RequestContextHolder詳解

一、概述

RequestContextHolder是Spring框架中作為HTTP請求上下文綁定的持有者，可以方便地隨時獲取當前請求的HttpServletRequest、HttpServletResponse等信息，這對於Web開發來說是不可或缺的。

在Web應用程序中，一個請求可能會經過多個線程，每個線程都會處理請求的某一個環節。RequestContextHolder可以確保在整個請求處理過程中，我們都能夠訪問到該請求的相關信息。

二、RequestContextHolder分類

Spring框架提供了兩種不同類型的RequestContextHolder：

1、RequestContextHolder使用ThreadLocal保存請求信息

public class RequestContextHolder {
 
    private static final ThreadLocal requestAttributesHolder =
            new NamedThreadLocal("Request attributes");
 
    public static void resetRequestAttributes() {
        requestAttributesHolder.remove();
    }
 
    public static void setRequestAttributes(RequestAttributes attributes) {
        Assert.notNull(attributes, "Only non-null request attributes are permitted");
        requestAttributesHolder.set(attributes);
    }
 
    public static RequestAttributes getRequestAttributes() {
        RequestAttributes attributes = requestAttributesHolder.get();
        if (attributes == null) {
            attributes = new NativeWebRequestAttributes(new MockHttpServletRequest());
            requestAttributesHolder.set(attributes);
        }
        return attributes;
    }
 
    public static void setRequestAttributes(ServletRequest request, ServletResponse response) {
        setRequestAttributes(new ServletWebRequest(request, response));
    }
 
    public static void setRequestAttributes(ServletRequestAttributes attributes) {
        Assert.notNull(attributes, "ServletRequestAttributes must not be null");
        requestAttributesHolder.set(attributes);
    }
 
    public static ServletRequestAttributes getRequestAttributes() {
        RequestAttributes attributes = getRequestAttributes();
        Assert.state(attributes instanceof ServletRequestAttributes,
                "Request attribute does not support request access");
        return (ServletRequestAttributes) attributes;
    }
}

上述代碼中，使用ThreadLocal保存請求的信息，如從HttpServletRequest中獲取的請求信息，這樣可以保證每個線程都能夠訪問到已綁定請求的信息，線程之間不會相互影響。

2、RequestContextHolder使用InheritableThreadLocal保存請求信息

public class RequestContextHolder {
 
    private static final InheritableThreadLocal requestAttributesHolder =
            new NamedInheritableThreadLocal("Request attributes");
 
    public static void resetRequestAttributes() {
        requestAttributesHolder.remove();
    }
 
    public static void setRequestAttributes(RequestAttributes attributes) {
        Assert.notNull(attributes, "Only non-null request attributes are permitted");
        requestAttributesHolder.set(attributes);
    }
 
    public static RequestAttributes getRequestAttributes() {
        RequestAttributes attributes = requestAttributesHolder.get();
        if (attributes == null) {
            attributes = new NativeWebRequestAttributes(new MockHttpServletRequest());
            requestAttributesHolder.set(attributes);
        }
        return attributes;
    }
 
    public static void setRequestAttributes(ServletRequest request, ServletResponse response) {
        setRequestAttributes(new ServletWebRequest(request, response));
    }
 
    public static void setRequestAttributes(ServletRequestAttributes attributes) {
        Assert.notNull(attributes, "ServletRequestAttributes must not be null");
        requestAttributesHolder.set(attributes);
    }
 
    public static ServletRequestAttributes getRequestAttributes() {
        RequestAttributes attributes = getRequestAttributes();
        Assert.state(attributes instanceof ServletRequestAttributes,
                "Request attribute does not support request access");
        return (ServletRequestAttributes) attributes;
    }
}

與第一種方式不同的是，該代碼中使用InheritableThreadLocal保存請求的信息，InheritableThreadLocal是ThreadLocal類的子類，它所保存的值可以被子線程繼承，在子線程中可以訪問到保存的信息，由於InheritableThreadLocal是可繼承的，所以是跨線程的，內存消耗很大，開發時需慎重考慮。

三、RequestContextHolder的用法

1、獲取當前請求的HttpServletRequest

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

2、獲取當前請求的HttpServletResponse

HttpServletResponse response = ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getResponse();

3、獲取當前請求的RequestAttributes

RequestAttributes attributes = RequestContextHolder.getRequestAttributes();

4、通過RequestContextHolder為當前請求創建的子線程傳遞請求上下文

在多個線程的情況下，如果想把同一個請求傳遞給同一個線程來進行處理，可以先在原請求線程中獲取當前請求上下文，並傳遞到另外一個線程中，代碼示例如下：

原請求線程中：

RequestAttributes attributes = RequestContextHolder.getRequestAttributes();

新線程中：

RequestContextHolder.setRequestAttributes(attributes);

5、在Controller層使用RequestContextHolder的示例：

@RestController
@RequestMapping("/test")
public class TestController {
    @GetMapping("/getRequest")
    public String getRequest(HttpServletRequest request) {
        HttpSession session = request.getSession();
        request.setAttribute("name", "test");
        return ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getRequest().getAttribute("name").toString();
    }
}

四、小結

RequestContextHolder是Spring框架中作為HTTP請求上下文綁定的持有者，可以方便地隨時獲取當前請求的HttpServletRequest、HttpServletResponse等信息，能夠確保在整個請求處理過程中，我們都能夠訪問到該請求的相關信息，對Web開發來說非常重要。