SpringMVC源码解析

时间：2018-12-12 22:39:36 阅读：252 评论：0 收藏：0 [点我收藏+]

1. SpringMVC重要组件

1. DispatcherServlet

SpringMVC的中央Servlet，所有请求的入口，重写了doService()方法。核心方法：doService()、doDispatch()。

2. HandlerMapping

处理器映射，负责根据HttpServletRequest找到对应的Handler，这里返回Handler的辅助类HandlerExecutionChain。

public interface HandlerMapping {

    @Nullable
    HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception;

}

3. HandlerExecutionChain

Handler execution chain, consisting of handler object and any handler interceptors.

处理器执行链，Handler辅助类，由Handler对象和一些HandlerInterceptor组成，主要功能为执行拦截器，核心属性和方法：

    private final Object handler;

    private HandlerInterceptor[] interceptors;

    // 一般情况下返回的是一个HandlerMethod对象
    Object getHandler()

    boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response)

    void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv)

4. HandlerMethod

Encapsulates information about a handler method consisting of a method and a bean. Provides convenient access to method parameters, the method return value, method annotations, etc.

保存了Controller Bean对象和对应的Method对象。

    // 对应的Controller实例
    private final Object bean;

    // 处理请求的方法
    private final Method method;

    // 方法的参数
    private final MethodParameter[] parameters;

5. HandlerInterceptor

拦截器，自定义拦截器实现此接口。preHandle()和postHandle()方法分别在实际的请求处理之前和之后执行。

public interface HandlerInterceptor {

    default boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
        return true;
    }

    default void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler,
            @Nullable ModelAndView modelAndView) throws Exception {
    }

    default void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler,
            @Nullable Exception ex) throws Exception {
    }

}

6. HandlerAdapter

处理器适配器。handle()方法负责执行实际的请求处理并返回一个ModelAndView对象。

public interface HandlerAdapter {

    boolean supports(Object handler);

    ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

    long getLastModified(HttpServletRequest request, Object handler);
}

2. SpringMVC请求处理流程

DispatcherServlet作为入口且重写了doService()方法，所以请求会首先到doService()方法。下面步骤1、2、3在doService()方法完成，接着doService()方法调用doDispatch()方法，doDispatch()方法完成剩下的所有步骤。
WebApplicationContext对象绑定到Request，key为DispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE。
locale resolver绑定到Request。
theme resolver绑定到Request。
如果是multipart request，包装成MultipartHttpServletRequest。
调用HandlerMapping的getHandler()方法获取对应的Handler，这里是HandlerExecutionChain对象。
调用HandlerExecutionChain的getHandler()方法，这里返回HandlerMethod对象，根据Handler获取HandlerAdapter。
如果是GET或者HEAD方法执行HandlerAdapter的getLastModified()方法。
调用HandlerExecutionChain的applyPreHandle()方法执行HandlerInterceptor的preHandle()方法。
调用HandlerAdapter的handle()方法执行实际的请求处理，实际执行HandlerMethod的对应方法，返回ModelAndView对象，
根据ModelAndView视图解析器找到对应的View。
渲染视图返回用户。

3. HandlerMethod收集

请求处理流程中的第5步中提到了HandlerMapping，这个核心第实现是RequestMappingHandlerMapping类，当使用SpringMVC时此类就会被注入到spring容器中，此类实现了InitializingBean接口，所以在实例化过程中会执行afterPropertiesSet()方法，此方法调用initHandlerMethods()，这个方法负责收集HandlerMethod，其相关源码如下：

    /**
     * Detects handler methods at initialization.
     * @see #initHandlerMethods
     */
    @Override
    public void afterPropertiesSet() {
        initHandlerMethods();
    }

    /**
     * Scan beans in the ApplicationContext, detect and register handler methods.
     * @see #getCandidateBeanNames()
     * @see #processCandidateBean
     * @see #handlerMethodsInitialized
     */
    protected void initHandlerMethods() {
        // 1. 遍历所有beanName
        for (String beanName : getCandidateBeanNames()) {
            if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
                processCandidateBean(beanName);
            }
        }
        handlerMethodsInitialized(getHandlerMethods());
    }

    /**
     *  2. 判断beanName对应的Class是否为Controller
     */
    protected void processCandidateBean(String beanName) {
        Class<?> beanType = null;
        try {
            beanType = obtainApplicationContext().getType(beanName);
        }
        catch (Throwable ex) {
            // An unresolvable bean type, probably from a lazy bean - let‘s ignore it.
            if (logger.isTraceEnabled()) {
                logger.trace("Could not resolve type for bean ‘" + beanName + "‘", ex);
            }
        }
        // 3. 判断逻辑为是否有@Controller或@RequestMapping注解
        if (beanType != null && isHandler(beanType)) {
            // 检测Controller类中的HandlerMethod
            detectHandlerMethods(beanName);
        }
    }

    @Override
    protected boolean isHandler(Class<?> beanType) {
        // 判断逻辑为是否有@Controller或@RequestMapping注解
        return (AnnotatedElementUtils.hasAnnotation(beanType, Controller.class) ||
                AnnotatedElementUtils.hasAnnotation(beanType, RequestMapping.class));
    }

    protected void detectHandlerMethods(Object handler) {
        Class<?> handlerType = (handler instanceof String ?
                obtainApplicationContext().getType((String) handler) : handler.getClass());

        if (handlerType != null) {
            Class<?> userType = ClassUtils.getUserClass(handlerType);

            // 4. 检测到所有Method
            Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
                    (MethodIntrospector.MetadataLookup<T>) method -> {
                        try {
                            return getMappingForMethod(method, userType);
                        }
                        catch (Throwable ex) {
                            throw new IllegalStateException("Invalid mapping on handler class [" +
                                    userType.getName() + "]: " + method, ex);
                        }
                    });
            if (logger.isTraceEnabled()) {
                logger.trace("Mapped " + methods.size() + " handler method(s) for " + userType + ": " + methods);
            }

            // 5. 对检测到到Method进行注册缓存。
            methods.forEach((method, mapping) -> {
                Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
                // 这里把解析出到HanderMethod对象进行组册
                registerHandlerMethod(handler, invocableMethod, mapping);
            });
        }
    }

    protected void registerHandlerMethod(Object handler, Method method, T mapping) {
        // 6. 这里组册到MappingRegistry的对应的集合中缓存起来
        this.mappingRegistry.register(mapping, handler, method);
    }