HandlerAdapter的用途

请求处理器处理方法入参出参多种多样，为了方便调用，以及后续流程使用处理结果，对这一过程进行了封装，由handler对应类型的HandlerAdapter进行入参解析，出参封装等操作。

public interface HandlerAdapter {

  //是否支持该处理器
    boolean supports(Object handler);

  //执行处理器方法，并将返回值封装成ModelAndView
    @Nullable
    ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

  //返回请求的最近更新时间  如果不支持可以返回-1
    long getLastModified(HttpServletRequest request, Object handler);

}

与HandlerMapping的初始化策略类似，在未指定HandlerAdapter实现类时加载DispatcherServlet.properties中指定的类

org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
    org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter,\
    org.springframework.web.servlet.function.support.HandlerFunctionAdapter

HttpRequestHandlerAdapter

处理HttpRequestHandler类型的请求处理器，直接调用HttpRequestHandler的handleRequest方法

public class HttpRequestHandlerAdapter implements HandlerAdapter {

    @Override
    public boolean supports(Object handler) {
        return (handler instanceof HttpRequestHandler);
    }

    @Override
    @Nullable
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {

        ((HttpRequestHandler) handler).handleRequest(request, response);
        return null;
    }

    @Override
    public long getLastModified(HttpServletRequest request, Object handler) {
        if (handler instanceof LastModified) {
            return ((LastModified) handler).getLastModified(request);
        }
        return -1L;
    }

}

SimpleControllerHandlerAdapter

处理实现了Controller接口的请求处理器，直接调用handleRequest方法。

public class SimpleControllerHandlerAdapter implements HandlerAdapter {

    @Override
    public boolean supports(Object handler) {
        return (handler instanceof Controller);
    }

    @Override
    @Nullable
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {

        return ((Controller) handler).handleRequest(request, response);
    }

    @Override
    public long getLastModified(HttpServletRequest request, Object handler) {
        if (handler instanceof LastModified) {
            return ((LastModified) handler).getLastModified(request);
        }
        return -1L;
    }

}

RequestMappingHandlerAdapter

也是实现了一些Aware接口和InitializingBean接口，不过它的Aware接口只是简单的设值。重点看InitializingBean接口的实现

public void afterPropertiesSet() {
    // 初始化controller增强器
    initControllerAdviceCache();
    //初始化默认的方法参数解析器 用于将请求参数绑定到处理方法的入参上
    if (this.argumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
        this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    //初始化默认的初始化绑定器(@InitBinder)参数解析器
    if (this.initBinderArgumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
        this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    //初始化默认的返回值解析器
    if (this.returnValueHandlers == null) {
        List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
        this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
    }
}

RequestMappingHandlerAdapter#initControllerAdviceCache

private void initControllerAdviceCache() {
    if (getApplicationContext() == null) {
        return;
    }
    //找出容器中所有@ControllerAdvice标注的Bean
    List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());

    List<Object> requestResponseBodyAdviceBeans = new ArrayList<>();

    for (ControllerAdviceBean adviceBean : adviceBeans) {
        Class<?> beanType = adviceBean.getBeanType();
        if (beanType == null) {
            throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
        }
        //寻找有@ModelAttribute且无@RequestMapping标注的方法
        Set<Method> attrMethods = MethodIntrospector.selectMethods(beanType, MODEL_ATTRIBUTE_METHODS);
        if (!attrMethods.isEmpty()) {
            this.modelAttributeAdviceCache.put(adviceBean, attrMethods);
        }
        //寻找@InitBinder标注的方法
        Set<Method> binderMethods = MethodIntrospector.selectMethods(beanType, INIT_BINDER_METHODS);
        if (!binderMethods.isEmpty()) {
            this.initBinderAdviceCache.put(adviceBean, binderMethods);
        }
        if (RequestBodyAdvice.class.isAssignableFrom(beanType) || ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
            // 收集实现了RequestBodyAdvice接口或ResponseBodyAdvice接口的类
            requestResponseBodyAdviceBeans.add(adviceBean);
        }
    }
    if (!requestResponseBodyAdviceBeans.isEmpty()) {
        this.requestResponseBodyAdvice.addAll(0, requestResponseBodyAdviceBeans);
    }
}