由SpringMVC中RequetContextListener说起

零、引言

RequetContextListener从名字结尾Listener来看就知道属于监听器。
所谓监听器就是监听某种动作，在其开始（初始化）和结束（销毁）的时候进行某些操作。
由此可以猜测：该类用于在RequetContext（请求上下文对象）创建和销毁的时候进行某些操作（哪些操作？结尾总结!）

一、web.xml配置

要使用该listener对象需要在web.xml中进行如下配置。

<!-- 此监听器是监听HttpRequest对象，方便ContextHolderUtils程序调用HttpRequest对象 -->
<listener>
<description>request监听器</description>
<listener-class>org.springframework.web.context.request.RequestContextListener</listener-class>
</listener>

二、三个重要类解读

2.1 RequetContextListener

public class RequestContextListener implements ServletRequestListener {
private static final String REQUEST_ATTRIBUTES_ATTRIBUTE =
RequestContextListener.class.getName() + ".REQUEST_ATTRIBUTES";
// 在请求进入的时候，初始化变量放入ThreadLocal<T>中
@Override
public void requestInitialized(ServletRequestEvent requestEvent) {
//判定当前的requetEvent中获取的ServletRequest()对象是否是HttpServletRequet对象
if (!(requestEvent.getServletRequest() instanceof HttpServletRequest)) {
throw new IllegalArgumentException(
"Request is not an HttpServletRequest: " + requestEvent.getServletRequest());
}
//强制转型为 HttpServletRequest
HttpServletRequest request = (HttpServletRequest) requestEvent.getServletRequest();
// ServletRequestAttributes 保存了HttpServletRequet、Response、Session等变量
ServletRequestAttributes attributes = new ServletRequestAttributes(request);
request.setAttribute(REQUEST_ATTRIBUTES_ATTRIBUTE, attributes);
LocaleContextHolder.setLocale(request.getLocale());
//RequestContextHolder里面有一个ThreadLocal，当前线程共享
RequestContextHolder.setRequestAttributes(attributes);
}
//在请求被销毁的时候，将在初始化时候的ThreadLocal变量清空。
@Override
public void requestDestroyed(ServletRequestEvent requestEvent) {
...
}
}

2.2 RequetContextHolder

public abstract class RequestContextHolder  {
//ThreadLocal<T>变量用于保存当前线程的共享变量
private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
new NamedThreadLocal<RequestAttributes>("Request attributes");
private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
new NamedInheritableThreadLocal<RequestAttributes>("Request context");
/**
* 将线程中的共享变量清除掉，会在RequetContextListner的destory()方法中调用。
*/
public static void resetRequestAttributes() {
//清空变量
requestAttributesHolder.remove();
inheritableRequestAttributesHolder.remove();
}
//过渡方法
public static void setRequestAttributes(RequestAttributes attributes) {
setRequestAttributes(attributes, false);
}
// 核心的方式：将RequetAttrubutes(Request/Response/Session)放入到ThreadLocal<T>中进行共享
public static void setRequestAttributes(RequestAttributes attributes, boolean inheritable) {
if (attributes == null) {
resetRequestAttributes();
}
else {
if (inheritable) {
inheritableRequestAttributesHolder.set(attributes);
requestAttributesHolder.remove();
}
else {
requestAttributesHolder.set(attributes);
inheritableRequestAttributesHolder.remove();
}
}
}

2.3 ServletRequestAttributes

public class ServletRequestAttributes extends AbstractRequestAttributes {
private final HttpServletRequest request;
private HttpServletResponse response;
private volatile HttpSession session;
private final Map<String, Object> sessionAttributesToUpdate = new ConcurrentHashMap<String, Object>(1);
}

三、使用方法

从以上可以知道RuquetAttribute是放在了ThreadLocal中，则在该次请求中，可以在任意的代码位置中获取该该对象，由此拿到HttpServletRequet等对象。

HttpServletRequest request =((ServletRequestAttributes)RequestContextHolder.getRequestAttributes()).getRequest();

利用该方法可以在任意位置获取request对象.

四、总结

RequetContextListner主要作用就一个：
将本次请求的 ServletRequestAttributes 对象 保存在ThreadLocal中，方便在某一次请求的任意代码位置获取（包括直接在service层获取）。 http://www.cnblogs.com/LiuChunfu/p/7067828.html