JEP 321: HTTP Client API

Summary

Standardize the incubated HTTP Client API introduced in JDK 9, via JEP 110, and updated in JDK 10.

Goals

In addition to the goals of JEP 110, this JEP will:

• Take into account feedback received on the incubated API,
• Provide a standardized API, in the java.net.http package, based upon the incubated API, and
• Remove the incubated API.

Motivation

The motivation of this JEP remains the same as that of the motivation of JEP 110.

Description

This JEP proposes to standardize the HTTP Client API that was introduced as an incubating API in JDK 9 and updated in JDK 10. The incubating API has received a number of rounds of feedback that have resulted in significant improvements, but at a high level it remains largely the same. The API provides non-blocking request and response semantics through CompletableFutures, which can be chained to trigger dependent actions. Back-pressure and flow-control of request and response bodies is provided for via the Platform's reactive-streams support in the java.util.concurrent.Flow API.

While incubating in JDK 9 and JDK 10, the implementation has been almost completely rewritten. The implementation is now completely asynchronous (the previous HTTP/1.1 implementation was blocking). Use of the RX Flow concept has been pushed down into the implementation, which eliminated many of the original custom concepts needed to support HTTP/2. The flow of data can now be more easily traced, from the user-level request publishers and response subscribers all the way down to the underlying socket. This significantly reduces the number of concepts and complexity in the code, and maximizes the possibility of reuse between HTTP/1.1 and HTTP/2.

The module name and the package name of the standard API will be java.net.http.

Changes over what was incubated in JDK 10

1. The predefined implementation of BodyPublisher, BodyHandler, and BodySubscriber, created through static factory methods, have been moved out to separate non-instantiable utility factory classes, following the pluralized naming convention. This improves readability of these relatively small interfaces.

2. The names of the static factory methods have also been updated along the following broad categories:

• fromXxx: Adapters from standard Subscriber, e.g. takes a Flow.Subscriber returns a BodySubscriber.

• ofXxx: Factories that create a new pre-defined Body[Publisher|Handler|Subscriber] that perform useful common tasks, such as handling the response body as a String, or streaming the body to a File.

• other: Combinators (takes a BodySubscriber returns a BodySubscriber) and other useful operations.

3. A few BodyHandlers and corresponding BodySubscribers have been added, to improve usability in common scenarios:

• discard(Object replacement) combined discarding/ignoring the response body and allowing a given replacement. Feedback has indicated that this could appear confusing. It has been removed and replaced with two separate handlers: 1) discarding(), and 2) replacing(Object replacement).

• Added ofLines() that returns a BodyHandler<Stream<String>>, to support streaming of response body as a Stream of lines, line by line. Provides similar semantics to that of BufferedReader.lines().

• Added fromLineSubscriber​, that supports adaptation of response body to a Flow.Subscriber of String lines.

• Added BodySubscriber.mapping for general purpose mapping from one response body type to another.

4. The push promise support has been re-worked to reduce its impact on the API and bring it more in line with regular request/responses. Specifically, the MultiSubscriber and MultiResultMap have been removed. Push promises are now handled through a functional interface, PushPromiseHandler, that is optionally given during a send operation.

5. The HttpClient.Redirect policy has been simplified, by replacing SAME_PROTOCOL and SECURE policies, with NORMAL. It has been observed that the previously named SECURE was not really appropriately named and should be renamed to NORMAL, since it will likely be suitable for most normal cases. Given the newly named, aforementioned, NORMAL, SAME_PROTOCOL appears oddly named, possibly confusing, and not likely to be used.

6. WebSocket.MessagePart has been removed. This enum was used on the receiving side to indicate whether the delivery of a message is complete, or not. It is asymmetric with the sending side, which uses a simple boolean for this purpose. Additionally, it has been observed that handling received messages with a simple boolean significantly reduces and simplifies the receiving code logic. Determination of messages being delivered as a WHOLE, one of the benefits and the main purposes for the aforementioned MessagePart, has proved to not carry its own weight.

Further specifics on the API can be found in JEP 110, at the latest API javadoc, or the networking group's JDK HTTP Client page.

Testing

Existing tests for the incubated API will be updated to use the new standard API. Additional tests will be added to cover all scenarios supported, specifically the upgrade and downgrade between HTTP/1.1 and HTTP/2.

Risks and Assumptions

Code that currently depends upon the incubated HTTP Client API will need to be updated, at the very minimum to change its package imports. This is no different than for any other incubated feature. Code depending upon incubating modules already receives an appropriate warning at both compile time and run time.