Spring版本5.1.4.release.

    内容协商是用在Springmvc返回Controller方法结果序列化时使用，而不是解析mvc参数时使用。

    Springmvc支持４种内容协商，拓展名、固定值、Http的头部Accept、请求参数format，那Springmvc中怎么实现的呢，怎么使用已经有很多人分析了，这里来分析下怎么实现的。

    RequestResponseBodyMethodProcessor#handleReturnValue中调用了父类的writeWithMessageConverters，AbstractMessageConverterMethodProcessor#writeWithMessageConverters()中调用了getAcceptableMediaTypes，如下List-1

    List-1

private List<MediaType> getAcceptableMediaTypes(HttpServletRequest request)
        throws HttpMediaTypeNotAcceptableException {
    return this.contentNegotiationManager.resolveMediaTypes(new ServletWebRequest(request));
}

    List-2中，ContentNegotiationManager的resolveMediaTypes方法中循坏遍历ContentNegotiationStrategy，分别调用其resolveMediaTypes方法。

    List-2

@Override
public List<MediaType> resolveMediaTypes(NativeWebRequest request) throws HttpMediaTypeNotAcceptableException {
    for (ContentNegotiationStrategy strategy : this.strategies) {
        List<MediaType> mediaTypes = strategy.resolveMediaTypes(request);
        if (mediaTypes.equals(MEDIA_TYPE_ALL_LIST)) {
            continue;
        }
        return mediaTypes;
    }
    return MEDIA_TYPE_ALL_LIST;
}

   ContentNegotiationManager是由ContentNegotiationManagerFactoryBean创建的，如下图1所示，ContentNegotiationManagerFactoryBean实现了FactoryBean，通过getObject方法获取创建好后的ContentNegotiationManager，实现接口ServletContextAware是为了获取ServletContext从而构造ServletPathExtensionContentNegotiationStrategy，至于实现InitializingBean则是用afterPropertiesSet进行初始化ContentNegotiationManager

        

                                                                                    图1

    List-3

public ContentNegotiationManager build() {
    List<ContentNegotiationStrategy> strategies = new ArrayList<>();

    if (this.strategies != null) {
        strategies.addAll(this.strategies);
    }
    else {
        //1
        if (this.favorPathExtension) {
            PathExtensionContentNegotiationStrategy strategy;
            if (this.servletContext != null && !useRegisteredExtensionsOnly()) {
                strategy = new ServletPathExtensionContentNegotiationStrategy(this.servletContext, this.mediaTypes);
            }
            else {
                strategy = new PathExtensionContentNegotiationStrategy(this.mediaTypes);
            }
            strategy.setIgnoreUnknownExtensions(this.ignoreUnknownPathExtensions);
            if (this.useRegisteredExtensionsOnly != null) {
                strategy.setUseRegisteredExtensionsOnly(this.useRegisteredExtensionsOnly);
            }
            strategies.add(strategy);
        }
        //2
        if (this.favorParameter) {
            ParameterContentNegotiationStrategy strategy = new ParameterContentNegotiationStrategy(this.mediaTypes);
            strategy.setParameterName(this.parameterName);
            if (this.useRegisteredExtensionsOnly != null) {
                strategy.setUseRegisteredExtensionsOnly(this.useRegisteredExtensionsOnly);
            }
            else {
                strategy.setUseRegisteredExtensionsOnly(true);  // backwards compatibility
            }
            strategies.add(strategy);
        }
        //3
        if (!this.ignoreAcceptHeader) {
            strategies.add(new HeaderContentNegotiationStrategy());
        }
        //4
        if (this.defaultNegotiationStrategy != null) {
            strategies.add(this.defaultNegotiationStrategy);
        }
    }

    this.contentNegotiationManager = new ContentNegotiationManager(strategies);
    return this.contentNegotiationManager;
}

    如上List-3所示，

1. favorPathExtension是true，构造PathExtensionContentNegotiationStrategy，并加到结果结合strategies中，默认ServletPathExtensionContentNegotiationStrategy是不会构造的，除非我们手动的设置
2. favorParameter是false，如果我们设置为true后，会构造ParameterContentNegotiationStrategy，即我们设置的format=json会生效
3. ignoreAcceptHeader是false，所以会把HeaderContentNegotiationStrategy加入到结果集合中，即Http头部的Accept
4. 如果设置了defaultNegotiationStrategy，就会把我们添加的自定义Strategy加入到结果集合中

    如下List-4是ContentNegotiationManager的resolveMediaTypes方法，顺序遍历strategy，如果resolveMediaTypes返回的值不等于MEDIA_TYPE_ALL_LIST，那么就直接返回，结合List-3中添加的顺序，这就是路径拓展第一生效，第二format固定值，第三Http头部的Accept

  List-4

@Override
public List<MediaType> resolveMediaTypes(NativeWebRequest request) throws HttpMediaTypeNotAcceptableException {
    for (ContentNegotiationStrategy strategy : this.strategies) {
        List<MediaType> mediaTypes = strategy.resolveMediaTypes(request);
        if (mediaTypes.equals(MEDIA_TYPE_ALL_LIST)) {
            continue;
        }
        return mediaTypes;
    }
    return MEDIA_TYPE_ALL_LIST;
}

    ContentNegotiationStrategy使用了策略模式，在HeaderContentNegotiationStrategy中，从Http头部拿到Accept值后，还对结果按权重进行排序，这是有对应的JSR规范描述这个权重的计算的