【转】在SpringMVC Controller中注入Request成员域

原文链接：https://www.cnblogs.com/abcwt112/p/7777258.html

原文作者：abcwt112

主题

　　在工作中遇到1个问题....我们定义了一个Controller基类,所有Springmvc自定义的controller都继承它....在它内部定义一个@Autowired HttpServletRequest request;可不可以? 能不能从这个对象里取requestParamters和attributes? 多线程之间会不会影响?

思考

初次思考,我想这应该是不行的.为什么呢?

注入bean是在spring容器启动的时候...request的实现类是在tomcat里(我使用的servlet容器是tomcat)....我又没在spring的容器里配置这个bean.注入应该是失败的...

退一步说,就算是成功了....那注入的也就是1个对象而已.每次servlet接受到请求都会重新生成1个request...这明显和之前启动的那个对象不同吧....怎么想都不可能成功...

如果确实是这样的....那就没有这篇文章了....后来实践了一下..发现这个注入是可以的.使用起来取数据也没任何问题....

其实我那个时候debug看了一下,基本就知道为什么可以取到数据了..但是我并不知道原理和Spring(Springmvc)的处理流程...所以现在研究了一下并记录下来...

原理

首先给大家看一下在方法中注入request作为参数和在成员域中注入request的 注入的request对象之间的区别....

成员域注入的时候注入的是1个代理对象.是 AutowireUtils.ObjectFactoryDelegatingInvocationHandler的实例.

方法注入的就是一般tomcat原生的requestFacade对象.

所以这是不同的...

/**
     * Reflective InvocationHandler for lazy access to the current target object.
     */
    @SuppressWarnings("serial")
    private static class ObjectFactoryDelegatingInvocationHandler implements InvocationHandler, Serializable {

        private final ObjectFactory<?> objectFactory;

        public ObjectFactoryDelegatingInvocationHandler(ObjectFactory<?> objectFactory) {
            this.objectFactory = objectFactory;
        }

        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            String methodName = method.getName();
            if (methodName.equals("equals")) {
                // Only consider equal when proxies are identical.
                return (proxy == args[0]);
            }
            else if (methodName.equals("hashCode")) {
                // Use hashCode of proxy.
                return System.identityHashCode(proxy);
            }
            else if (methodName.equals("toString")) {
                return this.objectFactory.toString();
            }
            try {
                return method.invoke(this.objectFactory.getObject(), args);
            }
            catch (InvocationTargetException ex) {
                throw ex.getTargetException();
            }
        }
    }

当代理对象(就是成员域request)的大部分方法被调用的时候,ObjectFactoryDelegatingInvocationHandler会使用objectFactory获取对象(原生request),再调用对象上的方法.

然后我们来看下XmlWebApplicationContext初始化到请求到进入controller里几个对注入request成员域有影响的步骤.

refresh方法和postProcessBeanFactory方法

ApplicationContext的抽象实现类AbstractApplicationContext(基本是所有ac的父类)里定义了ac的refresh方法(包含了使用BeanFactory注入bean)的流程..

@Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            // 记录开始wac开始初始化的时间,设置激活标记,servlet的相关param设置到env(之前做过1次),校验env中必须的props
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            // 将旧的BF里的bean删掉,新建1个BF,设置部分属性,加载XML配置文件
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            // 1.设置BF解析bean配置需要用到的一些对象比如env. 2.注册一些BeanPostProcessor比如ApplicationContextAwareProcessor去设置Aware需要的对象
            // 3.忽略一些特定class注入的对象,设置一些特定class注入的对象为指定值
            // 4.将一些env中的properties map当做bean注册到BF中
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                // 1.设置一个BeanPostProcess为ServletContextAware的实现类注入servlet相关对象
                // 2.在BF中增加requetsScope等Scope
                // 3.把servletContext,Config,ServletInitParams,ServletAttribute当做Bean注册到BF中
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                // 初始化并调用BeanFactoryPostProcessor
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                // 注册BeanPostProcessors并注册到BF中去
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            } catch (BeansException ex) {
                logger.warn("Exception encountered during context initialization - cancelling refresh attempt", ex);

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }
        }
    }

其中有1个模板方法

postProcessBeanFactory(beanFactory);

这个方法允许AbstractApplicationContext的子类覆盖它并实现对BF的定制(这个时候bean的defination路径已经指定了,但是bean还没加载).

AbstractRefreshableWebApplicationContext覆盖了这个方法

/**
     * Register request/session scopes, a {@link ServletContextAwareProcessor}, etc.
     * 1.设置一个BeanPostProcess为ServletContextAware的实现类注入servlet相关对象
     * 2.在BF中增加requetsScope等Scope
     * 3.把servletContext,Config,ServletInitParams,ServletAttribute当做Bean注册到BF中
     *
     */
    @Override
    protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        // 设置一个BeanPostProcess为ServletContextAware的实现类注入servlet相关对象
        beanFactory.addBeanPostProcessor(new ServletContextAwareProcessor(this.servletContext, this.servletConfig));
        beanFactory.ignoreDependencyInterface(ServletContextAware.class);
        beanFactory.ignoreDependencyInterface(ServletConfigAware.class);

        // 在BF中增加requetsScope等Scope
        WebApplicationContextUtils.registerWebApplicationScopes(beanFactory, this.servletContext);
        // 把servletContext,Config,ServletInitParams,ServletAttribute当做Bean注册到BF中
        WebApplicationContextUtils.registerEnvironmentBeans(beanFactory, this.servletContext, this.servletConfig);
    }

其中有一步

WebApplicationContextUtils.registerWebApplicationScopes(beanFactory, this.servletContext);

这里设置了一些特殊的bean的scope,比如request,session,globalSession,application.(当然这个不是我这篇文章的主题.)

同时设置了一些特殊的autowired bean

beanFactory.registerResolvableDependency(ServletRequest.class, new RequestObjectFactory());
beanFactory.registerResolvableDependency(ServletResponse.class, new ResponseObjectFactory());
beanFactory.registerResolvableDependency(HttpSession.class, new SessionObjectFactory());
beanFactory.registerResolvableDependency(WebRequest.class, new WebRequestObjectFactory());

ServletRequest的实现类(比如HttpServletRequest)被指定使用RequestObjectFactory注入.

RequestObjectFactory

RequestObjectFactory就是1个ObjectFactory就是前面ObjectFactoryDelegatingInvocationHandler里的ObjectFactory.所以在成员域request对象上调用方法其实就是通过RequestObjectFactory获取对象再调用方法.

    /**
     * Factory that exposes the current request object on demand.
     */
    @SuppressWarnings("serial")
    private static class RequestObjectFactory implements ObjectFactory<ServletRequest>, Serializable {

        @Override
        public ServletRequest getObject() {
            return currentRequestAttributes().getRequest();
        }

        @Override
        public String toString() {
            return "Current HttpServletRequest";
        }
    }

    /**
     * Return the current RequestAttributes instance as ServletRequestAttributes.
     * @see RequestContextHolder#currentRequestAttributes()
     */
    private static ServletRequestAttributes currentRequestAttributes() {
        RequestAttributes requestAttr = RequestContextHolder.currentRequestAttributes();
        if (!(requestAttr instanceof ServletRequestAttributes)) {
            throw new IllegalStateException("Current request is not a servlet request");
        }
        return (ServletRequestAttributes) requestAttr;
    }

RequestObjectFactory的getObject方法很简单,就是调用静态方法

RequestContextHolder.currentRequestAttributes().getRequest()

RequestContextHolder

    public static RequestAttributes currentRequestAttributes() throws IllegalStateException {
        RequestAttributes attributes = getRequestAttributes();
        if (attributes == null) {
            if (jsfPresent) {
                attributes = FacesRequestAttributesFactory.getFacesRequestAttributes();
            }
            if (attributes == null) {
                throw new IllegalStateException("No thread-bound request found: " +
                        "Are you referring to request attributes outside of an actual web request, " +
                        "or processing a request outside of the originally receiving thread? " +
                        "If you are actually operating within a web request and still receive this message, " +
                        "your code is probably running outside of DispatcherServlet/DispatcherPortlet: " +
                        "In this case, use RequestContextListener or RequestContextFilter to expose the current request.");
            }
        }
        return attributes;
    }

    /**
     * Return the RequestAttributes currently bound to the thread.
     * @return the RequestAttributes currently bound to the thread,
     * or {@code null} if none bound
     */
    public static RequestAttributes getRequestAttributes() {
        RequestAttributes attributes = requestAttributesHolder.get();
        if (attributes == null) {
            attributes = inheritableRequestAttributesHolder.get();
        }
        return attributes;
    }

    private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
            new NamedThreadLocal<RequestAttributes>("Request attributes");

    private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
            new NamedInheritableThreadLocal<RequestAttributes>("Request context");

上面是一些关键方法

所以最终其实request是从threadlocal中取...

FrameworkServlet

那么request是什么时候设置到threadlocal中去的呢?

是在Springmvc的dispatcherServlet的父类FrameworkServlet里操作的.

    @Override
    protected final void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        processRequest(request, response);
    }

    @Override
    protected final void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        processRequest(request, response);
    }

不管你是doGet还是doPost还是doXXX方法都是委托processRequest方法去做的.

    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();
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

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

        initContextHolders(request, localeContext, requestAttributes);

        try {
            doService(request, response);
        }
        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);
            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, response, startTime, failureCause);
        }
    }

其中调用了

initContextHolders(request, localeContext, requestAttributes);

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

就是在这里设置到RequestContextHolder的threadlocal中去的...

小结

1.在controller中注入的request是jdk动态代理对象,ObjectFactoryDelegatingInvocationHandler的实例.当我们调用成员域request的方法的时候其实是调用了objectFactory的getObject()对象的相关方法.这里的objectFactory是RequestObjectFactory.

2.RequestObjectFactory的getObject其实是从RequestContextHolder的threadlocal中去取值的.

3.请求刚进入springmvc的dispatcherServlet的时候会把request相关对象设置到RequestContextHolder的threadlocal中去.

对于原文的补充

原文没有详细描述成员域request注入的具体过程，补充如下：

ApplicationContext的初始化调用过程如下：

在refresh方法被调用后，会调用org.springframework.beans.factory.support.DefaultListableBeanFactory#findAutowireCandidates

    protected Map<String, Object> findAutowireCandidates(
            String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {

        String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                this, requiredType, true, descriptor.isEager());
        Map<String, Object> result = new LinkedHashMap<String, Object>(candidateNames.length);
        for (Class<?> autowiringType : this.resolvableDependencies.keySet()) {
            if (autowiringType.isAssignableFrom(requiredType)) {
                Object autowiringValue = this.resolvableDependencies.get(autowiringType);
                autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
                if (requiredType.isInstance(autowiringValue)) {
                    result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
                    break;
                }
            }
        }
        for (String candidateName : candidateNames) {
            if (!isSelfReference(beanName, candidateName) && isAutowireCandidate(candidateName, descriptor)) {
                result.put(candidateName, getBean(candidateName));
            }
        }
        if (result.isEmpty()) {
            DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch();
            for (String candidateName : candidateNames) {
                if (!candidateName.equals(beanName) && isAutowireCandidate(candidateName, fallbackDescriptor)) {
                    result.put(candidateName, getBean(candidateName));
                }
            }
        }
        return result;
    }

该方法内部会调用：

org.springframework.beans.factory.support.AutowireUtils#resolveAutowiringValue

    public static Object resolveAutowiringValue(Object autowiringValue, Class<?> requiredType) {
        if (autowiringValue instanceof ObjectFactory && !requiredType.isInstance(autowiringValue)) {
            ObjectFactory<?> factory = (ObjectFactory<?>) autowiringValue;
            if (autowiringValue instanceof Serializable && requiredType.isInterface()) {
                autowiringValue = Proxy.newProxyInstance(requiredType.getClassLoader(),
                        new Class<?>[] {requiredType}, new ObjectFactoryDelegatingInvocationHandler(factory));
            }
            else {
                return factory.getObject();
            }
        }
        return autowiringValue;
    }

可以看到，如果需要注入的对象是接口类型的话，则为其创建代理对象。