为什么HttpServletRequest能够autowired注入？为什么多请求不混乱？

小咪咪 2022-11-27 08:48 145阅读 0赞

# 问题的引入 #

最近看代码里发现有将`HttpServletRequest` 直接通过@Autowired注入。我有点想不明白，注入的默认是单例，这样不会出问题吗？为了研究明白为什么，我写了个简单的demo研究了下。

# demo代码 #

以下是springmvc里的一个controller注入了一个httpServletRequest。  
有个简单的get方法，将2个request里的header 同时打印出来，查看结果。

@RestController
    public class SimpleController { 
    
    
        //注入了一个httpServletRequest
        @Autowired
        HttpServletRequest autoRequest;
        
         @RequestMapping("/get")
        public String get(HttpServletRequest request){ 
            System.out.println("------------------------------------------");
            Enumeration<String> en=autoRequest.getHeaderNames();
            System.out.println(autoRequest);
            while(en.hasMoreElements()){ 
                String key;
                System.out.println((key=en.nextElement()) +" = "+autoRequest.getHeader(key));
            }
            System.out.println("=========================================");
            Enumeration<String> en2=request.getHeaderNames();
            System.out.println(request);
            while(en2.hasMoreElements()){ 
                String key;
                System.out.println((key=en2.nextElement()) +" = "+request.getHeader(key));
            }
    
            String uuid=(String) request.getSession().getAttribute("uuid");
            System.out.println(request.getSession().getId());
            return uuid==null?"none":uuid;
        }
    
    }

2个浏览器分别call后打印出结果对比，发现打印的cookie会改变，打印出的结果并没有想象中autoRequest不变的结果。  
带着疑问，第一个问题就是

# HttpServletRequest 是怎么注入进去的？ #

由于知道，所有注入的的bean都是需要通过beanfactory的 getBean获取得到。于是断点`AbstractBeanFactory.doGetBean`然后通过条件断点，期望能够断到HttpServletRequest的获取的过程。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1b194bA_size_16_color_FFFFFF_t_70_pic_center]  
遗憾的是不能够获取到。于是换了思路,写了个after

@RestController
    public class SimpleController { 
    
        @Autowired
        HttpServletRequest autoRequest;
        @PostConstruct
        public void after(){ 
            //断点这里查看
            System.out.println(this.autoRequest);
        }
    }

发现有个ObjectFactory,顾名思义是生产出 request的工厂类  
![在这里插入图片描述][202008141600092.png_pic_center]  
进入到WebApplicatonContextUtils.java，发现这里放入了一个生产出HttpServletRequest的beanFactory  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1b194bA_size_16_color_FFFFFF_t_70_pic_center 1]

private static class RequestObjectFactory implements ObjectFactory<ServletRequest>, Serializable { 
    
    		@Override
    		public ServletRequest getObject() { 
    			return currentRequestAttributes().getRequest();
    		}
    
    		@Override
    		public String toString() { 
    			return "Current HttpServletRequest";
    		}
    	}

至此应该就能明白注入的地方了。那下个疑问，

# @autowired request 是怎么找到这个RequestObjectFactory #

我们知道 AutowiredAnnotationBeanPostProcessor 是用来处理bean里的属性的注入的. 是众多的BeanPostProcessor里的一种  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1b194bA_size_16_color_FFFFFF_t_70_pic_center 2]  
从`AutowiredAnnotationBeanPostProcessor`最后到`DefaultListableBeanFactory.findAutowireCandidates`

protected Map<String, Object> findAutowireCandidates(
    			@Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) { 
    
    		String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
    				this, requiredType, true, descriptor.isEager());
    		Map<String, Object> result = new LinkedHashMap<>(candidateNames.length);
       //重点在这里this.resolvableDependencies
       //之前beanFactory.registerResolvableDependency(ServletRequest.class, new RequestObjectFactory());
       //实际上就是this.resolvableDependencies.put(dependencyType, autowiredValue);
       
    		for (Map.Entry<Class<?>, Object> classObjectEntry : this.resolvableDependencies.entrySet()) { 
    			Class<?> autowiringType = classObjectEntry.getKey();
    			//循环到这里发现有个key的class 是ServletRequest的派生类 
    			if (autowiringType.isAssignableFrom(requiredType)) { 
    			     //拿到autowiringValue ，就是RequestObjectFactory
    				Object autowiringValue = classObjectEntry.getValue();
    				//调用AutowireUtils.resolveAutowiringValue创建出一个对象
    				autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
    				if (requiredType.isInstance(autowiringValue)) { 
    					result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
    					break;
    				}
    			}
    		}
    		for (String candidate : candidateNames) { 
    			if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) { 
    				addCandidateEntry(result, candidate, descriptor, requiredType);
    			}
    		}
    		if (result.isEmpty()) { 
    			boolean multiple = indicatesMultipleBeans(requiredType);
    			// Consider fallback matches if the first pass failed to find anything...
    			DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch();
    			for (String candidate : candidateNames) { 
    				if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, fallbackDescriptor) &&
    						(!multiple || getAutowireCandidateResolver().hasQualifier(descriptor))) { 
    					addCandidateEntry(result, candidate, descriptor, requiredType);
    				}
    			}
    			if (result.isEmpty() && !multiple) { 
    				// Consider self references as a final pass...
    				// but in the case of a dependency collection, not the very same bean itself.
    				for (String candidate : candidateNames) { 
    					if (isSelfReference(beanName, candidate) &&
    							(!(descriptor instanceof MultiElementDescriptor) || !beanName.equals(candidate)) &&
    							isAutowireCandidate(candidate, fallbackDescriptor)) { 
    						addCandidateEntry(result, candidate, descriptor, requiredType);
    					}
    				}
    			}
    		}
    		return result;
    	}

结论就是  
1.beanFactory.registerResolvableDependency(ServletRequest.class, new RequestObjectFactory()); 实际上就是往resolvableDependencies这个map里扔  
2.AutowiredAnnotationBeanPostProcessor 处理的时候要去找一个ServletRequest，会在resolvableDependencies找，找到一个key为ServletRequest.class的类就是RequestObjectFactory

至此怎么找到 RequestObjectFactory 也明白了。  
也到了快要揭开为什么多请求不混乱？

# 为什么多请求不混乱？ #

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;
    	}

public static RequestAttributes currentRequestAttributes() throws IllegalStateException { 
    		RequestAttributes attributes = getRequestAttributes();
    		...
    		return attributes;
    	}

//答案在这里，这个是一个ThreadLocal对象，也就是每个request的请求都会将封装好的RequestAttributes放到这个ThreadLocal里。
         private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
    			new NamedThreadLocal<>("Request attributes");
    
    	 private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
    			new NamedInheritableThreadLocal<>("Request context");
         public static RequestAttributes getRequestAttributes() { 
    		RequestAttributes attributes = requestAttributesHolder.get();
    		if (attributes == null) { 
    			attributes = inheritableRequestAttributesHolder.get();
    		}
    		return attributes;
    	}

既然是ThreadLocal一定就会有set的地方，很容易看到RequestContextHolder的setRequestAttributes方法，断点到这里，在请求一次。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1b194bA_size_16_color_FFFFFF_t_70_pic_center 3]  
可以看出请求过来会进入RequestContextFilter这个filter,这里会将request给设置到当前的线程里。

protected void doFilterInternal(
    			HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
    			throws ServletException, IOException { 
    
    		ServletRequestAttributes attributes = new ServletRequestAttributes(request, response);
    		//将request设到当前的线程里
    		initContextHolders(request, attributes);
    
    		try { 
    		//继续走filterChain里的其它filter
    			filterChain.doFilter(request, response);
    		}
    		finally { 
    		  //最后当请求返回后，需要将清除线程里的threadlocal
    			resetContextHolders();
    			if (logger.isTraceEnabled()) { 
    				logger.trace("Cleared thread-bound request context: " + request);
    			}
    			attributes.requestCompleted();
    		}
    	}

至此，完整的明白了HttpServletRequest能autowired注入，并且多请求不混乱。

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[202008141600092.png_pic_center]: /images/20221124/5a3088429c07471da5e6c40a72e83562.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1b194bA_size_16_color_FFFFFF_t_70_pic_center 1]: /images/20221124/85cbcc3e43ea4b3e8ba8d84c09e37bfa.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1b194bA_size_16_color_FFFFFF_t_70_pic_center 2]: /images/20221124/71c081534abd4a7285a48690734ab0c3.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1b194bA_size_16_color_FFFFFF_t_70_pic_center 3]: /images/20221124/b2e8a440258f445ca41e60fef1f60d41.png