Spring5源码解析-Spring中的应用上下文
之前讲到过，Spring中的 beans生活(用这俩字觉得更形象具体)在其应用程序的上下文环境中。在本文中，我们将详细介绍应用程序上下文,另外此篇同样是由域联系到的逃逸分析的关于Spring容器的续篇。
关于 Spring5源码解析-@Autowired
这篇文章讲了通过 @Autowired
注解进行依赖注入。这一次我们来探讨 应用程序上下文(application context)
的概念。在第一部分中，我们来看看所有Spring管理的bean生活在什么样的环境中。在第二部分，来分析下到负责上下文管理的类。在最后一部分中，我们来进行一些实践操作。
什么是Spring的应用程序上下文？
众所周知，Spring管理的这些类被称为bean，并且生活在Spring容器中。bean处理程序的最基本实现是 bean factory
。作为 org.springframework.beans.factory.BeanFactory
接口的 实现类
，这是一个初始化，配置和管理bean的容器。但通常在Spring应用程序中仅使用 BeanFactory
是不够的。它出现在 应用程序上下文中
。
应用程序上下文(Application context)是一种面向企业化( 其实Spring文档里也有面向企业这一说，不过这不就是企业里流水线的工厂里才能有的东西么
)的bean工厂。作为标准bean工厂，它是bean class生活的空间。但与标准bean工厂不同，应用程序上下文提供了一个补充企业层(也就是通用的东西了，比如企业里的胸牌，服装等)。又迷糊了吧，举个例子 :例如，通过提供国际化，转换服务或事件传播，使我们省去很多麻烦去亲自处理。通常，应用程序上下文优于bean工厂。但它的唯一缺点是内存消耗比bean工厂大，出现这种情况是由于补充的服务。如果内存的使用对于你的程序要求非常苛刻(例如在applet或移动环境中)，请考虑更多使用bean factory。否则，在更标准的应用程序中，应使用应用程序上下文(application context)。
Spring的应用程序上下文类
想要了解Spring中应用程序上下文，关键部分就是 org.springframework.context.ApplicationContext 接口。它扩展了一些其他接口:
- org.springframework.core.env.EnvironmentCapable :用于标记对象来对外暴露自己说我实现了 Environment 接口。根据这个接口的注释可以知道，它主要用于完成类型的检查。
/**
* Interface indicating a component that contains and exposes an {@link Environment} reference.
* 注释说了很清晰明了了，就不多废话了
* <p>All Spring application contexts are EnvironmentCapable, and the interface is used primarily
* for performing {@code instanceof} checks in framework methods that accept BeanFactory
* instances that may or may not actually be ApplicationContext instances in order to interact
* with the environment if indeed it is available.
*
* <p>As mentioned, {@link org.springframework.context.ApplicationContext ApplicationContext}
* extends EnvironmentCapable, and thus exposes a {@link #getEnvironment()} method; however,
* {@link org.springframework.context.ConfigurableApplicationContext ConfigurableApplicationContext}
* redefines {@link org.springframework.context.ConfigurableApplicationContext#getEnvironment
* getEnvironment()} and narrows the signature to return a {@link ConfigurableEnvironment}.
* The effect is that an Environment object is 'read-only' until it is being accessed from
* a ConfigurableApplicationContext, at which point it too may be configured.
*
* @author Chris Beams
* @since 3.1
* @see Environment
* @see ConfigurableEnvironment
* @see org.springframework.context.ConfigurableApplicationContext#getEnvironment()
*/
public interface EnvironmentCapable{
/**
* Return the {@link Environment} associated with this component
* (may be {@code null} or a default environment).
*/
EnvironmentgetEnvironment();
}
- org.springframework.beans.factory.ListableBeanFactory :通过继承该interface可以列出所有bean，也可以只列出与预期类型相对应的bean。
- org.springframework.beans.factory.HierarchicalBeanFactory :支持分层bean的管理。
- org.springframework.context.MessageSource :用来解决消息支持国际化。
- org.springframework.context.ApplicationEventPublisher :通过该接口，可以允许通知所有类来监听到某些应用程序上下文事件。
- org.springframework.core.io.support.ResourcePatternResolver :是一个有助于将资源地址(例如:classpath:/WEB-INF/web.xml)解析到 org.springframework.core.io.Resource 对象中的策略接口。
/**
* Central interface to provide configuration for an application.
* This is read-only while the application is running, but may be
* reloaded if the implementation supports this.
*
* <p>An ApplicationContext provides:
* <ul> 请看所扩展相关各个接口功能的描述
* <li>Bean factory methods for accessing application components.
* Inherited from {@link org.springframework.beans.factory.ListableBeanFactory}.
* <li>The ability to load file resources in a generic fashion.
* Inherited from the {@link org.springframework.core.io.ResourceLoader} interface.
* <li>The ability to publish events to registered listeners.
* Inherited from the {@link ApplicationEventPublisher} interface.
* <li>The ability to resolve messages, supporting internationalization.
* Inherited from the {@link MessageSource} interface.
* <li>Inheritance from a parent context. Definitions in a descendant context
* will always take priority. This means, for example, that a single parent
* context can be used by an entire web application, while each servlet has
* its own child context that is independent of that of any other servlet.
* </ul>
*
* <p>In addition to standard {@link org.springframework.beans.factory.BeanFactory}
* lifecycle capabilities, ApplicationContext implementations detect and invoke
* {@link ApplicationContextAware} beans as well as {@link ResourceLoaderAware},
* {@link ApplicationEventPublisherAware} and {@link MessageSourceAware} beans.
*
* @author Rod Johnson
* @author Juergen Hoeller
* @see ConfigurableApplicationContext
* @see org.springframework.beans.factory.BeanFactory
* @see org.springframework.core.io.ResourceLoader
*/
public interface ApplicationContextextends EnvironmentCapable,ListableBeanFactory,HierarchicalBeanFactory,
MessageSource, ApplicationEventPublisher, ResourcePatternResolver {
对于我们来说，实现这些接口使应用程序上下文比一个简单的bean工厂更有用。我们通过 org.springframework.web.context.support.XmlWebApplicationContext
这个实现类来看其在Web应用程序中使用。此类扩展了同一个包下 AbstractRefreshableWebApplicationContext
这个抽象类。
XmlWebApplicationContext
实现了 AbstractRefreshableApplicationContext
中的抽象方法 loadBeanDefinitions
，用于读取所有bean。从这个方法实现，可以看出，所有的bean都是通过 org.springframework.beans.factory.xml.XmlBeanDefinitionReader
从XML文件读取的。
/**
* Loads the bean definitions via an XmlBeanDefinitionReader.
* @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
* @see #initBeanDefinitionReader
* @see #loadBeanDefinitions
*/
@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory)throwsBeansException, IOException{
// Create a new XmlBeanDefinitionReader for the given BeanFactory.
//只能说Spring源码注释的太详细了，英文确实很重要
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// Configure the bean definition reader with this context's
// resource loading environment.
beanDefinitionReader.setEnvironment(getEnvironment());
beanDefinitionReader.setResourceLoader(this);
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
// Allow a subclass to provide custom initialization of the reader,
// then proceed with actually loading the bean definitions.
initBeanDefinitionReader(beanDefinitionReader);
loadBeanDefinitions(beanDefinitionReader);
}
另一个有趣的方法，继承自 AbstractRefreshableWebApplicationContext
，是 postProcessBeanFactory
。它在加载所有bean定义之后并在其实例化之前被调用( postProcess
就是bean构造函数之后调用即实例化之前)。 AbstractRefreshableWebApplicationContext
使用它来注册请求和会话作用域以及环境bean(具体看下面源码)。另外，这个抽象类实现了 ConfigurableWebApplicationContext
接口，这样一来就可以定义 servlet
的上下文和一些本地的配置。
/**
* Register request/session scopes, a {@link ServletContextAwareProcessor}, etc.
*/
@Override
protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory){
beanFactory.addBeanPostProcessor(new ServletContextAwareProcessor(this.servletContext, this.servletConfig));
beanFactory.ignoreDependencyInterface(ServletContextAware.class);
beanFactory.ignoreDependencyInterface(ServletConfigAware.class);
WebApplicationContextUtils.registerWebApplicationScopes(beanFactory, this.servletContext);
WebApplicationContextUtils.registerEnvironmentBeans(beanFactory, this.servletContext, this.servletConfig);
}
@Override
@Nullable
publicServletContextgetServletContext(){
return this.servletContext;
}
@Override
public void setServletConfig(@Nullable ServletConfig servletConfig){
this.servletConfig = servletConfig;
if (servletConfig != null && this.servletContext == null) {
setServletContext(servletConfig.getServletContext());
}
}
@Override
@Nullable
publicServletConfiggetServletConfig(){
return this.servletConfig;
}
/**
* {@inheritDoc}
* <p>Replace {@code Servlet}-related property sources.
*/
@Override
protected void initPropertySources(){
ConfigurableEnvironment env = getEnvironment();
if (env instanceof ConfigurableWebEnvironment) {
((ConfigurableWebEnvironment) env).initPropertySources(this.servletContext, this.servletConfig);
}
}
由 XmlWebApplicationContext
间接继承的另一个抽象类是 AbstractRefreshableApplicationContext
。它有几种处理上下文刷新的方法。处理事件通知的类是 org.springframework.context.support.AbstractApplicationContext
，由 XmlWebApplicationContext
间接继承。它包含一个将事件( ApplicationEvent
类的实例)发送到所有侦听对象的 publishEvent
方法。
但是我们的重中之重是处理生命周期，是 AbstractApplicationContext
类的 public void refresh() throws BeansException, IllegalStateException
方法来做到的。
@Override
public void refresh()throwsBeansException, 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) {
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();
}
}
}
通过阅读源码，我们可以注意到以下操作:
- 上下文准备刷新(属性源初始化)
- bean工厂准备好用来一系列操作(classloader定义，基本bean注册)
- bean后置处理(postProcessBeanFactory方法)被调用
- 消息源(消息管理)被初始化
- event multicaster初始化(event multicaster是将事件分派到合适的侦听对象的对象)
- 在特定的上下文子类中初始化其他特殊的bean。
- 监听器的注册
- 所有剩余的bean的实例化(例如:转换服务)
在非Web环境中，我们可以使用标准应用程序上下文，如 FileSystemXmlApplicationContext
， ClassPathXmlApplicationContext
或 GenericXmlApplicationContext
。
关于Spring的应用程序环境的一些实践
关于此 部分，我们将看到:如何在一个控制器中获得一个上下文，查找得到一些bean配置并来解析一个消息。在进入正式的代码之前，我们需要做一些上下文的配置:
<!-- activate configuration by annotations, for example enable @Controller annotation -->
<context:annotation-config/>
<!-- indicates where Spring should looking for application services as services, controllers or components, annotated respectively with @Service, @Controller and @Component -->
<context:component-scanbase-package="com.mysite.test"/>
<!-- registers RequestMappingHandlerMapping, RequestMappingHandlerAdapter and ExceptionHandlerExceptionResolver; thanks to it, Spring can resolve requests annotated with @RequestMapping and @ExceptionHandler -->
<mvc:annotation-driven/>
<!-- represents a bean which will resolve the messages -->
<beanclass="org.springframework.context.support.ReloadableResourceBundleMessageSource"id="messageSource">
<propertyname="basenames">
<list>
<value>classpath:messages</value>
<value>classpath:errors</value>
</list>
</property>
<propertyname="defaultEncoding"value="UTF-8">
<propertyname="fallbackToSystemLocale"value="false">
</property>
</property>
</bean>
通过上面的配置，我们可以编写一个测试controller和一个类ApplicationContextProvider，它将保存一个应用程序上下文实例并按需返回:
// controller, TestController.java
@Controller
public class TestController{
@Autowired
private ApplicationContext context;
@RequestMapping(value = "/test", method = RequestMethod.GET)
publicStringtest(){
LOGGER.debug("[TestController] Received application context :"+context);
ApplicationContext providerContext = ApplicationContextProvider.getApplicationContext();
LOGGER.debug("[TestController] Provider context is :"+providerContext);
if (this.context == providerContext) {
LOGGER.debug("[TestController] Both contextes are the same");
}
LOGGER.debug("[TestController] Message is :"+this.context.getMessage("testMessage", new Object[] {}, Locale.ENGLISH));
return "test";
}
}
// context provider, ApplicationContextProvider.java
@Component
public class ApplicationContextProviderimplements ApplicationContextAware{
private static ApplicationContext context;
@Override
public void setApplicationContext(ApplicationContext c)throwsBeansException{
context = c;
}
public staticApplicationContextgetApplicationContext(){
return context;
}
}
ApplicationContext
实例由 Spring
管理。这就是为什么我们可以使用 @Autowired
注解将其注入另一个Spring管理的bean(在我们的例子中是一个controller )。这是通过注入的从一个bean得到上下文第一种方法。
第二种方法是使 ApplicationContextProvider
类实现 org.springframework.context.ApplicationContextAware
接口。这里需要说一下，该接口实现后可以获取当前正在运行的 ApplicationContext
的这个事件的通知。所以实现类必须实现这个方法: void setApplicationContext(ApplicationContext applicationContext)throws BeansException
。该方法允许设置当前的 ApplicationContext
实例并用来使用。上下文通过 org.springframework.context.support.ApplicationContextAwareProcessor
传递给 ApplicationContextAware
实现，在 AbstractApplicationContext
类中注册(见下面源码)。需要注意的是， ApplicationcontextAwareProcessor
也用于设置bean工厂或应用程序的上下文环境，见下面此类源码中的 private final StringValueResolver embeddedValueResolver;
的 StringValueResolver
接口的实现。可以知道，要实现这两种功能，这些类必须分别从 org.springframework.context
包中实现 EmbeddedValueResolverAware
和 EnvironmentAware
接口。
/**
* Configure the factory's standard context characteristics,
* such as the context's ClassLoader and post-processors.
* @param beanFactory the BeanFactory to configure
*/
protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory){
// Tell the internal bean factory to use the context's class loader etc.
beanFactory.setBeanClassLoader(getClassLoader());
beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));
// Configure the bean factory with context callbacks.
//将applicationContext实例扔进去，见下面对ApplicationContextAwareProcessor的源码注释
beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);
// BeanFactory interface not registered as resolvable type in a plain factory.
// MessageSource registered (and found for autowiring) as a bean.
beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
beanFactory.registerResolvableDependency(ResourceLoader.class, this);
beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
beanFactory.registerResolvableDependency(ApplicationContext.class, this);
// Register early post-processor for detecting inner beans as ApplicationListeners.
beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));
// Detect a LoadTimeWeaver and prepare for weaving, if found.
if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
// Set a temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
}
// Register default environment beans.
if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
}
if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
}
if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
}
}
/**
* {@link org.springframework.beans.factory.config.BeanPostProcessor}
* 看下面这句注释:
* implementation that passes the ApplicationContext to beans that
* implement the {@link EnvironmentAware}, {@link EmbeddedValueResolverAware},
* {@link ResourceLoaderAware}, {@link ApplicationEventPublisherAware},
* {@link MessageSourceAware} and/or {@link ApplicationContextAware} interfaces.
*
* <p>Implemented interfaces are satisfied in order of their mention above.
*
* <p>Application contexts will automatically register this with their
* underlying bean factory. Applications do not use this directly.
*
* @author Juergen Hoeller
* @author Costin Leau
* @author Chris Beams
* @since 10.10.2003
* @see org.springframework.context.EnvironmentAware
* @see org.springframework.context.EmbeddedValueResolverAware
* @see org.springframework.context.ResourceLoaderAware
* @see org.springframework.context.ApplicationEventPublisherAware
* @see org.springframework.context.MessageSourceAware
* @see org.springframework.context.ApplicationContextAware
* @see org.springframework.context.support.AbstractApplicationContext#refresh()
*/
class ApplicationContextAwareProcessorimplements BeanPostProcessor{
private final ConfigurableApplicationContext applicationContext;
private final StringValueResolver embeddedValueResolver;
/**
* Create a new ApplicationContextAwareProcessor for the given context.
* 要创建此实例，必须要有ConfigurableApplicationContext的上下文实例才行
*/
public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext){
this.applicationContext = applicationContext;
this.embeddedValueResolver = new EmbeddedValueResolver(applicationContext.getBeanFactory());
}
@Override
publicObjectpostProcessBeforeInitialization(finalObject bean, String beanName)throwsBeansException{
AccessControlContext acc = null;
if (System.getSecurityManager() != null &&
(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {
acc = this.applicationContext.getBeanFactory().getAccessControlContext();
}
if (acc != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareInterfaces(bean);
return null;
}, acc);
}
else {
invokeAwareInterfaces(bean);
}
return bean;
}
在我们的例子中，对于 ApplicationContextAware
的实现是只是一个简单的上下文提供者。但是在别的地方，我们定义的这个provider可能是用来得到上下文资源的对象。 这就是关于获取应用程序上下文的两种方式
。
最后,我们通过一个方法来使用context(上下文)的getMessage方法来对消息解析。在我们的 message_en.properties
文件中，可以事先声明消息的模板: testMessage =It’s our test message with content。
然后我们会在日志文件中看到相应的输出。
顺便说一下，从 ApplicationContextProvider
获得的对象和 @Autowired
的对象之间的上下文是相同的:
[TestController] Received application context :Root WebApplicationContext: startup date [Wed Apr 02 20:00:23 CEST 2014]; root of context hierarchy [TestController] Provider context is :Root WebApplicationContext: startup date [Wed Apr 02 20:00:23 CEST 2014]; root of context hierarchy [TestController] All contexts are the same [TestController] Message is :It's our test message with content.
正文到此结束
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