HTTP Model

Overview

Since akka-http-core provides the central HTTP data structures you will find the following import in quite a few places around the code base (and probably your own code as well):

import akka.http.javadsl.model.*;
import akka.http.javadsl.model.headers.*;

import java.util.Optional;

This brings all of the most relevant types in scope, mainly:

• HttpRequest and HttpResponse, the central message model
• headers, the package containing all the predefined HTTP header models and supporting types
• Supporting types like Uri, HttpMethods, MediaTypes, StatusCodes, etc.

A common pattern is that the model of a certain entity is represented by an immutable type (class or trait), while the actual instances of the entity defined by the HTTP spec live in an accompanying object carrying the name of the type plus a trailing plural 's'.

For example:

• Defined HttpMethod instances are defined as static fields of the HttpMethods class.
• Defined HttpCharset instances are defined as static fields of the HttpCharsets class.
• Defined HttpEncoding instances are defined as static fields of the HttpEncodings class.
• Defined HttpProtocol instances are defined as static fields of the HttpProtocols class.
• Defined MediaType instances are defined as static fields of the MediaTypes class.
• Defined StatusCode instances are defined as static fields of the StatusCodes class.

HttpRequest

HttpRequest and HttpResponse are the basic immutable classes representing HTTP messages.

An HttpRequest consists of

• a method (GET, POST, etc.)
• a URI
• a seq of headers
• an entity (body data)
• a protocol

Here are some examples how to construct an HttpRequest:

// construct a simple GET request to `homeUri`
Uri homeUri = Uri.create("/home");
HttpRequest request1 = HttpRequest.create().withUri(homeUri);

// construct simple GET request to "/index" using helper methods
HttpRequest request2 = HttpRequest.GET("/index");

// construct simple POST request containing entity
ByteString data = ByteString.fromString("abc");
HttpRequest postRequest1 = HttpRequest.POST("/receive").withEntity(data);

// customize every detail of HTTP request
//import HttpProtocols._
//import MediaTypes._
Authorization authorization = Authorization.basic("user", "pass");
HttpRequest complexRequest =
    HttpRequest.PUT("/user")
        .withEntity(HttpEntities.create(ContentTypes.TEXT_PLAIN_UTF8, "abc"))
        .addHeader(authorization)
        .withProtocol(HttpProtocols.HTTP_1_0);

In its basic form HttpRequest.create creates an empty default GET request without headers which can then be transformed using one of the withX methods, addHeader, or addHeaders. Each of those will create a new immutable instance, so instances can be shared freely. There exist some overloads for HttpRequest.create that simplify creating requests for common cases. Also, to aid readability, there are predefined alternatives for create named after HTTP methods to create a request with a given method and uri directly.

HttpResponse

An HttpResponse consists of

• a status code
• a list of headers
• an entity (body data)
• a protocol

Here are some examples how to construct an HttpResponse:

// simple OK response without data created using the integer status code
HttpResponse ok = HttpResponse.create().withStatus(200);

// 404 response created using the named StatusCode constant
HttpResponse notFound = HttpResponse.create().withStatus(StatusCodes.NOT_FOUND);

// 404 response with a body explaining the error
HttpResponse notFoundCustom =
    HttpResponse.create()
        .withStatus(404)
        .withEntity("Unfortunately, the resource couldn't be found.");

// A redirecting response containing an extra header
Location locationHeader = Location.create("http://example.com/other");
HttpResponse redirectResponse =
    HttpResponse.create()
        .withStatus(StatusCodes.FOUND)
        .addHeader(locationHeader);

In addition to the simple HttpEntities.create methods which create an entity from a fixed String or ByteString as shown here the Akka HTTP model defines a number of subclasses of HttpEntity which allow body data to be specified as a stream of bytes. All of these types can be created using the method on HttpEntites.

HttpEntity

An HttpEntity carries the data bytes of a message together with its Content-Type and, if known, its Content-Length. In Akka HTTP there are five different kinds of entities which model the various ways that message content can be received or sent:

HttpEntityStrict

The simplest entity, which is used when all the entity are already available in memory. It wraps a plain ByteString and represents a standard, unchunked entity with a known Content-Length.

HttpEntityDefault

The general, unchunked HTTP/1.1 message entity. It has a known length and presents its data as a Source[ByteString] which can be only materialized once. It is an error if the provided source doesn't produce exactly as many bytes as specified. The distinction of HttpEntityStrict and HttpEntityDefault is an API-only one. One the wire, both kinds of entities look the same.

HttpEntityChunked

The model for HTTP/1.1 chunked content (i.e. sent with Transfer-Encoding: chunked). The content length is unknown and the individual chunks are presented as a Source[ChunkStreamPart]. A ChunkStreamPart is either a non-empty chunk or the empty last chunk containing optional trailer headers. The stream consists of zero or more non-empty chunks parts and can be terminated by an optional last chunk.

HttpEntityCloseDelimited

An unchunked entity of unknown length that is implicitly delimited by closing the connection (Connection: close). Content data is presented as a Source[ByteString]. Since the connection must be closed after sending an entity of this type it can only be used on the server-side for sending a response. Also, the main purpose of CloseDelimited entities is compatibility with HTTP/1.0 peers, which do not support chunked transfer encoding. If you are building a new application and are not constrained by legacy requirements you shouldn't rely on CloseDelimited entities, since implicit terminate-by-connection-close is not a robust way of signaling response end, especially in the presence of proxies. Additionally this type of entity prevents connection reuse which can seriously degrade performance. Use HttpEntityChunked instead!

HttpEntityIndefiniteLength

A streaming entity of unspecified length for use in a Multipart.BodyPart.

Entity types HttpEntityStrict, HttpEntityDefault, and HttpEntityChunked are a subtype of RequestEntity which allows to use them for requests and responses. In contrast, HttpEntityCloseDelimited can only be used for responses.

Streaming entity types (i.e. all but HttpEntityStrict) cannot be shared or serialized. To create a strict, sharable copy of an entity or message use HttpEntity.toStrict or HttpMessage.toStrict which returns a CompletionStage of the object with the body data collected into a ByteString.

The class HttpEntities contains static methods to create entities from common types easily.

You can use the isX` methods of ``HttpEntity to find out of which subclass an entity is if you want to provide special handling for each of the subtypes. However, in many cases a recipient of an HttpEntity doesn't care about of which subtype an entity is (and how data is transported exactly on the HTTP layer). Therefore, the general method HttpEntity.getDataBytes() is provided which returns a Source<ByteString, ?> that allows access to the data of an entity regardless of its concrete subtype.

Header Model

Akka HTTP contains a rich model of the most common HTTP headers. Parsing and rendering is done automatically so that applications don't need to care for the actual syntax of headers. Headers not modelled explicitly are represented as a RawHeader (which is essentially a String/String name/value pair).

See these examples of how to deal with headers:

        // create a ``Location`` header
        Location locationHeader = Location.create("http://example.com/other");

        // create an ``Authorization`` header with HTTP Basic authentication data
        Authorization authorization = Authorization.basic("user", "pass");

    // a method that extracts basic HTTP credentials from a request
	private Optional<BasicHttpCredentials> getCredentialsOfRequest(HttpRequest request) {
        Optional<Authorization> auth = request.getHeader(Authorization.class);
        if (auth.isPresent() && auth.get().credentials() instanceof BasicHttpCredentials)
            return Optional.of((BasicHttpCredentials) auth.get().credentials());
        else
            return Optional.empty();
    }

HTTP Headers

When the Akka HTTP server receives an HTTP request it tries to parse all its headers into their respective model classes. Independently of whether this succeeds or not, the HTTP layer will always pass on all received headers to the application. Unknown headers as well as ones with invalid syntax (according to the header parser) will be made available as RawHeader instances. For the ones exhibiting parsing errors a warning message is logged depending on the value of the illegal-header-warnings config setting.

Some headers have special status in HTTP and are therefore treated differently from "regular" headers:

Content-Type

The Content-Type of an HTTP message is modeled as the contentType field of the HttpEntity. The Content-Type header therefore doesn't appear in the headers sequence of a message. Also, a Content-Type header instance that is explicitly added to the headers of a request or response will not be rendered onto the wire and trigger a warning being logged instead!

Transfer-Encoding

Messages with Transfer-Encoding: chunked are represented as a HttpEntityChunked entity. As such chunked messages that do not have another deeper nested transfer encoding will not have a Transfer-Encoding header in their headers list. Similarly, a Transfer-Encoding header instance that is explicitly added to the headers of a request or response will not be rendered onto the wire and trigger a warning being logged instead!

Content-Length

The content length of a message is modelled via its HttpEntity. As such no Content-Length header will ever be part of a message's header sequence. Similarly, a Content-Length header instance that is explicitly added to the headers of a request or response will not be rendered onto the wire and trigger a warning being logged instead!

Server

A Server header is usually added automatically to any response and its value can be configured via the akka.http.server.server-header setting. Additionally an application can override the configured header with a custom one by adding it to the response's header sequence.

User-Agent

A User-Agent header is usually added automatically to any request and its value can be configured via the akka.http.client.user-agent-header setting. Additionally an application can override the configured header with a custom one by adding it to the request's header sequence.

Date

The Date response header is added automatically but can be overridden by supplying it manually.

Connection

On the server-side Akka HTTP watches for explicitly added Connection: close response headers and as such honors the potential wish of the application to close the connection after the respective response has been sent out. The actual logic for determining whether to close the connection is quite involved. It takes into account the request's method, protocol and potential Connection header as well as the response's protocol, entity and potential Connection header. See this test for a full table of what happens when.

Parsing / Rendering

Parsing and rendering of HTTP data structures is heavily optimized and for most types there's currently no public API provided to parse (or render to) Strings or byte arrays.

The URI model

Akka HTTP offers its own specialised URI model class which is tuned for both performance and idiomatic usage within other types of the HTTP model. For example, an HTTPRequest's target URI is parsed into this type, where all character escaping and other URI specific semantics are applied.

Sometimes it may be needed to obtain the "raw" value of an incoming URI, without applying any escaping or parsing to it. While this use-case is rare, it comes up every once in a while. It is possible to obtain the "raw" request URI in Akka HTTP Server side by turning on the akka.http.server.raw-request-uri-header flag. When enabled, a Raw-Request-URI header will be added to each request. This header will hold the original raw request's URI that was used. For an example check the reference configuration.