从content-type设置看Spring MVC处理header的一个坑

转载自：https://www.cnblogs.com/kaiblog/p/7565231.html

我们经常需要在HttpResponse中设置一些headers，我们使用Spring MVC框架的时候我们如何给Response设置Header呢？

Sooooooooooooo easy，看下面的代码：

@RequestMapping(value = "/rulelist", method = RequestMethod.GET)
@ResponseBody
public String getRuleList(HttpServletRequest request,
        HttpServletResponse response) {
    response.addHeader("test", "test"); 
    return service.getRuleList();
}

通过验证，我们可以看到test项已经被成功添加到response的头部信息

Content-Length: 2 kilobytes
Content-Type:   text/plain;charset=ISO-8859-1
Server: Apache-Coyote/1.1
test: test

接下来，我们希望修改Content-Type，从而统一服务器端和客户端的内容编码。我们继续修改代码，

@RequestMapping(value = "/rulelist", method = RequestMethod.GET)
@ResponseBody
public String getRuleList(HttpServletRequest request,
        HttpServletResponse response) {
    response.addHeader("Content-Type", "application/json;charset=UTF-8");
    return service.getRuleList();
}

接下来，我们验证一下结果：

Content-Length: 2 kilobytes
Content-Type:   text/plain;charset=ISO-8859-1
Server: Apache-Coyote/1.1

和我们预想的并一样，response的content-type header没有被设置成"application/json;charset=UTF-8"，很令人困惑。

那么，接下来让我们来探索下Spring MVC内部是如何处理这一过程的。首先我们先要对Spring MVC框架处理Http请求的流程有一个整体的了解。

下图清晰地向大家展示了Spring MVC处理HTTP请求的流程，（图片来自网络）

具体流程如下：

1. DispatcherServlet接收到Request请求

2. HandlerMapping选择一个合适的Handler处理Request请求

3-4. 选择合适的HandlerAdapter，调用用户编写的Controller处理业务逻辑。（HandlerAdapter主要是帮助Spring MVC支持多种类型的Controller）

5. Controller将返回结果放置到Model中并且返回view名称给Handler Adapter

6. DispatcherServlet选择合适的ViewResolver来生成View对象

7-8. View对象利用Model中的数据进行渲染并返回数据

相信大家对于上面的处理流程并不陌生，上面的流程图向我们展示了SpringMVC生成ModelAndView并返回response的大体流程。

下面我们来看看我们上面代码片段的处理流程是如何进行的？

从上面的流程图我们可以看到，content-type header是单独被处理的，具体过程可以参考下面的源码(AbstractMessageConverterMethodProcessor)：

protected <T> void writeWithMessageConverters(T returnValue, MethodParameter returnType,
        ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
        throws IOException, HttpMediaTypeNotAcceptableException {
 
    Class<?> returnValueClass = getReturnValueType(returnValue, returnType);
    HttpServletRequest servletRequest = inputMessage.getServletRequest();
    List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(servletRequest); //适合的兼容media types类型实际上，我们可以使用produces = {}来指定我们需要的mediatype
    List<MediaType> producibleMediaTypes = getProducibleMediaTypes(servletRequest, returnValueClass);
 
    Set<MediaType> compatibleMediaTypes = new LinkedHashSet<MediaType>();
    for (MediaType requestedType : requestedMediaTypes) {
        for (MediaType producibleType : producibleMediaTypes) {
            if (requestedType.isCompatibleWith(producibleType)) {
                compatibleMediaTypes.add(getMostSpecificMediaType(requestedType, producibleType));
            }
        }
    }
    if (compatibleMediaTypes.isEmpty()) {
        if (returnValue != null) {
            throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
        }
        return;
    }
 
    List<MediaType> mediaTypes = new ArrayList<MediaType>(compatibleMediaTypes);
    MediaType.sortBySpecificityAndQuality(mediaTypes);
 
    MediaType selectedMediaType = null;　　　//选择最匹配的mediaType
    for (MediaType mediaType : mediaTypes) {
        if (mediaType.isConcrete()) {
            selectedMediaType = mediaType;
            break;
        }
        else if (mediaType.equals(MediaType.ALL) || mediaType.equals(MEDIA_TYPE_APPLICATION)) {
            selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
            break;
        }
    }
 
    if (selectedMediaType != null) {
        selectedMediaType = selectedMediaType.removeQualityValue();
        for (HttpMessageConverter<?> messageConverter : this.messageConverters) {         //遍历messageConvertors, 寻找可以处理相应返回类型和mediatype的HttpMessageConvertor
            if (messageConverter.canWrite(returnValueClass, selectedMediaType)) {
                returnValue = this.adviceChain.invoke(returnValue, returnType, selectedMediaType,
                        (Class<HttpMessageConverter<?>>) messageConverter.getClass(), inputMessage, outputMessage);
                if (returnValue != null) {         //这里将会填充mediatype到header，并将httpmessage发送给请求者
                    ((HttpMessageConverter<T>) messageConverter).write(returnValue, selectedMediaType, outputMessage);
                    if (logger.isDebugEnabled()) {
                        logger.debug("Written [" + returnValue + "] as \"" + selectedMediaType + "\" using [" +
                                messageConverter + "]");
                    }
                }
                return;
            }
        }
    }
 
    if (returnValue != null) {
        throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
    }
}

接下来，将选择好的mediatype写入到HttpOutputMessage中

public final void write(final T t, MediaType contentType, HttpOutputMessage outputMessage)
        throws IOException, HttpMessageNotWritableException {
 
    final HttpHeaders headers = outputMessage.getHeaders();     //设置contenttype到HttpOutputMessage
    if (headers.getContentType() == null) {
        MediaType contentTypeToUse = contentType;
        if (contentType == null || contentType.isWildcardType() || contentType.isWildcardSubtype()) {
            contentTypeToUse = getDefaultContentType(t);
        }
        if (contentTypeToUse != null) {
            headers.setContentType(contentTypeToUse);
        }
    }
    if (headers.getContentLength() == -1) {
        Long contentLength = getContentLength(t, headers.getContentType());
        if (contentLength != null) {
            headers.setContentLength(contentLength);
        }
    }
　　　　　　/* 省略了不相干代码 */
}

最终的Headers设置在ServletServerHttpResponse类中完成，

private void writeHeaders() {
    if (!this.headersWritten) {
        for (Map.Entry<String, List<String>> entry : this.headers.entrySet()) {
            String headerName = entry.getKey();
            for (String headerValue : entry.getValue()) {         //将复合类中之前设置的header(content-type)内容补充到servletResponse
                this.servletResponse.addHeader(headerName, headerValue);
            }
        }
        // HttpServletResponse exposes some headers as properties: we should include those if not already present
        if (this.servletResponse.getContentType() == null && this.headers.getContentType() != null) {
            this.servletResponse.setContentType(this.headers.getContentType().toString());
        }
        if (this.servletResponse.getCharacterEncoding() == null && this.headers.getContentType() != null &&
                this.headers.getContentType().getCharSet() != null) {
            this.servletResponse.setCharacterEncoding(this.headers.getContentType().getCharSet().name());
        }
        this.headersWritten = true;
    }
}