一、概述
-
RequestContextHolder：持有上下文的Request容器
-
获取Request对象常见的几种方式, 通过IOC进行获取, 通过传参获取, 通过RequestContextHolder获取
-
通过RequestContextHolder的静态方法可以随时随地取到当前请求的request对象
public class RequestContext { public static void setAttr(String key, Object value) { HttpServletRequest request = RequestContext.getRequest(); request.setAttribute(key, value); } public static Object getAttr(String key) { HttpServletRequest request = RequestContext.getRequest(); return request.getAttribute(key); } protected static ServletRequestAttributes getRequestAttributes() { try { RequestAttributes attributes = RequestContextHolder.getRequestAttributes(); return (ServletRequestAttributes) attributes; } catch (Exception e) { throw new BusinessException("获取RequestAttributes失败!"); } } public static HttpServletRequest getRequest() { try { return getRequestAttributes().getRequest(); } catch (Exception e) { throw new BusinessException("获取request对象失败!"); } } }
二、RequestContextHolder 源码
RequestAttributes 存储在TL中, 这样就可以保证用户线程每次请求的时候都能够拿到一个新的对象, 从而保证在每一次请求中, RequestAttributes都是唯一的
public abstract class RequestContextHolder {
private static final boolean jsfPresent = ClassUtils.isPresent("javax.faces.context.FacesContext", RequestContextHolder.class.getClassLoader());
/** 得到存储进去的request */
private static final ThreadLocal<RequestAttributes> requestAttributesHolder = new NamedThreadLocal("Request attributes");
/** 可被子线程继承的request */
private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder = new NamedInheritableThreadLocal("Request context");
public RequestContextHolder() {
}
public static void resetRequestAttributes() {
requestAttributesHolder.remove();
inheritableRequestAttributesHolder.remove();
}
public static void setRequestAttributes(@Nullable RequestAttributes attributes) {
setRequestAttributes(attributes, false);
}
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();
}
}
//直接获取ThreadLocal里面的值,这样就保证了每一次获取到的Request是该请求的request.
@Nullable
public static RequestAttributes getRequestAttributes() {
RequestAttributes attributes = (RequestAttributes)requestAttributesHolder.get();
if (attributes == null) {
attributes = (RequestAttributes)inheritableRequestAttributesHolder.get();
}
return attributes;
}
public static RequestAttributes currentRequestAttributes() throws IllegalStateException {
RequestAttributes attributes = getRequestAttributes();
if (attributes == null) {
if (jsfPresent) {
attributes = RequestContextHolder.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: In this case, use RequestContextListener or RequestContextFilter to expose the current request.");
}
}
return attributes;
}
private static class FacesRequestAttributesFactory {
private FacesRequestAttributesFactory() {
}
@Nullable
public static RequestAttributes getFacesRequestAttributes() {
FacesContext facesContext = FacesContext.getCurrentInstance();
return facesContext != null ? new FacesRequestAttributes(facesContext) : null;
}
}
}
三、问题
1. request和response怎么和当前请求挂钩
getRequestAttributes()：直接获取ThreadLocal里面的值，保证了每一次获取到的Request是该请求的request.
2. request和response等是什么时候设置进去的
FrameworkServlet类重写了service()、doGet()、doPost() 等方法，方法里面都有一个预处理方法 processRequest(request, response)； 查看 processRequest(request, response) 的实现：
- 获取上一个请求的参数
- 重新建立新的参数
- 设置到XXContextHolder
- 处理父类的service()处理请求
- 恢复request 发布事件
processRequest 源码
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
// 获取上一个请求保存的LocaleContext
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
// 建立新的LocaleContext
LocaleContext localeContext = buildLocaleContext(request);
// 获取上一个请求保存的RequestAttributes
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
// 建立新的ServletRequestAttributes
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
// 将ServletRequestAttributes设置到RequestContextHolder中, 这也是为什么RequestAttributes可以强转的原因
initContextHolders(request, localeContext, requestAttributes);
try {
// 执行对应的业务方法
doService(request, response);
}
catch (ServletException ex) {
failureCause = ex;
throw ex;
}
catch (IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}
finally {
// 移除TL和ITL中的数据
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
if (logger.isDebugEnabled()) {
if (failureCause != null) {
this.logger.debug("Could not complete request", failureCause);
}
else {
if (asyncManager.isConcurrentHandlingStarted()) {
logger.debug("Leaving response open for concurrent processing");
}
else {
this.logger.debug("Successfully completed request");
}
}
}
//发布事件
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
initContextHolders(request, localeContext, requestAttributes)
把新的RequestAttributes设置进LocalThread，实际上保存的类型为ServletRequestAttributes，这也是为什么在使用的时候可以把RequestAttributes强转为ServletRequestAttributes。
private void initContextHolders(
HttpServletRequest request, LocaleContext localeContext, RequestAttributes requestAttributes) {
if (localeContext != null) {
LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);
}
if (requestAttributes != null) {
RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
}
if (logger.isTraceEnabled()) {
logger.trace("Bound request context to thread: " + request);
}
}
