OKHttp Add header only if it doesn't already exist

Question

I couldn't find a way to add an header only in case it doesn't exist. What I actually need is to add the default header "Content-Type": "application/json" but only in case the header doesn't exist already. One way to do it would be to have a different Http client or a different interceptor for when the default is necessary but I was expecting to be able to check whether the header is already there and add it only in case it is not.

Answer 1

This is definitely possible, but it also depends where you do it.

  val i = Interceptor {
    val request = if (it.request().header("A") != null) it.request() else it.request()
      .newBuilder()
      .header("A", "xxx")
      .build()
    
    val response = it.proceed(request)

    if (response.header("A") != null) response else response
      .newBuilder()
      .header("A", "xxx")
      .build()
  }

But Content-Type is special because it is usually carried on the RequestBody, or ResponseBody. The BridgeInterceptor sits between the application interceptors and network interceptors.

See https://github.com/square/okhttp/blob/5c62ed796d05682c969b2636d3419b5bc214eb11/okhttp/src/jvmMain/kotlin/okhttp3/internal/http/BridgeInterceptor.kt#L43-L46

      val contentType = body.contentType()
      if (contentType != null) {
        requestBuilder.header("Content-Type", contentType.toString())
      }

and

https://github.com/square/okhttp/blob/5c62ed796d05682c969b2636d3419b5bc214eb11/okhttp/src/jvmMain/kotlin/okhttp3/internal/http/BridgeInterceptor.kt#L102-L103