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
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，
- 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
- 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
- ignoreAcceptHeader是false，所以会把HeaderContentNegotiationStrategy加入到结果集合中，即Http头部的Accept
- 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
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