The number of proposed features for Java Development Kit (JDK) 23, the next planned version of standard Java, has climbed to 11. The latest additions include previews of structured concurrency and flexible constructor bodies and the deprecation of the memory access methods in sun.misc.UnSafe for removal in a future release of the JDK.
Due September 17, JDK 23 also will include a vector API, which will be incubated for the eighth time, a second preview of stream gatherers, a second preview of a class-file API, and a preview of primitive types in patterns, instanceof, and switch. Other features include module import declarations, Markdown documentation comments, and making the generational mode of the Z Garbage collector the default mode. Another proposed addition is a third preview of implicitly declared classes and instance methods. Early access builds for JDK 23 can be accessed from jdk.java.net.
Plans for JDK 23 call for deprecating the memory-access methods in sun.misc.Unsafe for removal in a future release. These methods are unsupported and they have been superseded by standard APIs, namely the VarHandle API and the foreign function and memory API. The OpenJDK encourages library developers to migrate from sun.misc.Unsafe to supported replacements, so applications can migrate smoothly to modern JDK releases. Goals of the proposal include preparing the ecosystem for removal of memory access methjods in sun.misc.Unsafe in a future JDK release, and helping developers realize when their applications rely, directly or indirectly, on those memory access methods. It is not a goal to remove sun.misc.Unsafe entirely, as a small number of its methods are not used for memory access. These methods will be deprecated and removed separately.
Structured concurrency would get a third preview in JDK 23, having been previewed in JDK 22 and JDK 21. Its purpose is to simplify concurrent programming by introducing an API for structured concurrency, which treats groups of related tasks in different threads as a single unit of work, streamlining error handling and cancellation to improve reliability and observability. The goals of this proposal include promoting a style of programming that can eliminate common risks arising from cancellation and shutdown, such as thread leaks and cancellation delays, and improving the observability of concurrent code.
Flexible constructor bodies would get a second preview in JDK 23. The feature was previewed in JDK 22 under the name of statements before super(…). This proposal would allow a constructor body to initialize fields in the same class before explicitly invoking a constructor. It is intended to give developers greater freedom to express the behavior of constructors, enabling more natural placement of logic that currently must be factored into auxiliary static methods, auxiliary intermediate constructors, or constructor arguments. The feature also is intended to preserve the guarantee that constructors run in a top-down order during class instantiation, ensuring that code in a subclass constructor cannot interfere with superclass instantiation.
Previously previewed in JDK 21 and JDK 22, implicitly declared classes and instance methods simplifies the Java language to allow beginners to write their first programmers without needing to understand language features designed for large programs. Beginners can write streamlined declarations for single-class programs and then seamlessly expand programs to use more advanced features as their skills grow. Two enhancements are being added in JDK 23: Implicitly declared classes will import automatically three static methods for simple text I/O with the console, and will import on demand all public top-level classes and interfaces of packages exported by the java.base module.
Module import declarations, which would be a preview feature in JDK 23, enhance Java with the ability to succinctly import all packages exported by the module. This proposal, which was created in August, simplifies the reuse of modular libraries, but does not require the imported code to be in a module itself. Goals include simplifying the use of modular libraries by allowing entire modules to be imported at once and avoiding the noise of multiple type-import-on-demand declarations when using diverse parts of the API exported by a module. Another goal is allowing beginners to more easily use third-party libraries and fundamental Java classes without having to learn where they are located in a package hierarchy.
Markdown documentation comments, a proposal created last September, would enable JavaDoc documentation comments to be written in Markdown rather than solely in a mixture of HTML and JavaDoc @tags. The proposal is intended to make API documentation comments easier to write and read in source form by introducing the ability to use Markdown syntax alongside HTML elements and JavaDoc tags. It also is intended to extend the Compiler Tree API to enable other tools that analyze documentation comments to handle Markdown content. The plan should not adversely affect the interpretation of existing documentation comments.
JDK 23 is also slated to switch the default mode of the Z Garbage Collector (ZGC) to the generational mode. Generational ZGC is intended to boost Java application performance by more frequently collecting young objects, which tend to die young. The non-generational mode would be deprecated, with the intent to remove it in a future release. Goals of this feature include signaling the intent that future development will focus on generational ZGC, and reducing the maintenance cost of supporting the two different modes. Maintaining non-generational ZGC slows the development of new features. ZGC was added to Java in JDK 15 in September 2020.
The vector API, which has been incubated in previous Java versions from JDK 16 to last month’s JDK 22, introduces an API to express vector computations that reliably compile at run time to optimal vector instructions on supported CPU architectures. Goals of the proposal include providing a clear and concise API, providing reliable runtime compilation and performance on x64 and AArch64 architectures, providing graceful degradation, being platform-agnostic, and aligning with Project Valhalla, which is intended to augment the Java object model with value objects.
Stream gatherers, previously previewed in JDK 22, would enhance the stream API to support custom intermediate operations. Stream gatherers would allow stream pipelines to transform data in ways not easily achievable with the existing built-in intermediate operations. Goals include making stream pipelines more flexible and expressive and allowing custom intermediate operations to manipulate streams of infinite size.
The class-file API is intended to provide an API for processing class files that tracks the class file format defined by the Java Virtual Machine specification. It also would enable JDK components to migrate to the standard API and eventually remove the JDK’s internal copy of the third-party ASM library. The class-file API would add refinements including streamlining the CodeBuilder class, which has factory methods for bytecode instructions, including low-level factories, mid-level factories, and high-level builders for basic blocks.
For JDK 23, Java’s builders removed mid-level methods that duplicated low-level methods or were infrequently used, while renaming the remaining mid-level methods to improve usability. They also refined the ClassSignature class model, which has been improved to model the generic signatures of superclasses and superinterfaces more accurately. According to the OpenJDK proposal behind this feature, the Java platform should define and implement a standard class-file API that evolves together with the class-file format, which can evolve every six months.
Also cited for inclusion in JDK 23 is another preview feature, primitive types in patterns, instanceof, and switch. This feature would enhance pattern matching by allowing primitive type patterns in all pattern contexts, and extend instanceof and switch to work with all primitive types. Goals include providing easy-to-use constructs that eliminate the risk of losing information due to unsafe casts, aligning pattern types with instanceof, aligning instanceof with safe casting, and allowing uniform data exploration by allowing type patterns for all types, whether primitive or reference types. Another goal is allowing pattern matching to use all primitive type patterns in nested and top-level contexts.
A number of other features previewed in JDK 22 could make it into JDK 23. These include statements before super(…), which would give developers greater freedom in expressing constructor behavior; string templates, which would make it easy to express strings that include values computed at run time; and scoped values, which would enable sharing of immutable data within and across threads. Derived record creation and a hot code heap proposal, which would reduce application execution times, also are possibilities.
Java steward Oracle also has revealed plans for Java in 2024. Oracle outlined improvements that involve OpenJDK projects ranging from Amber, for developing smaller, productivity-oriented features, to Babylon, for extending Java to foreign programming models such as GPUs, to Valhalla, for augmenting the Java object model with value objects to eliminate longstanding performance bottlenecks.