为什么80%的码农都做不了架构师？>>> 
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 httpServletRequestHolder =
new ThreadLocal();
@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 httpServletRequestHolder =
new ThreadLocal();
@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 httpServletRequestHolder =
new ThreadLocal();
@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 requestAttributesHolder =
new NamedThreadLocal("Request attributes"); // 重点
private static final ThreadLocal inheritableRequestAttributesHolder =
new NamedInheritableThreadLocal("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。