SpringMVC中RequetContextListener

来自：https://www.cnblogs.com/softidea/p/7068196.html

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 ，创建request对象
    @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）。