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一、RequestContextHolder的使用
在Web开发中，Service层或者某个工具类中需要获取到HttpServletRequest对象还是比较常见的。一种方式是将HttpServletRequest作为方法的参数从Controller层一直放下传递，不过这种有点费劲，且做起来不是优雅；还有另一种方式则是使用RequestContextHolder，可以新建一个工具类HttpUtils调用getRequest()获取HttpServletRequest即可。不把HttpServletRequest当作参数传过来，这意味着要在Service的方法中直接获取到HttpServletRequest对象。
我们知道，一次请求，Web应用服务器就会分配一个线程去处理。也就是说，在Service方法中获取到的HttpServletRequest对象需要满足：线程内共享，线程间隔离。这恰恰是ThreadLocal的应用场景。
RequestContextHolder的使用代码如下：
public class HttpUtils {
......
/**
* 获取session.
*
* @return
*/
public static HttpSession getSession() {
RequestAttributes ra = RequestContextHolder.getRequestAttributes();
HttpServletRequest request =
((ServletRequestAttributes) ra).getRequest();
return request.getSession();
}
/**
* 获取Request.
*
* @return
*/
public static HttpServletRequest getRequest() {
RequestAttributes ra = RequestContextHolder.getRequestAttributes();
HttpServletRequest request =
((ServletRequestAttributes) ra).getRequest();
return request;
}
......
}要理解上面的为何可以这么使用，需要理解两个问题：
- RequestContextHolder为什么能获取到当前的HttpServletRequest
- HttpServletRequest是在什么时候设置到RequestContextHolder中
第1个问题，熟悉ThreadLocal的人应该很容易看出来这个是ThreadLocal的应用。关于ThreadLocal可以参考我曾经看过的一篇博客：https://www.cnblogs.com/shuilangyizu/p/8621733.html
//得到存储进去的request
private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
new NamedThreadLocal<RequestAttributes>("Request attributes");
//可被子线程继承的request
private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
new NamedInheritableThreadLocal<RequestAttributes>("Request context");再看`getRequestAttributes()`方法,相当于直接获取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。
public static RequestAttributes getRequestAttributes() {
RequestAttributes attributes = requestAttributesHolder.get();
if (attributes == null) {
attributes = inheritableRequestAttributesHolder.get();
}
return attributes;
}第2个问题，应该属于spring-mvc的问题，这个是在spring-mvc执行时设置进去的
二、RequestContextHolder源码分析
首先我们先来看下RequestContextHolder的源码，源码如下：
public abstract class RequestContextHolder {
private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
new NamedThreadLocal<RequestAttributes>("Request attributes");
private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
new NamedInheritableThreadLocal<RequestAttributes>("Request context");
public static void resetRequestAttributes() {
requestAttributesHolder.remove();
inheritableRequestAttributesHolder.remove();
}
public static void setRequestAttributes(RequestAttributes attributes) {
setRequestAttributes(attributes, false);
}
//将RequestAttributes对象放入到ThreadLocal中，而HttpServletRequest和HttpServletResponse等则封装在RequestAttributes对象中，在此处就不对RequestAttributes这个类展开。反正我们需要知道的就是要获取RequestAttributes对象，然后再从RequestAttributes对象中获取到我们所需要的HttpServletRequest即可
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();
}
}
}
public static RequestAttributes getRequestAttributes() {
RequestAttributes attributes = requestAttributesHolder.get();
if (attributes == null) {
attributes = inheritableRequestAttributesHolder.get();
}
return attributes;
}
}那么在spring-mvc中是怎么实现的呢，我们来简单分析的，想了解具体机制的可以去看看spring-mvc的源码。
我们看下FrameworkServlet这个类，也就是DispatcherServlet的父类，里面有个processRequest方法，根据方法名称我们也可以大概了解到这个是方法用于处理请求的。
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContext localeContext = buildLocaleContext(request);
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
//将RequestAttributes设置到RequestContextHolder
initContextHolders(request, localeContext, requestAttributes);
try {
//具体的业务逻辑
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 {
//重置RequestContextHolder之前设置RequestAttributes
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");
}
}
}
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
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);
}
}简单看下源码，我们可以知道HttpServletRequest是在执行doService方法之前，也就是具体的业务逻辑前进行设置的，然后在执行完业务逻辑或者抛出异常时重置RequestContextHolder移除当前的HttpServletRequest。
希望大家能理解RequestContextHolder的使用，并希望以后能在业务代码中巧用该工具让自己的代码更加简洁优雅。