Spring MVC 406

使用Spring MVC返回 JSON 数据有时候会在页面报出以下 406 错误。具体错误信息如下：

这里写图片描述

最常见的问题就是缺少 Jackson 工具包，它的作用是把 Java 对象转换成 JSON 输入出页面。当然这是最常见的情况，下面我就来介绍一下项目中出现的问题。由于项目遗留原因，项目请求中 URI 都是以 .htm 结尾。之前都是使用 HttpServletResponse 操作原生 Servlet 来返回 JSON 数据，而不是使用 Spring MVC 提供的 @ResponseBody 注解。

    public void out(Object obj, HttpServletResponse response) {
        response.setContentType("text/html; charset=utf-8");

        PrintWriter out = null;
        try {
            out = response.getWriter();
        } catch (IOException e) {
            e.printStackTrace();
        }
        out.print(obj);
    }

重复的代码就是不好的

所以对于新添加的接口我打算使用 Spring MVC 提供的 @ResponseBody来返回 JSON 数据。使用方式很简单，定义 @RequestMapping 方法返回值为任意的 POJO 对象，然后再这个方法上面添加 @ResponseBody 注解就好了。

    @RequestMapping("uri路径")
    @ResponseBody
    public User user(){
        User user = new User();
        user.setId("1");
        user.setName("carl");
        return user;
    }

之前一直使用这个注解都可以解决这个问题，但是公司项目中居然不成功。我检查了一下 pom 文件是引用了 Jackson Jar包，排除这个原因。和之前使用 @ResponseBody 注解的的不同点就是请求 URI 里面包含了 .htm，然后我就做了以下的小实验。

请求URI	返回
test	成功返回JSON
test.htm	406
test.xxx	成功返回JSON

从上面的例子中我们可以看到请求 URI 的后缀对于 Spring MVC 的响应生成是有影响的。

我们知道在 Spring MVC 中 HandlerMethodArgumentResolver接口负责将 HttpServletRequest 里面的请求参数绑定到标注了 @RequestMapping 的@Controller 的方法中;而对于 @RequestMapping 方法的返回值 Spring MVC 通过HandlerMethodReturnValueHandler来处理。Spring MVC 支持 restful，通过 @RequestBody、@ResposeBody 就是通过实现了以上两个接口的 RequestResponseBodyMethodProcessor 来实现的，而处理 restful 底层是通过 HttpMessageConverters 接口来实现的，对于这个接口这里我们就不过多介绍了。

下面我们就从源码的角度来分析一下返回 JSON 报406 这个错误的原因。

在Spring MVC 中处理 @ResponseBody 的入口是RequestResponseBodyMethodProcessor#handleReturnValue，而主要核心处理逻辑是在AbstractMessageConverterMethodProcessor#writeWithMessageConverters

    protected <T> void writeWithMessageConverters(T value, MethodParameter returnType,
            ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
            throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

        Class<?> valueType = getReturnValueType(value, returnType);
        Type declaredType = getGenericType(returnType);
        HttpServletRequest request = inputMessage.getServletRequest();
        List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);
        List<MediaType> producibleMediaTypes = getProducibleMediaTypes(request, valueType, declaredType);

        if (value != null && producibleMediaTypes.isEmpty()) {
            throw new IllegalArgumentException("No converter found for return value of type: " + valueType);
        }

        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 (value != null) {
                throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
            }
            return;
        }

        List<MediaType> mediaTypes = new ArrayList<MediaType>(compatibleMediaTypes);
        MediaType.sortBySpecificityAndQuality(mediaTypes);

        MediaType selectedMediaType = null;
        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) {
                if (messageConverter instanceof GenericHttpMessageConverter) {
                    if (((GenericHttpMessageConverter<T>) messageConverter).canWrite(
                            declaredType, valueType, selectedMediaType)) {
                        value = (T) getAdvice().beforeBodyWrite(value, returnType, selectedMediaType,
                                (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
                                inputMessage, outputMessage);
                        if (value != null) {
                            addContentDispositionHeader(inputMessage, outputMessage);
                            ((GenericHttpMessageConverter<T>) messageConverter).write(
                                    value, declaredType, selectedMediaType, outputMessage);
                            if (logger.isDebugEnabled()) {
                                logger.debug("Written [" + value + "] as \"" + selectedMediaType +
                                        "\" using [" + messageConverter + "]");
                            }
                        }
                        return;
                    }
                }
                else if (messageConverter.canWrite(valueType, selectedMediaType)) {
                    value = (T) getAdvice().beforeBodyWrite(value, returnType, selectedMediaType,
                            (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
                            inputMessage, outputMessage);
                    if (value != null) {
                        addContentDispositionHeader(inputMessage, outputMessage);
                        ((HttpMessageConverter<T>) messageConverter).write(value, selectedMediaType, outputMessage);
                        if (logger.isDebugEnabled()) {
                            logger.debug("Written [" + value + "] as \"" + selectedMediaType +
                                    "\" using [" + messageConverter + "]");
                        }
                    }
                    return;
                }
            }
        }

        if (value != null) {
            throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
        }
    }

上面的代码看着很复杂其实逻辑很简单。

1) getAcceptableMediaTypes() 通过策略获取到请求可以接受的 MedieType
2) getProducibleMediaTypes() 根据返回值获取到可产生哪些 MedieType
3) isCompatibleWith() 匹配请求 MedieType 与响应产生的 MedieType，如果匹配就添加到匹配的 MedieType 列表当中。
4) HttpMessageConverter#write() 根据在 Medie 列表中找到的最合适的 MedieType 把它写入 HttpServletResponse 中

我们可以看到有 3 个地方会影响最终响应的生成：也就是第1、2、4 这 4 个步骤。

而在Spring MVC 找不到 Jackson 就属于第 4 步，因为处理 JSON 对应的HttpMessageConverter为 MappingJackson2HttpMessageConverter。而添加这个类的处理逻辑在WebMvcConfigurationSupport#addDefaultHttpMessageConverters。
这里写图片描述

它是根据jackson2Present这个参数来添加 JSON 处理器的。

    private static final boolean jackson2Present =
            ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", WebMvcConfigurationSupport.class.getClassLoader()) &&
                    ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator", WebMvcConfigurationSupport.class.getClassLoader());

上面这段代码的逻辑是在当前 ClassLoader 加载 ObjectMapper 或者 JsonGenerator，如果加载成功就添加 MappingJackson2HttpMessageConverter，而这两个类属于 Jackson。

下面我们来看第一点：

其实获取请求可接受的 MedieType 是根据 ContentNegotiationManager#resolveMediaTypes Spring MVC 内容协商来解析的。

默认有两种策略，也就是 ContentNegotiationManager#strategies：

ServletPathExtensionContentNegotiationStrategy：根据请求 URI 扩展名来获取 MedieType，它最终会调用javax.servlet.ServletContext#getMimeType从 ServletContext 里面获取支持的 URI 请求扩展，包含以下 170 种扩展：

 "css" -> "text/css"
 "ps" -> "application/postscript"
 "movie" -> "video/x-sgi-movie"
 "bin" -> "application/octet-stream"
 "xspf" -> "application/xspf+xml"
 "axa" -> "audio/annodex"
 "jad" -> "text/vnd.sun.j2me.app-descriptor"
 "xul" -> "application/vnd.mozilla.xul+xml"
 "midi" -> "audio/midi"
 "exe" -> "application/octet-stream"
 "java" -> "text/x-java-source"
 "texi" -> "application/x-texinfo"
 "mov" -> "video/quicktime"
 "dvi" -> "application/x-dvi"
 "xml" -> "application/xml"
 "jar" -> "application/java-archive"
 "axv" -> "video/annodex"
 "pict" -> "image/pict"
 "mpa" -> "audio/mpeg"
 "zip" -> "application/zip"
 "oth" -> "application/vnd.oasis.opendocument.text-web"
 "mpe" -> "video/mpeg"
 "otg" -> "application/vnd.oasis.opendocument.graphics-template"
 "qt" -> "video/quicktime"
 "cdf" -> "application/x-cdf"
 "mpg" -> "video/mpeg"
 "ras" -> "image/x-cmu-raster"
 "bcpio" -> "application/x-bcpio"
 "tex" -> "application/x-tex"
 "ai" -> "application/postscript"
 "png" -> "image/png"
 "eps" -> "application/postscript"
 "mathml" -> "application/mathml+xml"
 "otp" -> "application/vnd.oasis.opendocument.presentation-template"
 "odb" -> "application/vnd.oasis.opendocument.database"
 "oda" -> "application/oda"
 "texinfo" -> "application/x-texinfo"
 "ott" -> "application/vnd.oasis.opendocument.text-template"
 "pnm" -> "image/x-portable-anymap"
 "odc" -> "application/vnd.oasis.opendocument.chart"
 "ots" -> "application/vnd.oasis.opendocument.spreadsheet-template "
 "odf" -> "application/vnd.oasis.opendocument.formula"
 "odg" -> "application/vnd.oasis.opendocument.graphics"
 "au" -> "audio/basic"
 "odi" -> "application/vnd.oasis.opendocument.image"
 "pnt" -> "image/x-macpaint"
 "doc" -> "application/msword"
 "odm" -> "application/vnd.oasis.opendocument.text-master"
 "odp" -> "application/vnd.oasis.opendocument.presentation"
 "rm" -> "application/vnd.rn-realmedia"
 "jsf" -> "text/plain"
 "odt" -> "application/vnd.oasis.opendocument.text"
 "aif" -> "audio/x-aiff"
 "ods" -> "application/vnd.oasis.opendocument.spreadsheet"
 "aim" -> "application/x-aim"
 "xwd" -> "image/x-xwindowdump"
 "vsd" -> "application/vnd.visio"
 "flac" -> "audio/flac"
 "mpega" -> "audio/x-mpeg"
 "js" -> "application/javascript"
 "mid" -> "audio/midi"
 "mif" -> "application/x-mif"
 "mac" -> "image/x-macpaint"
 "cer" -> "application/pkix-cert"
 "sh" -> "application/x-sh"
 "pgm" -> "image/x-portable-graymap"
 "wml" -> "text/vnd.wap.wml"
 "jpeg" -> "image/jpeg"
 "man" -> "text/troff"
 "wmv" -> "video/x-ms-wmv"
 "art" -> "image/x-jg"
 "rtf" -> "application/rtf"
 "svg" -> "image/svg+xml"
 "snd" -> "audio/basic"
 "mpv2" -> "video/mpeg2"
 "ppm" -> "image/x-portable-pixmap"
 "txt" -> "text/plain"
 "pps" -> "application/vnd.ms-powerpoint"
 "abs" -> "audio/x-mpeg"
 "shar" -> "application/x-shar"
 "t" -> "text/troff"
 "xpm" -> "image/x-xpixmap"
 "asf" -> "video/x-ms-asf"
 "ppt" -> "application/vnd.ms-powerpoint"
 "rdf" -> "application/rdf+xml"
 "rtx" -> "text/richtext"
 "z" -> "application/x-compress"
 "dib" -> "image/bmp"
 "cpio" -> "application/x-cpio"
 "tr" -> "text/troff"
 "swf" -> "application/x-shockwave-flash"
 "bmp" -> "image/bmp"
 "xht" -> "application/xhtml+xml"
 "asx" -> "video/x-ms-asf"
 "oga" -> "audio/ogg"
 "roff" -> "text/troff"
 "wspolicy" -> "application/wspolicy+xml"
 "pic" -> "image/pict"
 "body" -> "text/html"
 "latex" -> "application/x-latex"
 "hqx" -> "application/mac-binhex40"
 "ogg" -> "audio/ogg"
 "tif" -> "image/tiff"
 "dv" -> "video/x-dv"
 "me" -> "text/troff"
 "wbmp" -> "image/vnd.wap.wbmp"
 "html" -> "text/html"
 "ogv" -> "video/ogg"
 "svgz" -> "image/svg+xml"
 "ogx" -> "application/ogg"
 "tar" -> "application/x-tar"
 "ms" -> "application/x-wais-source"
 "qti" -> "image/x-quicktime"
 "etx" -> "text/x-setext"
 "nc" -> "application/x-netcdf"
 "qtif" -> "image/x-quicktime"
 "mpeg" -> "video/mpeg"
 "spx" -> "audio/ogg"
 "pbm" -> "image/x-portable-bitmap"
 "psd" -> "image/vnd.adobe.photoshop"
 "ulw" -> "audio/basic"
 "xbm" -> "image/x-xbitmap"
 "tiff" -> "image/tiff"
 "aiff" -> "audio/x-aiff"
 "gif" -> "image/gif"
 "aifc" -> "audio/x-aiff"
 "ief" -> "image/ief"
 "rgb" -> "image/x-rgb"
 "jspf" -> "text/plain"
 "m3u" -> "audio/x-mpegurl"
 "xsl" -> "application/xml"
 "avi" -> "video/x-msvideo"
 "dtd" -> "application/xml-dtd"
 "htc" -> "text/x-component"
 "sv4crc" -> "application/x-sv4crc"
 "tsv" -> "text/tab-separated-values"
 "vxml" -> "application/voicexml+xml"
 "sv4cpio" -> "application/x-sv4cpio"
 "json" -> "application/json"
 "tcl" -> "application/x-tcl"
 "class" -> "application/java"
 "kar" -> "audio/midi"
 "jpe" -> "image/jpeg"
 "sit" -> "application/x-stuffit"
 "htm" -> "text/html"
 "jpg" -> "image/jpeg"
 "pct" -> "image/pict"
 "ustar" -> "application/x-ustar"
 "avx" -> "video/x-rad-screenplay"
 "src" -> "application/x-wais-source"
 "anx" -> "application/annodex"
 "wmls" -> "text/vnd.wap.wmlsc"
 "hdf" -> "application/x-hdf"
 "wav" -> "audio/x-wav"
 "gtar" -> "application/x-gtar"
 "mp2" -> "audio/mpeg"
 "mp1" -> "audio/mpeg"
 "xhtml" -> "application/xhtml+xml"
 "mp4" -> "video/mp4"
 "wrl" -> "model/vrml"
 "mp3" -> "audio/mpeg"
 "gz" -> "application/x-gzip"
 "pdf" -> "application/pdf"
 "pls" -> "audio/x-scpls"
 "wmlscriptc" -> "application/vnd.wap.wmlscriptc"
 "csh" -> "application/x-csh"
 "jnlp" -> "application/x-java-jnlp-file"
 "wmlc" -> "application/vnd.wap.wmlc"
 "xslt" -> "application/xslt+xml"
 "xls" -> "application/vnd.ms-excel"

因为 htm后缀对应 text/html，所以如果请求是 xxx.htm，不管第二步返回什么，服务端最多只能生成 html 页面。而使用test.xxx，并不在支持的扩展参数里面，所以没有影响。

HeaderContentNegotiationStrategy请求头策略，根据 http 的请求头来生成请求可接受的 MedieType。

第二步是获取到服务端支持的可响应的 MedieType，它的规则如下：

获取@RequestMapping 注解的 produces() 标注。
遍历所有的 HttpMessageConverter 获取支持 @RequestMapping 返回值的 MedieType

因为是 URI 扩展参数惹的祸，所以我首先想到的解决方案就是移除 ServletPathExtensionContentNegotiationStrategy 这个策略。

因为是 Spring IOC 来创建对象，所以我想根据 Spring IOC 容器扩展来解决这个问题。

方法一：使用BeanPostProcessor修改 Bean

因为是WebMvcConfigurationSupport#requestMappingHandlerAdapter来创建 RequestMappingHandlerAdapter并且WebMvcConfigurationSupport#mvcContentNegotiationManager创建的 ContentNegotiationManager。所以从容器中获取到 bean Id 为requestMappingHandlerAdapter的 Bean 对象RequestMappingHandlerAdapter，获取到 ContentNegotiationManager。获取直接根据 mvcContentNegotiationManager获取到 ContentNegotiationManager。然后通过移除 ContentNegotiationManager.strategies策略列表中的 URI 扩展参数策略就可以了。

因为 RequestMappingHandlerAdapter 对象里面没有 ContentNegotiationManager 的获取方法且 ContentNegotiationManager 类中没有策略列表的操作方法，所以这个方法不可行。

方法二：使用BeanFactoryPostProcessor修改 Bean

可以通过 BeanFactoryPostProcessor#postProcessBeanFactory 来修改 BeanDefinition 的属性来移除策略列表中的 URI 扩展参数策略。

因为 @Configuration 与 @Bean 生成的 BeanDefinition 是把这个 BeanDefinition 伪装成一个 Spring Factory Bean。创建实例直接调用这个方法，而不能通过 BeanDefinition 里面的参数来控制对象的创建。所以这个方法也不可行。

方法三：@EnableMvcConfig

在 WebMvcConfigurationSupport 类中调用 mvcContentNegotiationManager方法生成 ContentNegotiationManager 对象的时候，最终会调用 ContentNegotiationManagerFactoryBean的afterPropertiesSet() 而 favorPathExtension 参数可以控制是否添加 PathExtensionContentNegotiationStrategy，如果这个值为 true 就会添加，反之而不会。这个值的默认值是 true，那么我们可以不可修改这个参数的值呢？

答案是有的，因为在调用ContentNegotiationManagerFactoryBean#afterPropertiesSet方法之前，会调用 WebMvcConfigurationSupport#configureContentNegotiation而我们可以通过继承 WebMvcConfigurerAdapter 类使用 @EnableWebMvc 注解来修改这个值。

下面就是我的测试代码工程结构：

这里写图片描述

Bootstrap.java

@SpringBootApplication
public class Bootstrap {

    public static void main(String[] args) {
        SpringApplication.run(Bootstrap.class, args);
    }

}

MyMvcConfig.java

@Configuration
@EnableWebMvc
public class MyMvcConfig extends WebMvcConfigurerAdapter {

    @Override
    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
        configurer.favorPathExtension(false);
        super.configureContentNegotiation(configurer);
    }
}

TestController.java

@Controller
public class TestController {

    @RequestMapping("URI地址")
    @ResponseBody
    public User user(){
        User user = new User();
        user.setId("1");
        user.setName("carl");
        return user;
    }

}

然后再使用以上的请求 URI 做个实验：

请求URI	返回
test	成功返回JSON
test.htm	成功返回JSON
test.xxx	成功返回JSON

并且无论访问哪个 URI 生成的 requestedMediaTypes 都为：

这里写图片描述

并且 http 的请求头如下：

这里写图片描述

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