Spring HandlerMethodReturnValueHandler源碼解析

Spring 4.3.2

Spring的Controller調用業務邏輯返回的Object會經過HandlerMethodReturnValueHandler的轉換，接口HandlerMethodReturnValueHandler定義了兩個方法

package org.springframework.web.method.support;

import org.springframework.core.MethodParameter;
import org.springframework.web.context.request.NativeWebRequest;

public interface HandlerMethodReturnValueHandler {

    boolean supportsReturnType(MethodParameter returnType);

    void handleReturnValue(Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;

}

當supportsReturnType返回true時會調用handleReturnValue轉換

源代碼如下org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite

private HandlerMethodReturnValueHandler getReturnValueHandler(MethodParameter returnType) {
        for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
            if (handler.supportsReturnType(returnType)) {
                return handler;
            }
        }
        return null;
    }

默認有15個實現HandlerMethodReturnValueHandler的類，在org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter中添加

源代碼如下

注釋是HandlerMethodReturnValueHandler處理的返回值類型

private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {
        List<HandlerMethodReturnValueHandler> handlers = new ArrayList<HandlerMethodReturnValueHandler>();

        // Single-purpose return value types
        handlers.add(new ModelAndViewMethodReturnValueHandler()); // 處理返回類型ModelAndView
        handlers.add(new ModelMethodProcessor()); // 處理返回類型Model
        handlers.add(new ViewMethodReturnValueHandler()); // 處理返回類型View
        handlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters())); // 處理返回類型 ？
        handlers.add(new StreamingResponseBodyReturnValueHandler()); // 處理返回類型StreamingResponseBody
        handlers.add(new HttpEntityMethodProcessor(getMessageConverters(), // 處理返回類型HttpEntity|RequestEntity
                this.contentNegotiationManager, this.requestResponseBodyAdvice));
        handlers.add(new HttpHeadersReturnValueHandler()); // 處理返回類型HttpHeaders
        handlers.add(new CallableMethodReturnValueHandler()); // 處理返回類型Callable
        handlers.add(new DeferredResultMethodReturnValueHandler()); // 處理返回類型DeferredRequest | ListenableFuture
        handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory)); // 處理返回類型WebAsyncTask

        // Annotation-based return value types
        handlers.add(new ModelAttributeMethodProcessor(false)); //處理@ModelAttribute
        handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), // 處理@ResponseBody
                this.contentNegotiationManager, this.requestResponseBodyAdvice));

        // Multi-purpose return value types
        handlers.add(new ViewNameMethodReturnValueHandler()); // 處理返回類型Void.class | CharSequense.class
        handlers.add(new MapMethodProcessor()); // 處理返回類型Map

        // Custom return value types
        if (getCustomReturnValueHandlers() != null) {
            handlers.addAll(getCustomReturnValueHandlers()); // 自定義
        }

        // Catch-all
        if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {
            handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));
        }
        else {
            handlers.add(new ModelAttributeMethodProcessor(true)); // 處理返回類型ModelAttribute | 普通類型
        }

        return handlers;
    }