SpringMVC中出现的线程安全问题分析

(ps:前几个星期发生的事情)之前同事跟我说不要使用@Autowired方式注入HttpServletRequest(ps:我们的代码之前用的是第2种方式)。同事的意思大概是注入的HttpServletRequest对象是同一个而且存在线程安全问题。我保持质疑的态度，看了下源码，证明了@Autowired方式不存在线程安全问题，而@ModelAttribute方式存在线程安全问题。

观看本文章之前，最好看一下我上一篇写的文章：
1.通过循环引用问题来分析Spring源码
2.你真的了解Spring MVC处理请求流程吗?

public abstract class BaseController {

    @Autowired
    protected HttpSession httpSession;

    @Autowired
    protected HttpServletRequest request;

}


public abstract class BaseController1 {

    protected HttpServletRequest request;

    protected HttpServletResponse response;

    protected HttpSession httpSession;

    @ModelAttribute
    public void init(HttpServletRequest request,
                     HttpServletResponse response,
                     HttpSession httpSession) {
        this.request = request;
        this.response = response;
        this.httpSession = httpSession;
    }
}

线程安全测试

@RequestMapping("/test")
@RestController
public class TestController extends BaseController {

    @GetMapping("/1")
    public void test1() throws InterruptedException {
//        System.out.println("thread.id=" + Thread.currentThread().getId());
//        System.out.println("thread.name=" + Thread.currentThread().getName());

//        ServletRequestAttributes servletRequestAttributes =
//                ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes());
//
//        HttpServletRequest httpServletRequest = servletRequestAttributes.getRequest();
        TimeUnit.SECONDS.sleep(10);

//        System.out.println("base.request=" + request);
        System.out.println("base.request.name=" + request.getParameter("name"));
    }

    @GetMapping("/2")
    public void test2() throws InterruptedException {
//        System.out.println("thread.id=" + Thread.currentThread().getId());
//        System.out.println("thread.name=" + Thread.currentThread().getName());

//        ServletRequestAttributes servletRequestAttributes =
//                ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes());
//
//        HttpServletRequest httpServletRequest = servletRequestAttributes.getRequest();

//        System.out.println("base.request=" + request);
        System.out.println("base.request.name=" + request.getParameter("name"));

    }

    @InitBinder
    public void initBinder(WebDataBinder binder) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
        binder.registerCustomEditor(Date.class, new CustomDateEditor(sdf, true));
    }
}

通过JUC的CountDownLatch，模拟同一时刻100个并发请求。

public class Test {

    public static void main(String[] args) {
        CountDownLatch start = new CountDownLatch(1);
        CountDownLatch end = new CountDownLatch(100);

        CustomThreadPoolExecutor customThreadPoolExecutor = new CustomThreadPoolExecutor(
                100, 100, 0L,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<Runnable>(100)

        );

        for (int i = 0; i < 100; i++) {
            final int finalName = i;
            CustomThreadPoolExecutor.CustomTask task = new CustomThreadPoolExecutor.CustomTask(
                    new Runnable() {
                        @Override
                        public void run() {
                            try {
                                start.await();
                                HttpUtil.get("http://localhost:8081/test/2?name=" + finalName);
                            } catch (Exception ex) {
                                ex.printStackTrace();
                            } finally {
                                end.countDown();
                            }
                        }
                    }
            , "success");
            customThreadPoolExecutor.submit(task);
        }

        start.countDown();
        try {
            end.await();
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
        customThreadPoolExecutor.shutdown();
    }
}

通过观看base.request.name的值并没有null值和存在值重复的现象，很肯定的说@Autowired注入的HttpServletRequest不存在线程安全问题。

base.request.name=78
base.request.name=20
base.request.name=76
base.request.name=49
base.request.name=82
base.request.name=12
base.request.name=80
base.request.name=91
base.request.name=92
base.request.name=30
base.request.name=28
base.request.name=36
base.request.name=41
base.request.name=73
base.request.name=29
base.request.name=2
base.request.name=81
base.request.name=43
base.request.name=35
base.request.name=22
base.request.name=6
base.request.name=27
base.request.name=17
base.request.name=70
base.request.name=65
base.request.name=84
base.request.name=14
base.request.name=54
base.request.name=67
base.request.name=19
base.request.name=21
base.request.name=66
base.request.name=11
base.request.name=53
base.request.name=9
base.request.name=72
base.request.name=64
base.request.name=0
base.request.name=44
base.request.name=89
base.request.name=77
base.request.name=48
base.request.name=1
base.request.name=8
base.request.name=74
base.request.name=46
base.request.name=88
base.request.name=26
base.request.name=24
base.request.name=62
base.request.name=61
base.request.name=51
base.request.name=96
base.request.name=33
base.request.name=45
base.request.name=5
base.request.name=95
base.request.name=68
base.request.name=60
base.request.name=56
base.request.name=42
base.request.name=57
base.request.name=10
base.request.name=55
base.request.name=90
base.request.name=47
base.request.name=97
base.request.name=40
base.request.name=85
base.request.name=86
base.request.name=69
base.request.name=98
base.request.name=13
base.request.name=32
base.request.name=37
base.request.name=4
base.request.name=23
base.request.name=50
base.request.name=38
base.request.name=59
base.request.name=99
base.request.name=71
base.request.name=25
base.request.name=58
base.request.name=34
base.request.name=7
base.request.name=93
base.request.name=31
base.request.name=3
base.request.name=39
base.request.name=75
base.request.name=94
base.request.name=83
base.request.name=63
base.request.name=79
base.request.name=16
base.request.name=52
base.request.name=15
base.request.name=87
base.request.name=18

很明显发现base.request.name的值存在null或者重复的现象，说明通过@ModelAttribute注入的HttpServletRequest存在线程安全问题。

base.request.name=97
base.request.name=59
base.request.name=63
base.request.name=14
base.request.name=82
base.request.name=49
base.request.name=86
base.request.name=13
base.request.name=99
base.request.name=29
base.request.name=45
base.request.name=85
base.request.name=8
base.request.name=35
base.request.name=69
base.request.name=70
base.request.name=16
base.request.name=21
base.request.name=74
base.request.name=20
base.request.name=34
base.request.name=23
base.request.name=96
base.request.name=19
base.request.name=67
base.request.name=15
base.request.name=27
base.request.name=43
base.request.name=39
base.request.name=47
base.request.name=87
base.request.name=71
base.request.name=41
base.request.name=38
base.request.name=null
base.request.name=31
base.request.name=32
base.request.name=76
base.request.name=55
base.request.name=75
base.request.name=93
base.request.name=null
base.request.name=56
base.request.name=1
base.request.name=18
base.request.name=89
base.request.name=65
base.request.name=10
base.request.name=78
base.request.name=null
base.request.name=80
base.request.name=24
base.request.name=88
base.request.name=88
base.request.name=44
base.request.name=53
base.request.name=58
base.request.name=61
base.request.name=60
base.request.name=37
base.request.name=92
base.request.name=42
base.request.name=11
base.request.name=68
base.request.name=72
base.request.name=91
base.request.name=79
base.request.name=33
base.request.name=66
base.request.name=54
base.request.name=40
base.request.name=94
base.request.name=46
base.request.name=83
base.request.name=17
base.request.name=64
base.request.name=26
base.request.name=90
base.request.name=7
base.request.name=62
base.request.name=57
base.request.name=73
base.request.name=98
base.request.name=30
base.request.name=6
base.request.name=2
base.request.name=28
base.request.name=5
base.request.name=95
base.request.name=9
base.request.name=3
base.request.name=51
base.request.name=4
base.request.name=52
base.request.name=12
base.request.name=25
base.request.name=36
base.request.name=84
base.request.name=81
base.request.name=50

源码分析

1.在Spring容器初始化中，refresh()方法会调用postProcessBeanFactory(beanFactory);。它是个模板方法，在BeanDefinition被装载后(所有BeanDefinition被加载，但是没有bean被实例化)，提供一个修改beanFactory容器的入口。这里还是贴下AbstractApplicationContext中的refresh()方法吧。

    @Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // 1.Prepare this context for refreshing.
            prepareRefresh();

            // 2.Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // 3.Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // 4.Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // 5.Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // 6.Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // 7.Initialize message source for this context.
                initMessageSource();

                // 8.Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // 9.Initialize other special beans in specific context subclasses.
                onRefresh();

                //10. Check for listener beans and register them.
                registerListeners();

                // 11.Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);

                //12. Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    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;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

2.由于postProcessBeanFactory是模板方法，它会被子类AbstractRefreshableWebApplicationContext重写。在AbstractRefreshableWebApplicationContext的postProcessBeanFactory()做以下几件事情。

1.注册ServletContextAwareProcessor。
2.注册需要忽略的依赖接口ServletContextAware、ServletConfigAware。
3.注册Web应用的作用域和环境配置信息。

    @Override
    protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        beanFactory.addBeanPostProcessor(new ServletContextAwareProcessor(this.servletContext, this.servletConfig));
        beanFactory.ignoreDependencyInterface(ServletContextAware.class);
        beanFactory.ignoreDependencyInterface(ServletConfigAware.class);

        WebApplicationContextUtils.registerWebApplicationScopes(beanFactory, this.servletContext);
        WebApplicationContextUtils.registerEnvironmentBeans(beanFactory, this.servletContext, this.servletConfig);
    }
  1. WebApplicationContextUtils中的registerWebApplicationScopes()，beanFactory注册了request,application,session,globalSession作用域，也注册了需要解决的依赖：ServletRequest、ServletResponse、HttpSession、WebRequest。
    public static void registerWebApplicationScopes(ConfigurableListableBeanFactory beanFactory, ServletContext sc) {
        beanFactory.registerScope(WebApplicationContext.SCOPE_REQUEST, new RequestScope());
        beanFactory.registerScope(WebApplicationContext.SCOPE_SESSION, new SessionScope(false));
        beanFactory.registerScope(WebApplicationContext.SCOPE_GLOBAL_SESSION, new SessionScope(true));
        if (sc != null) {
            ServletContextScope appScope = new ServletContextScope(sc);
            beanFactory.registerScope(WebApplicationContext.SCOPE_APPLICATION, appScope);
            // Register as ServletContext attribute, for ContextCleanupListener to detect it.
            sc.setAttribute(ServletContextScope.class.getName(), appScope);
        }

        beanFactory.registerResolvableDependency(ServletRequest.class, new RequestObjectFactory());
        beanFactory.registerResolvableDependency(ServletResponse.class, new ResponseObjectFactory());
        beanFactory.registerResolvableDependency(HttpSession.class, new SessionObjectFactory());
        beanFactory.registerResolvableDependency(WebRequest.class, new WebRequestObjectFactory());
        if (jsfPresent) {
            FacesDependencyRegistrar.registerFacesDependencies(beanFactory);
        }
    }

4.RequestObjectFactory, ResponseObjectFactory, SessionObjectFactory都实现了ObjectFactory的接口，注入的值其实是getObject()的值。

    /**
     * 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";
        }
    }


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

        @Override
        public ServletResponse getObject() {
            ServletResponse response = currentRequestAttributes().getResponse();
            if (response == null) {
                throw new IllegalStateException("Current servlet response not available - " +
                        "consider using RequestContextFilter instead of RequestContextListener");
            }
            return response;
        }

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


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

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

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


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

        @Override
        public WebRequest getObject() {
            ServletRequestAttributes requestAttr = currentRequestAttributes();
            return new ServletWebRequest(requestAttr.getRequest(), requestAttr.getResponse());
        }

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

5.很明显，我们从getObject()中获取的值是从绑定当前线程的RequestAttribute中获取的,内部实现是通过ThreadLocal去完成的。看到这里，你应该明白了一点点。

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

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

    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;
    }

    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;
    }

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

6.我们再来捋一捋@Autowired注入HttpServletRequest对象的过程。这里以HttpServletRequest对象注入举例。首先调用DefaultListableBeanFactory中的findAutowireCandidates()方法，判断autowiringType类型是否和requiredType类型一致或者是autowiringType是否是requiredType的父接口(父类)。如果满足条件的话，我们会从resolvableDependencies中通过autowiringType（对应着上文的ServletRequest）拿到autowiringValue(对应着上文的RequestObjectFactory)。然后调用AutowireUtils.resolveAutowiringValue()对我们的ObjectFactory进行处理。

    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 candidate : candidateNames) {
            if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) {
                addCandidateEntry(result, candidate, descriptor, requiredType);
            }
        }
        if (result.isEmpty() && !indicatesMultipleBeans(requiredType)) {
            // Consider fallback matches if the first pass failed to find anything...
            DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch();
            for (String candidate : candidateNames) {
                if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, fallbackDescriptor)) {
                    addCandidateEntry(result, candidate, descriptor, requiredType);
                }
            }
            if (result.isEmpty()) {
                // Consider self references as a final pass...
                // but in the case of a dependency collection, not the very same bean itself.
                for (String candidate : candidateNames) {
                    if (isSelfReference(beanName, candidate) &&
                            (!(descriptor instanceof MultiElementDescriptor) || !beanName.equals(candidate)) &&
                            isAutowireCandidate(candidate, fallbackDescriptor)) {
                        addCandidateEntry(result, candidate, descriptor, requiredType);
                    }
                }
            }
        }
        return result;
    }
  1. 很明显，对我们的RequestObjectFactory进行了JDK动态代理。原来我们通过@Autowired注入拿到的HttpServletRequest对象是代理对象。
    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;
    }

    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();
            }
        }
    }

8.我们再来看SpringMVC是怎么把HttpServletRequest对象放入到ThreadLocal中。当用户发出请求后，会经过FrameworkServlet中的processRequest()方法做了一些骚操作，然后再交给子类DispatcherServlet中的doService()去处理这个请求。这些骚操作就包括把request,response对象包装成ServletRequestAttributes对象，然后放入到ThreadLocal中。

    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);
        }
    }
  1. buildRequestAttributes()方法将当前request和response对象包装成ServletRequestAttributes对象。initContextHolders()负责把RequestAttributes对象放入到requestAttributesHolder(ThreadLocal)中。一切真相大白。
    protected ServletRequestAttributes buildRequestAttributes(
            HttpServletRequest request, HttpServletResponse response, RequestAttributes previousAttributes) {

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

    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);
        }
    }

    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();
            }
        }
    }

    private static final boolean jsfPresent =
            ClassUtils.isPresent("javax.faces.context.FacesContext", RequestContextHolder.class.getClassLoader());

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

    private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
            new NamedInheritableThreadLocal<RequestAttributes>("Request context");
  1. SpringMVC会优先执行被@ModelAttribute注解的方法。也就是说我们每一次请求，都会去调用init()方法，对request，response，httpSession进行赋值操作，并发问题也由此产生。
    private void invokeModelAttributeMethods(NativeWebRequest request, ModelAndViewContainer container)
            throws Exception {

        while (!this.modelMethods.isEmpty()) {
            InvocableHandlerMethod modelMethod = getNextModelMethod(container).getHandlerMethod();
            ModelAttribute ann = modelMethod.getMethodAnnotation(ModelAttribute.class);
            if (container.containsAttribute(ann.name())) {
                if (!ann.binding()) {
                    container.setBindingDisabled(ann.name());
                }
                continue;
            }

            Object returnValue = modelMethod.invokeForRequest(request, container);
            if (!modelMethod.isVoid()){
                String returnValueName = getNameForReturnValue(returnValue, modelMethod.getReturnType());
                if (!ann.binding()) {
                    container.setBindingDisabled(returnValueName);
                }
                if (!container.containsAttribute(returnValueName)) {
                    container.addAttribute(returnValueName, returnValue);
                }
            }
        }
    }

public abstract class BaseController1 {

    protected HttpServletRequest request;

    protected HttpServletResponse response;

    protected HttpSession httpSession;

    @ModelAttribute
    public void init(HttpServletRequest request,
                     HttpServletResponse response,
                     HttpSession httpSession) {
        this.request = request;
        this.response = response;
        this.httpSession = httpSession;
    }
}

2019-04-18更新

最近的面试，让我想到了3个问题。假如有2个请求过来，一个是A请求，一个是B请求。A请求和B请求所处于不同线程(Spring中的Controller是单例多线程，也可以Controller的作用域设置为prototype，这样每次请求过来都会生成一个新的实例，不存在多线程问题，但同时也带来了更大的内存消耗，Struts2就是这样做的)，这时候会出现以下情况：

  • 1.A请求和B请求的HttpServletRequest对象都是ThreadLocal(@Autowire方式最终调用RequestContextHolder.currentRequestAttributes().getRequest())中获取到的。Tomcat的处理模型是Connector+Executor，最终是Executor复用线程来处理实际的业务。有没有可能会出现这种情况：A请求所在线程是A。当A请求结束后，B请求复用线程A，此时拿到的HttpServletRequest对象其实是A的。 其实仔细想一想就觉得不可能，当A请求完成时，应该会调用ThreadLocal.remove()方法清除线程A中绑定的HttpServletRequest对象。可以在RequestContextFilter看到一些猫腻！
RequestContextFilter.png
  • 2.我们的每次请求，Tomcat中的Executor都会分配一个线程来进行处理。每次请求都会创建HttpServletRequest 和 HttpServletResponse对象。正因为Controller是单例多线程，上述通过@ModelAndView方法初始化HttpServletRequest对象，会造成HttpServletRequest覆盖问题(后一次的请求覆盖前一次的请求)。对于在同一个Controller，为什么每次请求中的HttpServletRequest对象的hashCode都相同？因为HttpServletRequest是通过@Autowire注入来的，Controller中的HttpServletRequest实际上不是RequestFacade对象，而是Proxy(代理RequestObjectFactory)对象。所以这里的hashCode其实是Proxy对象在内存中的地址，真相大白！
AutowireUtils.png
  • 3.以方法参数注入的HttpServletRequest是没有线程安全问题的。但是我们在网页上请求/4接口100次，此时httpServletRequest对象的hashCode是一样的，这是为什么呢？服务器中每个thread所申请的request的内存空间在这个服务器启动的时候就是固定的，那么我每次请求，它都会在它所申请到的内存空间（可能是类似数组这样的结构）中新建一个request,（类似于数组的起点总是同一个内存地址）。那么我发起一个请求，他就会在起始位置新建一个Request传递给Servlet并开始处理，处理结束后就会销毁，那么他下一个请求所新建的Request。因为之前的request销毁了，所以又从起始地址开始创建，这样一切就解释得通了。
    image.png

我们先请求/4，再请求/5。两次请求中的HttpServletRequest的hashCode肯定不一样，因为/4处于阻塞状态，其HttpServletRequest不能被成功销毁。'/5'请求中的HttpServletRequest不能占据以前的内存地址，遂导致2次请求中的HttpServletRequest的hashCode不一致。

image.png

/4.png
/5.png

尾言

大家好，我是cmazxiaoma(寓意是沉梦昂志的小马)，希望和你们一起成长进步，感谢各位阅读本文章。

小弟不才。
如果您对这篇文章有什么意见或者错误需要改进的地方,欢迎与我讨论。
如果您觉得还不错的话,希望你们可以点个赞。
希望我的文章对你能有所帮助。
有什么意见、见解或疑惑，欢迎留言讨论。

最后送上：心之所向，素履以往。生如逆旅，一苇以航。


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