我们说过Akka-http是一个好的系统集成工具，集成是通过数据交换方式实现的。Http是个在网上传输和接收的规范协议。所以，在使用Akka-http之前，可能我们还是需要把Http模式的网上数据交换细节了解清楚。数据交换双方是通过Http消息类型Request和Response来实现的。在Akka-http中对应的是HttpRequest和HttpResponse。这两个类型都具备HttpEntity类型来装载需要交换的数据。首先，无论如何数据在线上的表现形式肯定是一串bytes。所以，数据交换两头Request,Response中的Entity也必须是以bytes来表达的。在Akka-http里我们把需要传输的数据转换成ByteString，通过网络发送給接收端、接收端再把收到消息Entity中的ByteString转换成目标类型的数据。这两个转换过程就是Akka-http的Marshalling和Unmarshalling过程了。我们先从HttpEntity的构建函数来了解它的定义：
object HttpEntity { implicit def apply(string: String): HttpEntity.Strict = apply(ContentTypes.`text/plain(UTF-8)`, string) implicit def apply(bytes: Array[Byte]): HttpEntity.Strict = apply(ContentTypes.`application/octet-stream`, bytes) implicit def apply(data: ByteString): HttpEntity.Strict = apply(ContentTypes.`application/octet-stream`, data) def apply(contentType: ContentType.NonBinary, string: String): HttpEntity.Strict = if (string.isEmpty) empty(contentType) else apply(contentType, ByteString(string.getBytes(contentType.charset.nioCharset))) def apply(contentType: ContentType, bytes: Array[Byte]): HttpEntity.Strict = if (bytes.length == 0) empty(contentType) else apply(contentType, ByteString(bytes)) def apply(contentType: ContentType, data: ByteString): HttpEntity.Strict = if (data.isEmpty) empty(contentType) else HttpEntity.Strict(contentType, data) def apply(contentType: ContentType, contentLength: Long, data: Source[ByteString, Any]): UniversalEntity = if (contentLength == 0) empty(contentType) else HttpEntity.Default(contentType, contentLength, data) def apply(contentType: ContentType, data: Source[ByteString, Any]): HttpEntity.Chunked = HttpEntity.Chunked.fromData(contentType, data) ...