通過RequestContextHolder直接獲取HttpServletRequest對象

問題

朋友遇到一個問題：他想在Service方法中使用HttpServletRequest的API，但是又不想把HttpServletRequest對象當作這個Service方法的參數傳過來，原因是這個方法被N多Controller調用，加一個參數就得改一堆代碼。一句話：就是他懶。不過，這個問題該這么解決呢？

思考

不把HttpServletRequest當作參數傳過來，這意味着要在Service的方法中直接獲取到HttpServletRequest對象。
我們知道，一次請求，Web應用服務器就會分配一個線程去處理。也就是說，在Service方法中獲取到的HttpServletRequest對象需要滿足：線程內共享，線程間隔離。
這恰恰是ThreadLocal的應用場景。

思路

那么，就需要在請求執行之前獲取到HttpServletRequest，把它set()到某個類的ThreadLocal類型的靜態成員中，使用的時候直接通過靜態方式訪問到這個ThreadLocal對象，調用它的get()方法，即可獲取到線程隔離的HttpServletRequest了。最后，在請求結束后，要調用ThreadLocal的remove()方法，清理資源引用。

實現

方式一 利用ServletRequestListener實現

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.http.HttpServletRequest;

public class RequestHolder implements ServletRequestListener {

    private static ThreadLocal<HttpServletRequest> httpServletRequestHolder = 
            new ThreadLocal<HttpServletRequest>();
    
    @Override
    public void requestInitialized(ServletRequestEvent requestEvent) {
        HttpServletRequest request = (HttpServletRequest) requestEvent.getServletRequest();
        httpServletRequestHolder.set(request); // 綁定到當前線程
    }
    
    @Override
    public void requestDestroyed(ServletRequestEvent requestEvent) {
        httpServletRequestHolder.remove(); // 清理資源引用
    }
    
    public static HttpServletRequest getHttpServletRequest() {
        return httpServletRequestHolder.get();
    }
    
}

方式二 利用Filter實現

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;

public class RequestHolder implements Filter {

    private static ThreadLocal<HttpServletRequest> httpServletRequestHolder = 
            new ThreadLocal<HttpServletRequest>();

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        httpServletRequestHolder.set((HttpServletRequest) request); // 綁定到當前線程
        try {
            chain.doFilter(request, response);
        } catch (Exception e) {
            throw e;
        } finally {
            httpServletRequestHolder.remove(); // 清理資源引用
        }
    }

    @Override
    public void destroy() {
    }

    public static HttpServletRequest getHttpServletRequest() {
        return httpServletRequestHolder.get();
    }
    
}

方式三 利用SpringMVC的攔截器實現

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.web.servlet.handler.HandlerInterceptorAdapter;

public class RequestHolder extends HandlerInterceptorAdapter {

    private static ThreadLocal<HttpServletRequest> httpServletRequestHolder = 
            new ThreadLocal<HttpServletRequest>();
    
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) 
        throws Exception {
        httpServletRequestHolder.set(request); // 綁定到當前線程
        return true;
    }
    
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, 
            Object handler, Exception ex) 
        throws Exception {
        httpServletRequestHolder.remove(); // 清理資源引用
    }
    
    public static HttpServletRequest getHttpServletRequest() {
        return httpServletRequestHolder.get();
    }
    
}

調用

無論是哪種方式，都可以直接在Service的方法中執行

HttpServletRequest request = RequestHolder.getHttpServletRequest();

即可直接獲取到線程隔離的HttpServletRequest了。

延伸

類似的功能，在SpringMVC中就有開箱即用的實現。代碼是

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

那么SpringMVC是如何實現的呢？
先看一下RequestContextHolder的源碼（精簡了一下）

public abstract class RequestContextHolder  {

    private static final ThreadLocal<RequestAttributes> requestAttributesHolder = 
            new NamedThreadLocal<RequestAttributes>("Request attributes"); // 重點
    private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder = 
            new NamedInheritableThreadLocal<RequestAttributes>("Request context");

    public static void resetRequestAttributes() {
        requestAttributesHolder.remove(); // 重點
        inheritableRequestAttributesHolder.remove();
    }

    public static void setRequestAttributes(RequestAttributes attributes) {
        setRequestAttributes(attributes, false);
    }

    public static void setRequestAttributes(RequestAttributes attributes, boolean inheritable) {
        if (attributes == null) {
            resetRequestAttributes();
        }
        else {
            if (inheritable) {
                inheritableRequestAttributesHolder.set(attributes);
                requestAttributesHolder.remove();
            }
            else {
                requestAttributesHolder.set(attributes); // 重點
                inheritableRequestAttributesHolder.remove();
            }
        }
    }

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

主要代碼就是把RequestAttributes對象ThreadLocal化，然后提供了setRequestAttributes()、getRequestAttributes()等靜態方法，來放入或取出ThreadLocal中線程隔離的RequestAttributes。
接下來看一下setRequestAttributes()方法是在什么時候調用的呢？

可以看到setRequestAttributes()被initContextHolders()調用，initContextHolders()又被processRequest()調用，而processRequest()在每次請求時都會被調用，無論是GET、POST、PUT、DELETE還是TRACE、OPTIONS等等。
先來看一下processRequest()方法

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(); // 重點1
    ServletRequestAttributes requestAttributes = 
            buildRequestAttributes(request, response, previousAttributes); // 重點2

    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

    initContextHolders(request, localeContext, requestAttributes); // 重點3

    try {
        doService(request, response); // 執行請求
    }
    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 {
        resetContextHolders(request, previousLocaleContext, previousAttributes); // 重點4
        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, startTime, failureCause); // 發布請求處理完成事件
    }
}

重點1

在set之前就先get，通常為null。

重點2

直接看buildRequestAttributes()方法的實現

protected ServletRequestAttributes buildRequestAttributes(HttpServletRequest request, HttpServletResponse response, 
            RequestAttributes previousAttributes) {
    if (previousAttributes == null || previousAttributes instanceof ServletRequestAttributes) {
        return new ServletRequestAttributes(request); // 重點
    }
    else {
        return null;  // preserve the pre-bound RequestAttributes instance
    }
}

ServletRequestAttributes的代碼不再去看了，它就是RequestAttributes接口的實現類，只是對HttpServletRequest對象（還有HttpSession）的一個包裝。

重點3

直接看initContextHolders()方法的實現

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); // 重點
    }
    if (logger.isTraceEnabled()) {
        logger.trace("Bound request context to thread: " + request);
    }
}

調用RequestContextHolder.setRequestAttributes()方法，把requestAttributes對象放入。this.threadContextInheritable默認是false。
即把HttpServletRequest的封裝對象ServletRequestAttributes與當前線程綁定。

重點4

private void resetContextHolders(HttpServletRequest request, LocaleContext prevLocaleContext, 
            RequestAttributes previousAttributes) {
    LocaleContextHolder.setLocaleContext(prevLocaleContext, this.threadContextInheritable);
    RequestContextHolder.setRequestAttributes(previousAttributes, this.threadContextInheritable); // 重點
    if (logger.isTraceEnabled()) {
        logger.trace("Cleared thread-bound request context: " + request);
    }
}

在請求執行完畢后，再次調用RequestContextHolder.setRequestAttributes()，但由於previousAttributes為null，所以，這里相當於調用RequestContextHolder.setRequestAttributes(null, false)。
再回顧一下setRequestAttributes()方法。

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();
        }
    }
}

參數attributes為null，就會調用resetRequestAttributes()，來清理當前線程引用的RequestAttributes。

至此，SpringMVC是如何實現直接獲取HttpServletRequest對象的源碼，就分析完了。和我們自己實現的思路差不多，只不過多繞了幾個彎而已。

 

原文鏈接：https://my.oschina.net/ojeta/blog/801640