[Servlet/Tomcat] HttpServletRequest#getHeader(headerNameWithIgnoreCase)（获取header时不区分大小写）

1 故事背景

• 最近项目上有个业务需求，翻译成技术需求，即：将request.headers中的几个header入参转换成request.body(pageRequest)中的内置参数。

为便于灵活配置，header 参数名称是动态可配置的（存放于nacos配置中心），比如：sysCode、Accept-Language

• 技术实现，主要就 springmvc的org.springframework.web.bind.WebDataBinder ，并结合 javax.servlet.http.HttpServletRequest，实现将header中的指定参数转发至request.body(pageRequest).params中

核心代码如下：

@RestController("cn.johnnyzen.bd.dataservice.biz.dataservice.controller.v2.CommonSearchController")
@Validated
@Api(tags = "DATA2API Controller # V2")
public class CommonSearchController implements DataServiceOpenApi {

    // ...

    @Autowired
    private ServiceConfig serviceConfig;

    // ...

    @InitBinder
    public void forwardedHeadersToParamsDataBinder(WebDataBinder binder, HttpServletRequest request) {
        //判断是否启用本特性
        ForwardedHeaders forwardedHeaders = serviceConfig.getForwardedHeaders();
        if(ObjectUtils.isEmpty(forwardedHeaders) || forwardedHeaders.getEnable().equals(Boolean.FALSE)){
            logger.warn("Fail to forward headers to body's params because that `service-config.forwardedHeaders` be empty or not enabled!");
            return ;
        }
        logger.info("Start to forward request's headers to request body params with `service-config.forwardedHeaders`,config data as follows : \n{}", forwardedHeaders);

        //获取被绑定对象----PageRequest
        PageRequest<Map<String,Object>> pageRequest = (PageRequest<Map<String, Object>>) binder.getTarget();
        if(ObjectUtils.isEmpty(pageRequest) || ObjectUtils.isEmpty(pageRequest.getParams())){
            logger.error("`request.body(pageRequest)` or `request.body(pageRequest).params` is empty!");
            return;
        }
        Map<String,Object> dynamicParams = pageRequest.getParams();

        List<ForwardHeaderToParamConfig> forwardHeaderToParamConfigList = forwardedHeaders.getHeaders();
        forwardHeaderToParamConfigList.stream().forEach(forwardedHeaderConfig -> {
            //获取目标header参数值，并转发至params中
            String headerName = forwardedHeaderConfig.getHeader();
            String headerValue = request.getHeader( forwardedHeaderConfig.getHeader() );
            String paramName = forwardedHeaderConfig.getParam();
            logger.debug("headerName:{}, headerValue:{}, paramName:{}", headerName, headerValue, paramName);
            dynamicParams.put(paramName, headerValue);
        });
    }

    // ...
}

• 那么，我写这篇博客的目的是什么呢？
• 你有没有这么一个疑惑：request.getHeader(headerName)，这个基于Tomcat.catalina实现的方法，是否区分headerName的大小写？

如果不区分大小写，那还好办；如果区分大小写，就尴尬了————我将需要将Accept-Language的每一种字母大小写的可能性都要一一进行配置！

经过源码分析，答案是：request.getHeader(headerName)不区分大小写！

感兴趣的朋友，可以进入第2章节，一起看看源码

2 源码分析

• springmvc: 5.2.15.RELEASE
• springboot: 2.3.12.RELEASE
• tomcat-embed: 9.0.46 (springboot内嵌的tomcat)
• 调试工具: IDEA

Step1 javax.servlet.http.HttpServletRequest : request.getHeader("Accept-Language")

import javax.servlet.http.HttpServletRequest;

//...
request.getHeader("Accept-Language")
//...

Step2 javax.servlet.http.HttpServletRequest#getHeader

javax.servlet.http.HttpServletRequest#getHeader

Step3 org.apache.catalina.connector.Request#getHeader

特别说明：org.apache.catalina.connector.Request#getHeader 其实现了接口：javax.servlet.http.HttpServletRequest#getHeader

org.apache.catalina.connector.Request#getHeader

Step4 org.apache.coyote.Request#getHeader

org.apache.coyote.Request#getHeader

Step5 org.apache.tomcat.util.http.MimeHeaders#getHeader

org.apache.tomcat.util.http.MimeHeaders#getHeader

Step6 org.apache.tomcat.util.http.MimeHeaders#getValue(java.lang.String)

org.apache.tomcat.util.http.MimeHeaders#getValue(java.lang.String)