Spring boot 之RequestContextHolder读取

在spring boot中，为了随时都能取到当前请求的request对象，可以通过RequestContextHolder的静态方法getRequestAttributes()获取Request相关的变量，如request, response等。如下：

public static HttpServletRequest getRequest() {

return 
 ((ServletRequestAttributes)RequestContextHolder.getRequestAttributes()).getRequest();

}

是什么原因，可以让我们随时都能取出request或者response对象呢，猜测是存在ThreadLocal：本地线程变量，存储在这个里面的数据可以在整个线程期都可见，线程一旦执行完成就回收，具体源码如下：

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<RequestAttributes>("Request attributes");

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


	/**
	 * Reset the RequestAttributes for the current thread.
	 */
	public static void resetRequestAttributes() {
		requestAttributesHolder.remove();
		inheritableRequestAttributesHolder.remove();
	}

	/**
	 * Bind the given RequestAttributes to the current thread,
	 * <i>not</i> exposing it as inheritable for child threads.
	 * @param attributes the RequestAttributes to expose
	 * @see #setRequestAttributes(RequestAttributes, boolean)
	 */
	public static void setRequestAttributes(RequestAttributes attributes) {
		setRequestAttributes(attributes, false);
	}

	/**
	 * Bind the given RequestAttributes to the current thread.
	 * @param attributes the RequestAttributes to expose,
	 * or {@code null} to reset the thread-bound context
	 * @param inheritable whether to expose the RequestAttributes as inheritable
	 * for child threads (using an {@link InheritableThreadLocal})
	 */
	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();
			}
		}
	}

	/**
	 * Return the RequestAttributes currently bound to the thread.
	 * @return the RequestAttributes currently bound to the thread,
	 * or {@code null} if none bound
	 */
	public static RequestAttributes getRequestAttributes() {
		RequestAttributes attributes = requestAttributesHolder.get();
		if (attributes == null) {
			attributes = inheritableRequestAttributesHolder.get();
		}
		return attributes;
	}

	/**
	 * Return the RequestAttributes currently bound to the thread.
	 * <p>Exposes the previously bound RequestAttributes instance, if any.
	 * Falls back to the current JSF FacesContext, if any.
	 * @return the RequestAttributes currently bound to the thread
	 * @throws IllegalStateException if no RequestAttributes object
	 * is bound to the current thread
	 * @see #setRequestAttributes
	 * @see ServletRequestAttributes
	 * @see FacesRequestAttributes
	 * @see javax.faces.context.FacesContext#getCurrentInstance()
	 */
	public static RequestAttributes currentRequestAttributes() throws IllegalStateException {
		RequestAttributes attributes = getRequestAttributes();
		if (attributes == null) {
			if (jsfPresent) {
				attributes = FacesRequestAttributesFactory.getFacesRequestAttributes();
			}
			if (attributes == null) {
				throw new IllegalStateException("No thread-bound request found: " +
						"Are you referring to request attributes outside of an actual web request, " +
						"or processing a request outside of the originally receiving thread? " +
						"If you are actually operating within a web request and still receive this message, " +
						"your code is probably running outside of DispatcherServlet/DispatcherPortlet: " +
						"In this case, use RequestContextListener or RequestContextFilter to expose the current request.");
			}
		}
		return attributes;
	}


	/**
	 * Inner class to avoid hard-coded JSF dependency.
 	 */
	private static class FacesRequestAttributesFactory {

		public static RequestAttributes getFacesRequestAttributes() {
			FacesContext facesContext = FacesContext.getCurrentInstance();
			return (facesContext != null ? new FacesRequestAttributes(facesContext) : null);
		}
	}

}

正如期望的一样使用ThreadLocal存储请求。

如果报空指针异常，需要在配置类中添加 RequestContextListener 的bean，如下：

@Bean
public RequestContextListener requestContextListener(){
	    return new RequestContextListener();
	}