Spring 单例 httprequest 线程安全

@Autowired HttpServletRequest之所以线程安全是因为， httpsevletRequest 储存在 RequestContextHolder中。

• 每次http请求的doXXX 都会被FrameworkServlet拦截，通过 RequestContextHolder.setxxxxx  写入TheadLocal。
• Autowired 获取request的时候，通过RequestContextHolder.getxxx 从ThreadLocal中获取。

为什么Autowired HttpServletRequest是线程安全的，获取的方式

1. 启动断点调试，查看request的来源是  WebApplicationContextUtils.RequestObjectFactory.

2. 查看源码 WebApplicationContextUtils.RequestObjectFactory, request 来自于 RequestContextHolder.currentRequestAttributes() 方法

class WebApplicationContextUtils {
    @SuppressWarnings("serial")
    private static class RequestObjectFactory implements ObjectFactory<ServletRequest>, Serializable {

        @Override
        public ServletRequest getObject() {
            return currentRequestAttributes().getRequest();
        }

        @Override
        public String toString() {
            return "Current HttpServletRequest";
        }
    }

    private static ServletRequestAttributes currentRequestAttributes() {
        RequestAttributes requestAttr = RequestContextHolder.currentRequestAttributes();
        if (!(requestAttr instanceof ServletRequestAttributes)) {
            throw new IllegalStateException("Current request is not a servlet request");
        }
        return (ServletRequestAttributes) requestAttr;
    }

}

3. 上述方法的attributes来自于线程安全的ThreadLocal中的当前线程的HttpServletRequest

public abstract class RequestContextHolder  {

    private static final boolean jsfPresent =
            ClassUtils.isPresent("javax.faces.context.FacesContext", RequestContextHolder.class.getClassLoader());

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

    public static void setRequestAttributes(@Nullable RequestAttributes attributes, boolean inheritable) {
        if (attributes == null) {
            resetRequestAttributes();
        }
        else {
            if (inheritable) {
                inheritableRequestAttributesHolder.set(attributes);
                requestAttributesHolder.remove();
            }
            else {
                requestAttributesHolder.set(attributes);
                inheritableRequestAttributesHolder.remove();
            }
        }
    }

设置的方式

request是什么时候设置到threadlocal中去的呢? 是在Springmvc的dispatcherServlet的父类FrameworkServlet里操作的.。 doGet 、doPost 、doXXX方法都是委托processRequest方法去做的. 也就是说请求方法会被FrameworkServlet的processRequest拦截。

class FrameworkServlet {
   protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
          .....
        initContextHolders(request, localeContext, requestAttributes);
   }

    private void initContextHolders(HttpServletRequest request,
            @Nullable LocaleContext localeContext, @Nullable RequestAttributes requestAttributes) {

        ......
        if (requestAttributes != null) {
            RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
        }
        ....
    }

}

参考

https://www.cnblogs.com/abcwt112/p/7777258.html