Due in March 2022, Java 18 continues to take shape, with eight feature proposals being considered for it so far. The latest feature proposals include a service-provider interface (SPI) for Internet address resolution and a second preview of pattern matching for switch statements.
The OpenJDK page for Java Development Kit (JDK) 18 lists the service-provider interface as proposed to target JDK 18 while a simple web server, a vector API, code snippets, a reimplementation of core reflection, and the UTF-8 charset officially target JDK 18 as of November 1.
A second incubation of a foreign function and memory API, which was incubated in Java 17, also is eyed for Java 18, as is another preview of pattern matching for switch. A record patterns and array patterns proposal, updated October 20 and previously anticipated for JDK 18, is not now officially targeted, although it theoretically could become part of the release.
While JDK 17, published September 14, was a long-term support (LTS) release that will receive at least eight years of support from Oracle, JDK 18, expected in March 2022, will be a short-term feature release that is supported for six months. Early-access builds of JDK 18 can be found for Linux, Windows, and MacOS at java.net.
Specifics of the JDK 18 proposals include:
- For the Internet-address resolution SPI, the proposal is to define an SPI for host and name address resolution so that
Inet.Addresscan make use of resolvers other than the platform’s built-in resolver. Motivations for this effort include better enablement of Project Loom, for concurrency and new programming models in Java, along with integrating new network protocols, customization, and enabling testing. The proposal does not involve developing an alternative resolver for the JDK. - A second preview of pattern matching for
switch, in which the Java language would be enhanced with pattern matching forswitchexpressions and statements, along with extensions to the language of patterns. This was previewed in JDK 17. Extending pattern matching toswitchallows an expression to be tested against a number of patterns, each with a specific action, so complex data-oriented queries can be expressed concisely and safely. - The reimplementation of core reflection with method handles would reimplement
lang.reflect.Method,Constructor, andFieldon top ofjava.lang.invokemethod handles. Having method handles serve as the underlying mechanism for reflection will reduce maintenance and development costs of both thejava.lang.reflectandjava.lang.invokeAPIs. - With the simple web server proposal, a command-line tool would be provided to start a minimal web server that serves static files only. No CGI or servlet-like functionality is available. The tool will be useful for prototyping, ad-hoc coding, and testing, particularly in educational contexts. Goals of the plan include offering an out-of-the-box static HTTP file server with easy setup and minimal functionality, reducing developer activation energy and making the JDK more approachable, and providing a default implementation via the command line together with a small API for programmatic creation and customization. Providing a feature-rich or commercial-grade server is not a goal of the proposal.
- Foreign function and memory API, in which an API is introduced through which Java programs can interoperate with code and data outside of the Java runtime. By invoking foreign functions – code outside the JVM – and by safely accessing foreign memory – memory not managed by the JVM – the API lets Java programs call native libraries and process native data without the brittleness and danger of JNI (Java Native Interface). The intent is to replace JNI with a superior, pure Java development model. For JDK 18, refinements would be incorporated, based on feedback, such as support for more carriers such as Boolean and MemoryAddress in memory access var handles, and a new API to copy Java arrays to and from memory segments.
- The vector API would be incubated for a third time in JDK 18, having previously been incubated in JDK 16 and JDK 17. This proposal would express vector computations that compile at run time to optimal vector instructions on supported CPU architectures, achieving performance superior to equivalent scalar computations. Vector operations express a degree of parallelization enabling more work to be done on a single CPU cycle, thus producing significant performance improvements. The platform-agnostic vector API aims to provide a way to write complex algorithms in Java, using the existing HotSpot auto-vectorizer but with a user model that makes vectorization more predictable. JDK 18 would also add support for the ARM Scalar Vector Extension platform and improve performance of vector operations that accept masks on architectures that support masking in hardware.
- Specifying UTF-8 as the default charset of the standard Java APIs. UTF-8 is a variable-wide character encoding for electronic communication and is considered the web’s standard charset. Charset is character encoding capable of encoding all characters on the web. Through this change, APIs that depend on the default charset will behave consistently across all implementations, operating systems, locales, and configurations. The proposal is not intended to define new Java-standard or JDK-specific APIs. Proponents of the proposal expect that applications in many environments will see no impact from Java’s choice of UTF-8, as MacOS, many Linux distributions, and many server applications already support UTF-8. However, there is risk in other environments, the most obvious being that applications depending on the default charset will behave incorrectly when processing data produced when the default charset was unspecified. Data corruption may silently occur. The main impact is expected to be fall on users of Windows systems in Asian locales and possibly some server environments in Asian and other locales.
- Code snippets in Java API documentation, involving the introduction of an
@snippettag for JavaDoc’s Standard Doclet, to simplify inclusion of example source code in API documentation. Among goals of the plan is facilitating the validation of source code fragments by providing API access to those fragments. While correctness is the responsibility of the author, enhanced support in JavaDoc and related tools can make it easier to achieve. Other goals include enabling modern styling, such as syntax highlighting, as well as the automatic linkage of names to declarations, and enabling better IDE support for creating and editing snippets. The proposal notes that authors of API documentation often include fragments of source code in documentation comments.