AWS CDK LambdaRestApi配置form/urlencoded及代理兼容问题

使用form/urlencoded格式请求ApiGateway时存在适配异常，当前ApiGateway已从原DotnetLambda架构调整为LambdaRestApi架构，所有常规JSON格式POST请求均可正常运行。
此前可通过AWS Web控制台手动为ApiGateway配置application/x-www-form-urlencoded; charset=utf-8请求处理规则解决该类适配问题，但该手动配置方式无法满足基础设施即代码的自动化部署要求。

• 其余所有ANY类型端点需保持proxy: false整体配置下的原有逻辑，未单独配置integrationParms/LambdaIntegration的路径要保障其他本地API调用正常运行
• 核心目标：让ApiGateway既保留proxy:true模式下的请求处理效果，又能支持需要proxy: false配置的自定义路径、自定义请求模型（处理form/urlencoded格式请求），解决当前配置抛出异常的问题

export class DotNetRestApiCustom extends Construct {
  lambda: DotnetLambda;
  api: LambdaRestApi;

  constructor(scope: Construct, id: string, props: DotNetHttpLambdaApiProps) {
    super(scope, id);

this.lambda = new DotnetLambda(
  this,
  `${props.name}ApiLambda`,
  `${props.name}Api`,
  props.lambdaPath,
  props.functionHandler,
  props.functionProps,
  props.extraHashCodePaths
);

let domainName = props.fullDomainName ?? `${Utils.stage.toLowerCase()}.${props.domainTag}.internal.${props.dnsZone.zoneName}`;
if (!props.fullDomainName && props.internalDomain === false) domainName = `${Utils.stage.toLowerCase()}.${props.domainTag}.${props.dnsZone.zoneName}`;

this.api = new LambdaRestApi(this, `${props.name}Api4`, {
  proxy: false,
  handler: this.lambda,
  domainName: {
    domainName: domainName,
    certificate: new Certificate(this, "Certificate", {
      domainName: domainName,
      validation: CertificateValidation.fromDns(props.dnsZone),
    }),
    endpointType: EndpointType.REGIONAL,
    securityPolicy: SecurityPolicy.TLS_1_2,
  },
});

const msgResources = this.api.root.addResource("person");
const msgResource = msgResources.addResource("{personId}");

const methodResponse: MethodResponse = {
  statusCode: "200", 
  responseModels: {"application/json": Model.EMPTY_MODEL}
}

const integrationResponse: IntegrationResponse = {
  statusCode: "200",
  contentHandling: ContentHandling.CONVERT_TO_TEXT
}

const requestTemplate = {
  "body": "$input.json('$')",
}

const integrationParams = new LambdaIntegration(this.lambda, {
  allowTestInvoke: true,
  proxy: false,
  integrationResponses: [integrationResponse],
  passthroughBehavior: PassthroughBehavior.WHEN_NO_TEMPLATES,
  requestTemplates: { "application/json": JSON.stringify(requestTemplate) },
});

this.api.root.addMethod("POST", integrationParams, {
  methodResponses: [methodResponse],
});

new ARecord(this, "ApiDnsRecord", {
  zone: props.dnsZone,
  recordName: domainName,
  target: RecordTarget.fromAlias(new ApiGateway(this.api)),
});

现有配置报错的核心原因是requestTemplates仅配置了application/json类型的映射规则，form/urlencoded格式请求进入时没有匹配的模板，透传后Lambda无法正确解析请求体。按以下两步修改即可满足需求：

• 第一步：在requestTemplates中新增application/x-www-form-urlencoded类型的映射模板，将表单参数自动解析为和proxy模式一致的Lambda事件结构，代码如下：

  const integrationParams = new LambdaIntegration(this.lambda, {
    allowTestInvoke: true,
    proxy: false,
    integrationResponses: [integrationResponse],
    passthroughBehavior: PassthroughBehavior.WHEN_NO_TEMPLATES,
    requestTemplates: {
      "application/json": JSON.stringify(requestTemplate),
      "application/x-www-form-urlencoded": `{
        "body": $util.urlDecode($input.body),
        "formParams": {
          #foreach( $token in $input.path('$').split('&') )
            #set( $keyVal = $token.split('=') )
            #if( $keyVal.size() >=2 )
              "$util.urlDecode($keyVal[0])": "$util.urlDecode($keyVal[1])"#if($foreach.hasNext),#end
            #end
          #end
        },
        "headers": {
          #foreach($header in $input.params().header.keySet())
          "$header": "$util.escapeJavaScript($input.params().header.get($header))" #if($foreach.hasNext),#end
          #end
        },
        "queryStringParameters": {
          #foreach($queryParam in $input.params().querystring.keySet())
          "$queryParam": "$util.escapeJavaScript($input.params().querystring.get($queryParam))" #if($foreach.hasNext),#end
          #end
        },
        "pathParameters": {
          #foreach($pathParam in $input.params().path.keySet())
          "$pathParam": "$util.escapeJavaScript($input.params().path.get($pathParam))" #if($foreach.hasNext),#end
          #end
        },
        "requestContext": {
          "httpMethod": "$context.httpMethod",
          "path": "$context.path",
          "requestId": "$context.requestId",
          "stage": "$context.stage"
        }
      }`
    },
  });
  

• 第二步：给所有非自定义处理的路径绑定默认代理集成，完全复现proxy:true模式的处理效果，不影响原有其他接口的正常运行：

  // 默认代理集成，和原proxy:true逻辑完全一致
  const defaultProxyIntegration = new LambdaIntegration(this.lambda, { proxy: true });
  // 给根路径、person路径、personId路径绑定ANY方法的默认集成
  this.api.root.addMethod("ANY", defaultProxyIntegration);
  msgResources.addMethod("ANY", defaultProxyIntegration);
  msgResource.addMethod("ANY", defaultProxyIntegration);
  // 后续新增其他资源路径时，同步绑定该默认集成即可
  

配置注意：表单映射模板里要完整透传头信息、查询参数、路径参数和上下文信息，保证传给后端Lambda的事件结构和proxy模式完全一致，避免业务代码读取参数报错。

内容的提问来源于stack exchange，提问作者MichaelEvanchik