spring源代码分析之org.springframework.web.method.support.HandlerMethodReturnValueHandler

该接口的设计用到了设计模式中的“策略模式”，spring提供了下面一些实现类：那么有这么多HandlerMethodReturnValueHandler实现类，在spring在处理请求时是如何选择合适的HandlerMethodReturnValueHandler实现类的呢，这里有一个重要的类：org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite，该类实现中使用了“组合模式”：public class HandlerMethodReturnValueHandlerComposite implements AsyncHandlerMethodReturnValueHandler {protected final Log logger = LogFactory.getLog(getClass());private final List<HandlerMethodReturnValueHandler> returnValueHandlers =new ArrayList<HandlerMethodReturnValueHandler>();选择合适的HandlerMethodReturnValueHandler的方法为selectHandler：private HandlerMethodReturnValueHandler selectHandler(Object value, MethodParameter returnType) {

//调用supportsReturnType方法来选择合适的HandlerMethodReturnValueHandler实现类
boolean isAsyncValue = isAsyncReturnValue(value, returnType);
for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {
continue;
}
if (handler.supportsReturnType(returnType)) {
return handler;
}
}
return null;
}

HandlerMethodReturnValueHandlerComposite类对外提供的处理http请求的方法如下:@Overridepublic void handleReturnValue(Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {//分两步：第一步找到具体的HandlerMethodReturnValueHandler；第二部调用该handler的

//handleReturnValue方法

HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
if (handler == null) {
throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
}
handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);

}

spring消息转换就是调用RequestResponseBodyMethodProcessor类的handleReturnValue方法来调用相关消息转换器

代码的：

	@Overridepublic void handleReturnValue(Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest)throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {mavContainer.setRequestHandled(true);ServletServerHttpRequest inputMessage = createInputMessage(webRequest);ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);// Try even with null return value. ResponseBodyAdvice could get involved.writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);}