SpringMVC之RequestContextHolder分析

看源碼的時候發現能夠在service獲取request和response,正常來講在service層是沒有request的,然而直接從controlller傳過來的話解決方法太粗暴,後來發現了SpringMVC提供的RequestContextHolder來研究一下,並藉此對SpringMVC的結構深刻了解一下

1.RequestContextHolder的使用

RequestContextHolder顧名思義,持有上下文的Request容器.使用是很簡單的,具體使用以下:

RequestAttributes requestAttributes = RequestContextHolder.currentRequestAttributes();
//RequestContextHolder.getRequestAttributes();
//從session裏面獲取對應的值
String str = (String) requestAttributes.getAttribute("name",RequestAttributes.SCOPE_SESSION);
HttpServletRequest request = ((ServletRequestAttributes)requestAttributes).getRequest();
HttpServletResponse response = ((ServletRequestAttributes)requestAttributes).getResponse();

看到這通常都會想到幾個問題:

request和response怎麼和當前請求掛鉤?
request和response等是何時設置進去的?

2.解決疑問

2.1 request和response怎麼和當前請求掛鉤?

首先分析RequestContextHolder這個類,裏面有兩個ThreadLocal保存當前線程下的request

//獲得存儲進去的request
private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
new NamedThreadLocal<RequestAttributes>("Request attributes");
//可被子線程繼承的request
private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
new NamedInheritableThreadLocal<RequestAttributes>("Request context");

再看getRequestAttributes()方法,至關於直接獲取ThreadLocal裏面的值,這樣就保證了每一次獲取到的Request是該請求的request.

public static RequestAttributes getRequestAttributes() {
        RequestAttributes attributes = requestAttributesHolder.get();
        if (attributes == null) {
            attributes = inheritableRequestAttributesHolder.get();
        }
        return attributes;
    }

2.2request和response等是何時設置進去的?

找這個的話須要對springMVC結構的DispatcherServlet的結構有必定了解才能準確的定位該去哪裏找相關代碼.

要分析的的就是三個類,簡單看下源碼

1. HttpServletBean 進行初始化工做

2. FrameworkServlet 初始化 WebApplicationContext,並提供service方法預處理請

3. DispatcherServlet 具體分發處理.

那麼就能夠在FrameworkServlet查看到該類重寫了service(),doGet(),doPost()...等方法,這些實現裏面都有一個預處理方法processRequest(request, response);,因此定位到了咱們要找的位置

查看processRequest(request, response);的實現,具體能夠分爲三步:

1. 獲取上一個請求的參數
2. 從新創建新的參數
3. 設置到XXContextHolder
4. 父類的service()處理請求
5. 恢復request
6. 發佈事