RequestContextHolder获取request

小灰灰 2022-03-25 05:30 368阅读 0赞

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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；还有另一种则是RequestContextHolder，直接在需要用的地方使用如下方式取HttpServletRequest即可，使用代码如下：

HttpServletRequest request = ((ServletRequestAttributes) RequestContextHolder
            .getRequestAttributes()).getRequest();

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

对于第1个问题，熟悉ThreadLocal的人应该很容易看出来这个是ThreadLocal的应用，这个类的原理在上一篇博文(ThreadLocal原理)有讲到，其实很类似上篇博文文末提到的UserContextHolder。  
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

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。

【多线程】ThreadLocal原理  
使用  
在每个线程的内部有个数据结构为Map的threadLocals变量，以<ThreadLocal,Value>的形式保存着线程变量和其对应的值。  
当使用set()方法时：

1.获取到当前线程的threadLocals，类型为Map  
2.将这值放到这个Map结构的变量中，key为ThreadLocal对象，value为所有存放的值

当使用get()方法时：

1.获取到当前线程的threadLocals，类型为Map。  
2.以ThreadLocal对象为Map的key获取到它的value值。

因为ThreadLocal对象作为Map的key，所以一个ThreadLocal对象只能存放一个值，当存放多个时，会将新值覆盖旧值。

数据结构：

public void set(T value) {
            Thread t = Thread.currentThread();
            ThreadLocalMap map = getMap(t);//当前线程为入参，获取当前线程的threadLocals变量
            if (map != null)
             //入参为this，也就是说key为ThreadLocal对象
                map.set(this, value);
            else
                createMap(t, value);
        }
    
        public T get() {
            Thread t = Thread.currentThread();
            ThreadLocalMap map = getMap(t);//当前线程为入参，获取当前线程的threadLocals
            if (map != null) {
                //入参为this，也就是说key为ThreadLocal
                ThreadLocalMap.Entry e = map.getEntry(this);
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    T result = (T)e.value;
                    return result;
                }
            }
            return setInitialValue();
        }
        
        ThreadLocalMap getMap(Thread t) {
            return t.threadLocals;//threadLocals为线程的变量
        }
        
        private Entry getEntry(ThreadLocal<?> key) {
            int i = key.threadLocalHashCode & (table.length - 1);
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);//避免内存泄漏，下文有提。
        }

[https://www.jianshu.com/p/6bf1adb775e0][https_www.jianshu.com_p_6bf1adb775e0]

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[https_www.jianshu.com_p_6bf1adb775e0]: https://www.jianshu.com/p/6bf1adb775e0