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摘要: 原创出处 https://muyinchen.github.io/2017/09/12/Spring5%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90 —— Spring%E4%B8%AD%E7%9A%84Context%20loader/ 「一叶知秋」欢迎转载,保留摘要,谢谢!


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我们已经知道,应用程序上下文是Spring管理的bean所在的容器。但是我们依然要问一个问题:这个上下文是如何创建的?那么在这篇文章中我们来探讨这个问题。

在第一部分中,会说下在Spring的应用程序上下文中所谓的**上下文加载器(context loader)**是什么。在第二部分,我们会讨论这个加载器的代码细节。最后一部分,老规矩,写我们自己的一个自定义的loader。在继续之前,需要说一下,loader(加载器) 将根据web application和dispatcher servlet来结合进行分析。其实这也是很多人一碰到源码就像无头苍蝇,不知道从何而起了,刚开始放下所有,从大体去思考该如何入手,这里对设计模式了解就很重要了,还有,源码的类注释很重要,不多说,接着走。

什么是Spring的上下文加载器(context loader)?

见名知意,上下文加载程序负责构建应用程序上下文。我们可以通过org.springframework.web.context.ContextLoaderListener的实例来对其分析(从我之前的设计模式的文章可以看到,Spring通过观察者模式,其实我自己总结的是电影院模式,声音和画面通过broadcaster发送到listener,listener再调用相应的adapter来处理,所以,这里就直接从listener来找了),它继承并扩展了同一个包下的ContextLoader类。同时还实现了javax.servlet.ServletContextListener接口。该接口旨在接收有关servlet上下文中更改变化的通知。只有当它们在(WEB-INF/web.xml)中注册时,这个接口的实现才能接收这些通知。

在Spring Web应用程序中,会在servlet上下文创建时调用上下文加载程序(context loader)。之后,开始初始化根Web应用程序上下文(Root WebApplicationContext)。Root非常重要,因为在加载的时候,可以创建两个或更多的上下文。第一个,也是最重要的,定义了整个bean的生存空间,被称为应用程序上下文(application context)。另一个是servlet应用程序上下文,其包含更多的是面向Web的元素,比如控制器(controllers)或视图解析器。然而我们需要记住的是,servlet的上下文是根应用程序上下文(Root WebApplicationContext)的子集,也就是父子容器一说。这意味着servlet可以从根应用程序上下文继承所有的bean。这就是为什么你可以在根配置文件中定义一些常见资源(例如:services,这也是我们的Spring xml配置文件为什么要分service和MVC两个的原因),并通过两个不同的servlet进行共享的原因。但是在另一方面,根应用程序上下文不能获取到特定于servlet的bean,看过我的逃逸分析的应该都清楚了吧。

我们可以将注意力拉回到关于上下文加载器的两个作用上:

  • 将根Web应用程序上下文(Root WebApplicationContext)绑定到调度程序特定的上下文中
  • 自动创建上下文(程序员不需要编写任何东西来使上下文工作)

Spring的上下文加载器详解

我们已经了解了上下文加载器的作用。现在,我们来更详细地介绍这其中的细节。web上下文加载器(context loader)类位于org.springframework.web.context包中。主类是ContextLoaderListener,它扩展了ContextLoader类。同时实现了ServletContextListener接口。

在上下文创建时调用的方法是public void contextInitialized(ServletContextEvent event)。它通过传递给它所接收到的servlet上下文(从事件参数获取event.getServletContext())来调用ContextLoaderinitWebApplicationContext方法。initWebApplicationContext方法进行的第一个操作是检查是否有另一个根上下文存在。如果至少存在另一个,则抛出IllegalStateException,并且初始化失败。否则,它继续初始化org.springframework.web.context.WebApplicationContext实例。如果初始化的实例实现了ConfigurableWebApplicationContext接口,则在设置当前应用程序上下文之前,加载器将进行一些设置服务(父上下文,应用程序上下文,servlet上下文等),并通过上下文的refresh()方法来准备bean,这已经在关于应用程序上下文的文章中介绍过了。

org.springframework.web.context.ContextLoaderListener:

/**
* Initialize the root web application context.
*/
@Override
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}

org.springframework.web.context.ContextLoader:

/**
* Initialize Spring's web application context for the given servlet context,
* using the application context provided at construction time, or creating a new one
* 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
* @see #ContextLoader(WebApplicationContext)
* @see #CONTEXT_CLASS_PARAM
* @see #CONFIG_LOCATION_PARAM
*/
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
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!");
}

Log logger = LogFactory.getLog(ContextLoader.class);
servletContext.log("Initializing Spring root WebApplicationContext");
if (logger.isInfoEnabled()) {
logger.info("Root WebApplicationContext: initialization started");
}
long startTime = System.currentTimeMillis();

try {

// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
//此处判断下初始化的实例实现了ConfigurableWebApplicationContext接口
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);
}
//refresh()准备生米煮熟饭了
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = this.context;
}
else if (ccl != null) {
currentContextPerThread.put(ccl, this.context);
}

if (logger.isDebugEnabled()) {
logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
}
if (logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
}

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;
}
}

ContextLoaderListener中第二个我们需要关注的方法是public void contextDestroyed(ServletContextEvent event)。每当加载程序的上下文关闭时都会调用它。这个方法干了两件事情:

  • 通过ContextLoader中的closeWebApplicationContext(),它关闭应用程序上下文。通过ConfigurableWebApplicationContext close()方法完成上下文关闭。上下文的销毁的过程其实就是销毁bean和关闭bean工厂,此处参考org.springframework.context.support.AbstractApplicationContext中的源码,下面相关部分已贴出。
  • 调用ContextCleanupListener.cleanupAttributes(event.getServletContext()),它将查找当前servlet上下文的所有实现org.springframework.beans.factory.DisposableBean接口的对象。之后,将调用它们的destroy()方法,以销毁不再使用的bean。

org.springframework.web.context.ContextLoaderListener:

/**
* Close the root web application context.
*/
@Override
public void contextDestroyed(ServletContextEvent event) {
closeWebApplicationContext(event.getServletContext());
ContextCleanupListener.cleanupAttributes(event.getServletContext());
}

org.springframework.web.context.ContextLoader:

/**
* Close Spring's web application context for the given servlet context. If
* the default {@link #loadParentContext(ServletContext)} implementation,
* which uses ContextSingletonBeanFactoryLocator, has loaded any shared
* parent context, release one reference to that shared parent context.
* <p>If overriding {@link #loadParentContext(ServletContext)}, you may have
* to override this method as well.
* @param servletContext the ServletContext that the WebApplicationContext runs in
*/
public void closeWebApplicationContext(ServletContext servletContext) {
servletContext.log("Closing Spring root WebApplicationContext");
try {
if (this.context instanceof ConfigurableWebApplicationContext) {
((ConfigurableWebApplicationContext) this.context).close();
}
}
finally {
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = null;
}
else if (ccl != null) {
currentContextPerThread.remove(ccl);
}
servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
if (this.parentContextRef != null) {
this.parentContextRef.release();
}
}
}

org.springframework.context.support.AbstractApplicationContext:

/**
* DisposableBean callback for destruction of this instance.
* Only called when the ApplicationContext itself is running
* as a bean in another BeanFactory or ApplicationContext,
* which is rather unusual.
* <p>The {@code close} method is the native way to
* shut down an ApplicationContext.
* @see #close()
* @see org.springframework.beans.factory.access.SingletonBeanFactoryLocator
*/
@Override
public void destroy() {
close();
}

/**
* Close this application context, destroying all beans in its bean factory.
* <p>Delegates to {@code doClose()} for the actual closing procedure.
* Also removes a JVM shutdown hook, if registered, as it's not needed anymore.
* @see #doClose()
* @see #registerShutdownHook()
*/
@Override
public void close() {
synchronized (this.startupShutdownMonitor) {
doClose();
// If we registered a JVM shutdown hook, we don't need it anymore now:
// We've already explicitly closed the context.
if (this.shutdownHook != null) {
try {
Runtime.getRuntime().removeShutdownHook(this.shutdownHook);
}
catch (IllegalStateException ex) {
// ignore - VM is already shutting down
}
}
}
}

/**
* Actually performs context closing: publishes a ContextClosedEvent and
* destroys the singletons in the bean factory of this application context.
* <p>Called by both {@code close()} and a JVM shutdown hook, if any.
* @see org.springframework.context.event.ContextClosedEvent
* @see #destroyBeans()
* @see #close()
* @see #registerShutdownHook()
*/
protected void doClose() {
if (this.active.get() && this.closed.compareAndSet(false, true)) {
if (logger.isInfoEnabled()) {
logger.info("Closing " + this);
}

LiveBeansView.unregisterApplicationContext(this);

try {
// Publish shutdown event.
publishEvent(new ContextClosedEvent(this));
}
catch (Throwable ex) {
logger.warn("Exception thrown from ApplicationListener handling ContextClosedEvent", ex);
}

// Stop all Lifecycle beans, to avoid delays during individual destruction.
try {
getLifecycleProcessor().onClose();
}
catch (Throwable ex) {
logger.warn("Exception thrown from LifecycleProcessor on context close", ex);
}

// Destroy all cached singletons in the context's BeanFactory.
destroyBeans();

// Close the state of this context itself.
closeBeanFactory();

// Let subclasses do some final clean-up if they wish...
onClose();

this.active.set(false);
}
}

/**
* Template method for destroying all beans that this context manages.
* The default implementation destroy all cached singletons in this context,
* invoking {@code DisposableBean.destroy()} and/or the specified
* "destroy-method".
* <p>Can be overridden to add context-specific bean destruction steps
* right before or right after standard singleton destruction,
* while the context's BeanFactory is still active.
* @see #getBeanFactory()
* @see org.springframework.beans.factory.config.ConfigurableBeanFactory#destroySingletons()
*/
protected void destroyBeans() {
getBeanFactory().destroySingletons();
}
/**
* Template method which can be overridden to add context-specific shutdown work.
* The default implementation is empty.
* <p>Called at the end of {@link #doClose}'s shutdown procedure, after
* this context's BeanFactory has been closed. If custom shutdown logic
* needs to execute while the BeanFactory is still active, override
* the {@link #destroyBeans()} method instead.
*/
protected void onClose() {
// For subclasses: do nothing by default.
}

在Spring Web应用程序中实现上下文加载程序

想象一下,你希望在系统的所有用户之间共享一个信息。你可以用传统的方式做到这一点,也可以使用你定义的上下文加载器。我们通过写一些简单的代码来达到这个目的。还有一个想要实现的功能会涉及多个上下文。我们的应用程序将同时处理guestconnected两种形式(请同时看下面源码)。可以看到他们的网页的URL匹配规则不一样。使用connected的用户将能够访问与guest规则下以.chtml扩展名结尾的相同的页面,也就是所谓的交集。需要说的是,他们不会共享相同的信息(两个不一样的上下文当然不会一样了)。还不懂的话看下面源码,对于这两者,我们将分别 指定两个servlet上下文。你会看到,因为它,访问connected用户将不会与访问guest共享相同的bean。

我们将从web.xml文件开始,请对比上面说的:

<!--?xml version="1.0" encoding="UTF-8"?-->
<web-app id="WebApp_ID" version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemalocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<servlet>
<servlet-name>guest</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:spring/guest-servlet.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>

<-- guest is the default servlet -->
<servlet-mapping>
<servlet-name>guest</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>

<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>
/WEB-INF/applicationContext.xml
</param-value>
</context-param>
<-- Customized listener which will put some personnalized data into servlet's context -->
<listener>
<listener-class>com.mysite.servlet.CustomizedContextLoader</listener-class>
</listener>

<servlet>
<servlet-name>connected</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:spring/connected-servlet.xml</param-value>
</init-param>
<load-on-startup>2</load-on-startup>
</servlet>

<servlet-mapping>
<servlet-name>connected</servlet-name>
<url-pattern>**.chtml</url-pattern>
</servlet-mapping>
</web-app>

两个指定的servlet的bean配置文件几乎相同。唯一的区别是connected-servlet.xml包含一个没有与guest servlet共享的bean的定义。这个bean的名字是secretData:

<bean id="secretData" class="com.migo.secret.SecretData">
<property name="question" value="How old are you ?"/>
<property name="answer" value="33"/>
</bean>
<context:annotation-config/>
<context:component-scan base-package="com.migo"/>
<mvc:annotation-driven/>

神秘豆的内容主要由setter和toString方法组成:

public class SecretData {

private String question;
private String answer;

public void setQuestion(String question) {
this.question = question;
}

public void setAnswer(String answer) {
this.answer = answer;
}

@Override
public String toString() {
return "SecretData {question: "+this.question+", answer: "+this.answer+"}";
}

}

其他Java代码也很简单。在CustomizedContextLoader中,我们重写contextInitialized方法来放置共享servlet的上下文属性:名字叫webappVersion。该属性是一个随机数,用于证明根应用程序上下文的加载程序仅被调用一次:

public class CustomizedContextLoader extends ContextLoaderListener  {

@Override
public void contextInitialized(ServletContextEvent event) {
System.out.println("[CustomizedContextLoader] Loading context");
// this value could be read from data source, but for the simplicity reasons, we put it statically
// number is random because we want to prove that the root context is loaded only once
Random random = new Random();
int version = random.nextInt(100001);
System.out.println("Version set into servlet's context :"+version);
event.getServletContext().setAttribute("webappVersion", version);
super.contextInitialized(event);
}
}

之后,我们传递给用来处理访问网址的TestController:

@Controller
public class TestController {

@Autowired
private ApplicationContext context;

@RequestMapping(value = "/test.chtml", method = RequestMethod.GET)
public String test(HttpServletRequest request) {
LOGGER.debug("[TestController] Webapp version from servlet's context :"+request.getServletContext().getAttribute("webappVersion"));
LOGGER.debug("[TestController] Found secretData bean :"+context.getBean("secretData"));
return "test";
}

@RequestMapping(value = "/test.html", method = RequestMethod.GET)
public String guestTest(HttpServletRequest request) {
LOGGER.debug("[TestController] Webapp version from servlet's context :"+request.getServletContext().getAttribute("webappVersion"));
LOGGER.debug("[TestController] Found secretData bean :"+context.getBean("secretData"));
return "test";
}
}

测试的时候,首先输入http://localhost:8080/test.chtml,然后输入http://localhost:8080/test.html。然后通过查看日志:

[CustomizedContextLoader] Loading context
Version set into servlet's context :38023
// ... test.chtml
[TestController] Webapp version from servlet's context :38023
[TestController] Found secretData bean :SecretData {question: How old are you ?, answer: 33}
// ... test.html
[TestController] Webapp version from servlet's context :38023
3 avr. 2014 14:01:02 org.apache.catalina.core.StandardWrapperValve invoke
GRAVE: Servlet.service() for servlet [guestServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No bean named 'secretData' is defined] with root cause
org.springframework.beans.factory.NoSuchBeanDefinitionException: No bean named 'secretData' is defined
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeanDefinition(DefaultListableBeanFactory.java:638)
at org.springframework.beans.factory.support.AbstractBeanFactory.getMergedLocalBeanDefinition(AbstractBeanFactory.java:1159)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:282)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:200)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:273)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:195)
at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:973)
at com.mysite.controller.TestController.guestTest(TestController.java:114)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:214)
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132)
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104)
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:748)
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:689)
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:83)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:945)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:876)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:931)
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:822)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:668)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:807)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:770)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:395)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:250)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

首先,将一个信息(“Version set into servlet’s context :”+version)放在servlet上下文中,并由两个servlet上下文继承。第二点是bean的可见性。Guestservlet没有看到secretData bean,因为它仅在connected (connected-servlet.xml)的配置中被定义。

总结

第一部分涉及了这个加载器的两个主要角色:将根Web应用程序上下文(Root WebApplicationContext)绑定到调度程序特定的上下文中并自动创建上下文。接下来,我们分析了关于上下文加载程序的代码的要点所涉及的细节,如所实现的接口和主要方法的细节实现。最后一部分是我们自定义扩展本地上下文加载器,然后对bean和servlet的属性继承方面进行一些测试。

666. 彩蛋

如果你对 Spring 感兴趣,欢迎加入我的知识星球一起交流。

知识星球

文章目录
  1. 1. 什么是Spring的上下文加载器(context loader)?
  2. 2. Spring的上下文加载器详解
  3. 3. 在Spring Web应用程序中实现上下文加载程序
  4. 4. 总结
  • 666. 彩蛋