通过RequestContextHolder直接获取HttpServletRequest对象

问题

朋友遇到一个问题：他想在Service方法中使用HttpServletRequest的API，但是又不想把HttpServletRequest对象当作这个Service方法的参数传过来，原因是这个方法被N多Controller调用，加一个参数就得改一堆代码。一句话：就是他懒。不过，这个问题该这么解决呢？

思考

不把HttpServletRequest当作参数传过来，这意味着要在Service的方法中直接获取到HttpServletRequest对象。
我们知道，一次请求，Web应用服务器就会分配一个线程去处理。也就是说，在Service方法中获取到的HttpServletRequest对象需要满足：线程内共享，线程间隔离。
这恰恰是ThreadLocal的应用场景。

思路

那么，就需要在请求执行之前获取到HttpServletRequest，把它set()到某个类的ThreadLocal类型的静态成员中，使用的时候直接通过静态方式访问到这个ThreadLocal对象，调用它的get()方法，即可获取到线程隔离的HttpServletRequest了。最后，在请求结束后，要调用ThreadLocal的remove()方法，清理资源引用。

实现

方式一 利用ServletRequestListener实现

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); // 绑定到当前线程
    }

    @Override
    public void requestDestroyed(ServletRequestEvent requestEvent) {
        httpServletRequestHolder.remove(); // 清理资源引用
    }

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

}

方式二 利用Filter实现

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); // 绑定到当前线程
        try {
            chain.doFilter(request, response);
        } catch (Exception e) {
            throw e;
        } finally {
            httpServletRequestHolder.remove(); // 清理资源引用
        }
    }

    @Override
    public void destroy() {
    }

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

}

方式三 利用SpringMVC的拦截器实现

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); // 绑定到当前线程
        return true;
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response,
            Object handler, Exception ex)
        throws Exception {
        httpServletRequestHolder.remove(); // 清理资源引用
    }

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

}

调用

无论是哪种方式，都可以直接在Service的方法中执行

HttpServletRequest request = RequestHolder.getHttpServletRequest();

即可直接获取到线程隔离的HttpServletRequest了。

延伸

类似的功能，在SpringMVC中就有开箱即用的实现。代码是

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

那么SpringMVC是如何实现的呢？
先看一下RequestContextHolder的源码（精简了一下）

public abstract class RequestContextHolder  {

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

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

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

主要代码就是把RequestAttributes对象ThreadLocal化，然后提供了setRequestAttributes()、getRequestAttributes()等静态方法，来放入或取出ThreadLocal中线程隔离的RequestAttributes。
接下来看一下setRequestAttributes()方法是在什么时候调用的呢？

可以看到setRequestAttributes()被initContextHolders()调用，initContextHolders()又被processRequest()调用，而processRequest()在每次请求时都会被调用，无论是GET、POST、PUT、DELETE还是TRACE、OPTIONS等等。
先来看一下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(); // 重点1
    ServletRequestAttributes requestAttributes =
            buildRequestAttributes(request, response, previousAttributes); // 重点2

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

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

    try {
        doService(request, response); // 执行请求
    }
    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); // 重点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); // 发布请求处理完成事件
    }
}

重点1

在set之前就先get，通常为null。

重点2

直接看buildRequestAttributes()方法的实现

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

ServletRequestAttributes的代码不再去看了，它就是RequestAttributes接口的实现类，只是对HttpServletRequest对象（还有HttpSession）的一个包装。

重点3

直接看initContextHolders()方法的实现

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

调用RequestContextHolder.setRequestAttributes()方法，把requestAttributes对象放入。this.threadContextInheritable默认是false。
即把HttpServletRequest的封装对象ServletRequestAttributes与当前线程绑定。

重点4

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

在请求执行完毕后，再次调用RequestContextHolder.setRequestAttributes()，但由于previousAttributes为null，所以，这里相当于调用RequestContextHolder.setRequestAttributes(null, false)。
再回顾一下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();
        }
    }
}

参数attributes为null，就会调用resetRequestAttributes()，来清理当前线程引用的RequestAttributes。

至此，SpringMVC是如何实现直接获取HttpServletRequest对象的源码，就分析完了。和我们自己实现的思路差不多，只不过多绕了几个弯而已。

原文链接：https://my.oschina.net/ojeta/blog/801640