
this
.resourceLocation +
"], factory key ["
+ factoryKey +
"]"
, ex);
}
}
try
{
BeanFactory beanFactory =
null
;
if
(factoryKey !=
null
) {
beanFactory = (BeanFactory) bfg.definition.getBean(factoryKey, BeanFactory.
class
);
}
else
if
(bfg.definition
instanceof
ListableBeanFactory) {
beanFactory = (BeanFactory) BeanFactoryUtils.beanOfType((ListableBeanFactory) bfg.definition, BeanFactory.
class
);
}
else
{
throw
new
IllegalStateException(
"Factory key is null, and underlying factory is not a ListableBeanFactory: "
+ bfg.definition);
}
return
new
CountingBeanFactoryReference(beanFactory, bfg.definition);
}
catch
(BeansException ex) {
throw
new
BootstrapException(
"Unable to return specified BeanFactory instance: factory key ["
+
factoryKey +
"], from group with resource name ["
+
this
.resourceLocation +
"]"
, ex);
}
}
}
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此方法分为两作了两件事,
第一,初始化上下文,主意这里初始化的是从web.xml配置参数里的Spring配置文件,也是上面讲loadParentContext方法里的
BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
这句指定的参数。这里初始化的是这个配置文件所有Bean。我们指定的factoryKey对应的Bean也是其中之一。
第二,从已经初始化的Spring上下文环境中获取Spring父环境。
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<
beans
>
<
bean
id
=
"factoryBeanId"
class
=
"org.springframework.context.support.ClassPathXmlApplicationContext"
>
<
constructor-arg
>
<
list
>
<
value
>sharebean.xml</
value
>
</
list
>
</
constructor-arg
>
</
bean
>
<
bean
id
=
"factoryBeanId2"
class
=
"org.springframework.context.support.ClassPathXmlApplicationContext"
>
<
constructor-arg
>
<
list
>
<
value
>sharebean2.xml</
value
>
</
list
>
</
constructor-arg
>
</
bean
>
</
beans
>
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<!—========================= web.xml ========================= -->
<
context-param
>
<
param-name
>locatorFactorySelector</
param-name
>
<
param-value
>beanRefFactory.xml</
param-value
>
</
context-param
>
<
context-param
>
<
param-name
>parentContextKey</
param-name
>
<
param-value
>factoryBeanId</
param-value
>
</
context-param
>
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这个一个典型的构造父环境的配置,web项目在启动的时候就会发现里面有Spring父环境的配置,那么Spring首先就会生成一个对应的配置文件为 beanRefFactory.xml的BeanFactory(web.xml中的locatorFactorySelector参数指定),同时 Spring在解析的时候,会发现factoryBeanId的配置同样为BeanFacotry(beanRefFactory.xml中 factoryBeanId对应的Bean),所以Spring在拿父环境时就会写成:
beanFactory = (BeanFactory) bfg.definition.getBean(factoryKey, BeanFactory.class);
方法实现里引入了BeanFactoryGroup类。类的结构很简单
refCount:用来记录实例被外部引用的记数,当调用locator.useBeanFactory(parentContextKey)方法时,引 用数就会加1,当调用CountingBeanFactoryReference#release方法时,引用数就会减1,当它变成0时,Spring就 会释放掉它占用的内存,同时也会销毁掉它definition变量引用的BeanFactory。下次再调用 locator.useBeanFactory(parentContextKey)就会重新初始化BeanFactory。说到release,请同学 们参考ContextLoader中如下的两条语句:
// 在调用CountingBeanFactoryReference#release后,即使对象已经销毁,这个Map仍然可以返回locator对象。
BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
// 如果对象已经销毁,再调用此方法会再一次初始化BeanFactory
this.parentContextRef = locator.useBeanFactory(parentContextKey);
bfgInstancesByKey:一个Map对象,以配置文件名为Key,配置文件解析后生成的BeanFactory构成的BeanFactoryGroup为值。
bfgInstancesByObj:一个Map对象,以BeanFactoryGroup.definitiion为Key,以 BeanFactoryGroup为值。这个对象主要还是在CountingBeanFactoryReference#release时使用。
下面,我看再看看另一个地方:
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if
(parentContextKey !=
null
) {
// locatorFactorySelector may be null, indicating the default "classpath*:beanRefContext.xml"
BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
this
.parentContextRef = locator.useBeanFactory(parentContextKey);
parentContext = (ApplicationContext)
this
.parentContextRef.getFactory();
}
BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
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上面这句仅仅是做了如下工作:
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BeanFactoryLocator bfl = (BeanFactoryLocator) instances.get(resourceLocation);
if
(bfl ==
null
) {
// 仅仅只是设置了ContextSingletonBeanFactoryLocator里的resourceLocation属性的值,并没有初始化工厂。
bfl =
new
ContextSingletonBeanFactoryLocator(resourceLocation);
instances.put(resourceLocation, bfl);
}
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而我们使用工厂模式的时候,一般是把对象初始化好了,再给外部使用,为什么Spring这里要多此一举,在调用getInstance这后还要去调用useBeanFactory来初始化父环境?为什么Spring开发者不写成如下:
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BeanFactoryLocator bfl = (BeanFactoryLocator) instances.get(resourceLocation);
if
(bfl ==
null
) {
bfl =
new
ContextSingletonBeanFactoryLocator(resourceLocation);
// 下面这句可能换成 initBeanFactory 类似语句,这里只是打个比方
bfl.useBeanFactory(parentContextKey);
instances.put(resourceLocation, bfl);
}
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本来我认为这个写法是必须的,后来想想也不是,不过这里体现了Spring的灵活设计。如果按排上面的方法进行改造有几点不妥,1,每次都会初始化,开销 比较大,可能有需求是需要延迟初始化的。2,每次都需要初始化都需要传入两个参数,分别为:配置文件名与父工厂名,3,类职责混乱,比如一个配置文件中可 能定义了多个父环境的Bean,采用Spring这种方法是很清晰的:
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// 返回BeanFactoryLocator方便定位某个配置文件。
BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(“classpath*: parentBeanFactory.xml”);
parentContextRef1 = locator.useBeanFactory(
"parent1Key"
);
parentContextRef2 = locator.useBeanFactory(
"parent2Key"
);
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而使用我们改造的方法,则要写成如下:
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parentContextRef1 = ContextSingletonBeanFactoryLocator.getInstance(
"parentBeanFactory.xml"
,
"parent1Key"
);
parentContextRef2 = ContextSingletonBeanFactoryLocator.getInstance(
"parentBeanFactory.xml"
,
"parent2Key"
);
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相当麻烦且无语,引用了也只是这个配置文件中的某一个Bean的引用,没什么意义。
这就是为什么BeanFactoryLocator接口存在的一个原因,用于查找某个配置文件中的一个BeanFactory。
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public
interface
BeanFactoryLocator {
BeanFactoryReference useBeanFactory(String factoryKey)
throws
BeansException;
}
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this.context = createWebApplicationContext(servletContext, parent);
我们来看看这个函数做了些什么:
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protected
WebApplicationContext createWebApplicationContext(
ServletContext servletContext, ApplicationContext parent)
throws
BeansException {
// 获得需要实例化的CONTEXT类名,确定ContextClass的类型。如果在web.xml中配置了contextClass这个parameter,
// 使用这个指定的类作为ContextClass,会抛出ClassNotFound的异常。反之,使用默认的XmlWebApplicationContext
Class contextClass = determineContextClass(servletContext);
// 所有的WebApplicationContext必须实现ConfigurableWebApplicationContext接口
if
(!ConfigurableWebApplicationContext.
class
.isAssignableFrom(contextClass)) {
throw
new
ApplicationContextException(
"Custom context class ["
+ contextClass.getName() +
"] is not of type ["
+ ConfigurableWebApplicationContext.
class
.getName() +
"]"
);
}
ConfigurableWebApplicationContext wac =
(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
// 设置父环境
wac.setParent(parent);
// 设置Servlet上下文环境
wac.setServletContext(servletContext);
// 设置Spring配置文件路径
wac.setConfigLocation(servletContext.getInitParameter(CONFIG_LOCATION_PARAM));
customizeContext(servletContext, wac);
wac.refresh();
return
wac;
}
protected
Class determineContextClass(ServletContext servletContext)
throws
ApplicationContextException {
// 获得需要实例化的CONTEXT类名,在web.xml中有设置,如果没有设置,那么为空
String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
if
(contextClassName !=
null
) {
try
{
return
ClassUtils.forName(contextClassName);
}
catch
(ClassNotFoundException ex) {
throw
new
ApplicationContextException(
"Failed to load custom context class."
, ex);
}
}
else
{
//如果在spring web.xml中没有设置context类位置,那么取得默认context
//取得defaultStrategies配置文件中的WebApplicationContext属性
contextClassName = defaultStrategies.getProperty(WebApplicationContext.
class
.getName());
try
{
return
ClassUtils.forName(contextClassName, ContextLoader.
class
.getClassLoader());
}
catch
(ClassNotFoundException ex) {
throw
new
ApplicationContextException(
"Failed to load default context class."
, ex);
}
}
}
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private
static
final
String DEFAULT_STRATEGIES_PATH =
"ContextLoader.properties"
;
static
{
// Load default strategy implementations from properties file.
// This is currently strictly internal and not meant to be customized
// by application developers.
try
{
ClassPathResource resource =
new
ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.
class
);
defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
}
catch
(IOException ex) {
throw
new
IllegalStateException(
"Could not load 'ContextLoader.properties': "
+ ex.getMessage());
}
}
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// 在ContextLoader.properties里定义如下
org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext
再来看看Spring是如果进行初始化ApplicationContext的。就以XmlWebApplicationContext来说,它继承了 ConfigurableWebApplicationContext这个接口,里面有个refresh()方法,我们可以看看它的实现 (AbstractApplicationContext):
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public
void
refresh()
throws
BeansException, IllegalStateException {
synchronized
(
this
.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
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);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
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.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch
(BeansException ex) {
// Destroy already created singletons to avoid dangling resources.
beanFactory.destroySingletons();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw
ex;
}
}
}
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这个方法的实现由于涉及的东西比较多,比较国际化,事件等等,等我们理解了后续的源代码分析之后再重新过来进行研究。这样效率更高点。
这样关于Spring在web项目中加载及初始化的方式我们大概也了解的比较清楚了,我们可以看到,Spring就第一步,加载都已经做了很多工作,不得不佩服Spring团队的智慧。
最后,Spring加载完成之前,会将ApplicationContext放入ServletContext中,方便程序进行访问。
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servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE,
this
.context);
currentContextPerThread.put(Thread.currentThread().getContextClassLoader(),
this
.context);
|
其中WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE定义如下:
String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
Spring环境
加载组件:ContextLoaderListener
配置路径:Servlet环境初始化参数contextConfigLocation指定的路径
缺省路径: 没有缺省路径
Spring环境的父环境
加载组件:ContextLoaderListener和ContextSingletonBeanFactoryLocator
配置路径:Servlet环境初始化参数locatorFactorySelector指定Bean工厂定位器使用的给BeanFactory,Servlet环境初始化参数parentContextKey指定Bean工厂定位器用于查找BeanFactory的关键字
缺省路径: parentContextKey的缺省路径是classpath*:beanRefFactory.xml
这里我们还有一个功能相近的类没有进行说明:
ContextJndiBeanFactoryLocator