【Spring】Spring IOC原理及源码解析之scope=request、session

时间:2023-03-08 16:09:10

一、容器

1. 容器

抛出一个议点:BeanFactory是IOC容器,而ApplicationContex则是Spring容器。

什么是容器?Collection和Container这两个单词都有存放什么东西的意思,但是放在程序猿的世界,却注定是千差万别。Collection,集合,存放obj instanceof Class为true的一类对象,重点在于存放;Container,容器,可以存放各种各样的obj,但不仅仅是存放,他被称为容器,更重要的是他能管理存放对象的生命周期和依赖。

容器:用于存放对象,并能对存放对象进行生命周期管理和依赖管理。

2. Spring IOC容器是BeanFactory

Spring IOC容器是BeanFactory,也正是基于【容器】的论点。

在逻辑和源码分析之前,先做一些铺垫。对于使用Spring的程序猿来说,常用是ApplicationContext接口及其实现子类,ClasPathXmlApplicationContext、FilesystemXmlApplicationContext、XmlWebApplicationContext和AnnotationConfigWebApplicationContext,对于这四个类来说,他们都有一个共同的抽象父类AbstractRefreshableApplicationContext,而正是在该抽象父类中完成对BeanFactory的装饰。

public abstract class AbstractRefreshableApplicationContext extends AbstractApplicationContext { private Boolean allowBeanDefinitionOverriding; private Boolean allowCircularReferences; /** Bean factory for this context */ private DefaultListableBeanFactory beanFactory;

所有通过ApplicationContext接入使用Spring服务的,都是使用该bean工厂,而且最重要的是这个bean工厂实现了所有BeanFactory的接口、抽象类,拥有完整的Spring IOC逻辑。

【Spring】Spring IOC原理及源码解析之scope=request、session

二、Spring IOC逻辑

  • BeanFactory在Spring容器初始化的时候创建,默认创建的是DefaultListableBeanFactory对象;
  • Bean的配置信息在Spring容器初始化的时候被加载,并被解析成BeanDefinition存放在DefaultListableBeanFactory的ConcurrentHashMap中,此时无论是单例、非单例Bean都没有被创建;
  • BeanFactory:直接通过BeanFactory来接入使用Spring,无论是单例、非单例的Bean都不会在Spring初始化的时候被创建,而是在第一次getBean的是时候才被创建,此时单例bean会被缓存,而非单例的bean不会被缓存;
  • ApplicationContext:直接通过ApplicationContext来介入使用Spring,单例并且非lazy-init的Bean在Spring初始化的时候被创建并缓存,非单例、lazy-init的Bean在第一次getBean的时候被创建(Spring容器初始化的refresh方法中,finishBeanFactoryInitialization(beanFactory)方法完成);
  • 所有Bean的依赖注入在getBean逻辑中完成,当然是在getBean实例化对象之后;
  • 对于web环境下scope为request、session、globalsession的Bean来说,通过RequestContextListener侦听器侦听Request的建立和销毁,从而进行Bean生命周期的管理,具体是RequestContextHolder中通过ThreadLocal将ServletRequestAttributes与当前线程绑定,ServletRequestAttributes构造方法传入HttpServletRequest进行绑定,所有Bean第一次获取的时候会被缓存到ServletRequestAttributes中(因为传入了request,实际上最终存入了request的map容器中),之后直接从ServletRequestAttributes中获取不再进行创建;

三、源码分析

1. BeanFactory的创建

第一节已经说过ApplicationContext常用的的四个子类都有一个公共的抽象父类AbstractRefreshableApplicationContext,在该类中对BeanFactory进行装饰,一个更重要的点是AbstractRefreshableApplicationContext的父类是AbstractApplicationContext,其refresh方法定义了整个Spring容器启动的过程。

也就是说,无论你采用哪一种ApplicationContext接入Spring容器,最终都会进入AbstractApplicationContext的refresh方法,完成Spring的启动。

源码分析是基于Spring IOC中提出的五点逻辑,发现一篇写的很全面的文章,因此决定不再写了,贴出来共享。

《Spring:源码解读Spring IOC原理》

作为连接文章中未提及的web部分,本文予以补充,进行原理和源码的分析。

2. web Scope(request、session、globalsession)

如上文继承关系所述内容,无论是哪一种ApplicationContext,最终都是通过AbstractBeanFactory.getBean(String)来获取Bean,节选部分代码。

    @Override
public Object get(String name, ObjectFactory<?> objectFactory) {
RequestAttributes attributes = RequestContextHolder.currentRequestAttributes();
Object scopedObject = attributes.getAttribute(name, getScope());
if (scopedObject == null) {
scopedObject = objectFactory.getObject();
attributes.setAttribute(name, scopedObject, getScope());
}
return scopedObject;
}

// 从BeanDefinition中获取scope配置内容 String scopeName = mbd.getScope();

// this.scopes是一个Map<String, Scope>,用于存放Scope对象实例

// 这里如果是request则获取到RequestScope

// 如果是Session则获取到SessionScope final Scope scope = this.scopes.get(scopeName);

if (scope == null){throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");}try {// 这里是重点// 通过Scope.get(String, ObjectFactory)接入到Spring IOC容器中 Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() { @Override public Object getObject() throws BeansException { beforePrototypeCreation(beanName); try { return createBean(beanName, mbd, args); } finally { afterPrototypeCreation(beanName); } } }); bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd); } catch (IllegalStateException ex) { throw new BeanCreationException(beanName, "Scope '" + scopeName + "' is not active for the current thread; " + "consider defining a scoped proxy for this bean if you intend to refer to it from a singleton", ex); }

关于Scope的类结构如下图所示。

【Spring】Spring IOC原理及源码解析之scope=request、session

AbstractRequestAttributeScope定义了get(beanName, ObjectFactory)方法,通过该方法接入Spring IOC容器,源码如下所示。

public Object get(String name, ObjectFactory<?> objectFactory) { // 这里很重要 // RequestContextHolder中通过ThreadLocal将RequestAttributes实例和当前线程绑定 // RequestAttributes在构造的时候需要传入HttpServletRequest,稍后会有源码分析和总结 RequestAttributes attributes = RequestContextHolder.currentRequestAttributes(); // 从当前线程绑定的RequestAttributes中获取对象(实际上是从HttpServletRequest的Map) // 如果已经实例化过了,则不再实例化 // 否则进入Spring IOC过程获取对象 Object scopedObject = attributes.getAttribute(name, getScope()); if (scopedObject == null) { scopedObject = objectFactory.getObject() attributes.setAttribute(name, scopedObject, getScope()); } return scopedObject; }

现在存在的疑问是RequestAttributes、RequestContextHolder是个什么鬼?从哪里来?

要想使用Spring web scope,在web.xml中要配置一个侦听器RequestContextListener,所有谜题的答案都在这里。

public class RequestContextListener implements ServletRequestListener {

    private static final String REQUEST_ATTRIBUTES_ATTRIBUTE =
RequestContextListener.class.getName() + ".REQUEST_ATTRIBUTES"; @Override
public void requestInitialized(ServletRequestEvent requestEvent) {
if (!(requestEvent.getServletRequest() instanceof HttpServletRequest)) {
throw new IllegalArgumentException(
"Request is not an HttpServletRequest: " + requestEvent.getServletRequest());
}
HttpServletRequest request = (HttpServletRequest) requestEvent.getServletRequest();
ServletRequestAttributes attributes = new ServletRequestAttributes(request);
request.setAttribute(REQUEST_ATTRIBUTES_ATTRIBUTE, attributes);
LocaleContextHolder.setLocale(request.getLocale());
RequestContextHolder.setRequestAttributes(attributes);
} @Override
public void requestDestroyed(ServletRequestEvent requestEvent) {
ServletRequestAttributes attributes = null;
Object reqAttr = requestEvent.getServletRequest().getAttribute(REQUEST_ATTRIBUTES_ATTRIBUTE);
if (reqAttr instanceof ServletRequestAttributes) {
attributes = (ServletRequestAttributes) reqAttr;
}
RequestAttributes threadAttributes = RequestContextHolder.getRequestAttributes();
if (threadAttributes != null) {
// We're assumably within the original request thread...
LocaleContextHolder.resetLocaleContext();
RequestContextHolder.resetRequestAttributes();
if (attributes == null && threadAttributes instanceof ServletRequestAttributes) {
attributes = (ServletRequestAttributes) threadAttributes;
}
}
if (attributes != null) {
attributes.requestCompleted();
}
} }

这句代码RequestContextHolder.setRequestAttributes(attributes)是将Spring web scope与Spring IOC结合的关键。

public abstract class RequestContextHolder {

// 将ServletRequestAttributes与当前线程绑定

private static final ThreadLocal<RequestAttributes> requestAttributesHolder = new NamedThreadLocal<RequestAttributes>("Request attributes");

private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder = new NamedInheritableThreadLocal<RequestAttributes>("Request context");

// 1 第一步 public static void setRequestAttributes(RequestAttributes attributes) { setRequestAttributes(attributes, false); }

// 2 第二步,inheritable=false public static void setRequestAttributes(RequestAttributes attributes, boolean inheritable)

{ if (attributes == null) { resetRequestAttributes(); }

else {

if (inheritable) { inheritableRequestAttributesHolder.set(attributes); requestAttributesHolder.remove(); }

else {

// 3 第三步,将ServletRequestAttributes与当前线程绑定 requestAttributesHolder.set(attributes); inheritableRequestAttributesHolder.remove(); } } } //其他源码,略 }

至此,整个过程结束。session的过程几乎和request一致,只是生命周期不一样,多了一把锁。总结一下上述源码分析的内容。

总结:Spring Bean所有scope定义为request、session、globalsession的Bean都不会主动初始化,当第一次通过Spring IOC来getBean的时候才会实例化并进行依赖注入,但是在实例化的时候需要借助于RequestContextHolder(ThreadLocal模式)来将Bean与当前线程中的HttpServletRequest来进行绑定,并在scope的生命周期中缓存在相应的域对象中(request或session)。

最后补充一点,ServletRequestAttributes的getAttribute和setAttribute。在getBean的时候,会先从ServletRequestAttributes.getAttribute中获取Bean,如果获取到则返回,否则进行IOC实例化,并调用ServletRequestAttributes.setArrtibute进行缓存,实际上在setAttribute方法中是调用request.setAttribute来完成。

public class ServletRequestAttributes extends AbstractRequestAttributes {

public static final String DESTRUCTION_CALLBACK_NAME_PREFIX = ServletRequestAttributes.class.getName() + ".DESTRUCTION_CALLBACK.";

private final HttpServletRequest request; private HttpServletResponse response;

private volatile HttpSession session;

private final Map<String, Object> sessionAttributesToupdate = new ConcurrentHashMap<String, Object>(1);

// 1 ServletRequestAttributes对象的创建,必须要一个HttpServletRequest实例

public ServletRequestAttributes(HttpServletRequest request) {

Assert.notNull(request, "Request must not be null"); this.request = request; }

public ServletRequestAttributes(HttpServletRequest request, HttpServletResponse response) { this(request); this.response = response; }

// 转调request.getAttribute或session.getAttribute

public Object getAttribute(String name, int scope) { if (scope == SCOPE_REQUEST) { if (!isRequestActive()) { throw new IllegalStateException( "Cannot ask for request attribute - request is not active anymore!"); } return this.request.getAttribute(name); } else { HttpSession session = getSession(false); if (session != null) { try { Object value =session.getAttribute(name); if (value != null) { this.sessionAttributesToUpdate.put(name, value); } return value; } catch (IllegalStateException ex) { // Session invalidated - shouldn't usually happen. } } return null; } }

// 转调reqeust.setAttribute或session。setAttribute public void setAttribute(String name, Object value, int scope) { if (scope == SCOPE_REQUEST) { if (!isRequestActive()) { throw new IllegalStateException( "Cannot set request attribute - request is not active anymore!"); } this.request.setAttribute(name, value); } else { HttpSession session = getSession(true); this.sessionAttributesToUpdate.remove(name); session.setAttribute(name, value); } }