SpringFramework之ContentNegotiation内容协商

    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

1private List<MediaType> getAcceptableMediaTypes(HttpServletRequest request)
2		throws HttpMediaTypeNotAcceptableException {
3	return this.contentNegotiationManager.resolveMediaTypes(new ServletWebRequest(request));
4}

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

    List-2

1@Override
2public List<MediaType> resolveMediaTypes(NativeWebRequest request) throws HttpMediaTypeNotAcceptableException {
3	for (ContentNegotiationStrategy strategy : this.strategies) {
4		List<MediaType> mediaTypes = strategy.resolveMediaTypes(request);
5		if (mediaTypes.equals(MEDIA_TYPE_ALL_LIST)) {
6			continue;
7		}
8		return mediaTypes;
9	}
10	return MEDIA_TYPE_ALL_LIST;
11}

   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

1public ContentNegotiationManager build() {
2	List<ContentNegotiationStrategy> strategies = new ArrayList<>();
3
4	if (this.strategies != null) {
5		strategies.addAll(this.strategies);
6	}
7	else {
8        //1
9		if (this.favorPathExtension) {
10			PathExtensionContentNegotiationStrategy strategy;
11			if (this.servletContext != null && !useRegisteredExtensionsOnly()) {
12				strategy = new ServletPathExtensionContentNegotiationStrategy(this.servletContext, this.mediaTypes);
13			}
14			else {
15				strategy = new PathExtensionContentNegotiationStrategy(this.mediaTypes);
16			}
17			strategy.setIgnoreUnknownExtensions(this.ignoreUnknownPathExtensions);
18			if (this.useRegisteredExtensionsOnly != null) {
19				strategy.setUseRegisteredExtensionsOnly(this.useRegisteredExtensionsOnly);
20			}
21			strategies.add(strategy);
22		}
23        //2
24		if (this.favorParameter) {
25			ParameterContentNegotiationStrategy strategy = new ParameterContentNegotiationStrategy(this.mediaTypes);
26			strategy.setParameterName(this.parameterName);
27			if (this.useRegisteredExtensionsOnly != null) {
28				strategy.setUseRegisteredExtensionsOnly(this.useRegisteredExtensionsOnly);
29			}
30			else {
31				strategy.setUseRegisteredExtensionsOnly(true);  // backwards compatibility
32			}
33			strategies.add(strategy);
34		}
35        //3
36		if (!this.ignoreAcceptHeader) {
37			strategies.add(new HeaderContentNegotiationStrategy());
38		}
39        //4
40		if (this.defaultNegotiationStrategy != null) {
41			strategies.add(this.defaultNegotiationStrategy);
42		}
43	}
44
45	this.contentNegotiationManager = new ContentNegotiationManager(strategies);
46	return this.contentNegotiationManager;
47}

    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

1@Override
2public List<MediaType> resolveMediaTypes(NativeWebRequest request) throws HttpMediaTypeNotAcceptableException {
3	for (ContentNegotiationStrategy strategy : this.strategies) {
4		List<MediaType> mediaTypes = strategy.resolveMediaTypes(request);
5		if (mediaTypes.equals(MEDIA_TYPE_ALL_LIST)) {
6			continue;
7		}
8		return mediaTypes;
9	}
10	return MEDIA_TYPE_ALL_LIST;
11}

    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