JDK 24: The new features in Java 24

Java Development Kit (JDK) 24 is now in an initial rampdown phase, with the feature set frozen at 24 features, ranging from a class-file API to a fourth preview of structured concurrency.

JDK 24 reached the rampdown phase on December 5. It is due as a production release on March 18, 2025. With its two dozen features, JDK 24 far outdoes predecessor JDK 23, which arrived arrived September 17 with 12 official new features.

The five most recent additions include warnings upon the use of memory-access methods in sun.misc.unsafe, a fourth preview of structured concurrency, deprecating the 32-bit x86 port for removal, and improving Java’s resistance to quantum computing attacks by providing Java implementations of a quantum-resistant module-latticed-based digital signature algorithm and a quantum-resistant module-latticed-based key encapsulation mechanism.

Previously proposed features include flexible constructor bodies; ahead-of-time class loading and linking; removing the Windows 32-bit x86 port; synchronizing virtual threads without pinning; simple source files and instance main methods; permanently disabling the security manager; module import declarations; an experimental version of compact object headers; primitive types in patterns, instanceof, and switch; linking runtime images without JMODs; the generational Shenandoah garbage collector; scoped values; a key derivation function API; removal of the non-generational mode in the Z Garbage Collector; stream gatherers; a vector API; a class-file API; warnings to prepare developers for future restrictions on the use of JNI (Java Native Interface); and a late barrier expansion for the G1 garbage collector.

JDK 24 has been designated a non-long-term support (LTS) release. (The current LTS release is JDK 21, which shipped in September 2023.) Like JDK 23, JDK 24 will receive only six months of premier-level support from Oracle. Early access builds of JDK 24 can be found at jdk.java.net. JDK 24 is slated to be followed next September by the next LTS release, JDK 25.

With a warning upon use of memory-access methods in sun.misc.Unsafe, Java would issue a warning at runtime on the first occasion that any memory-access method in sun.misc.unsafe is invoked. All of these unsupported methods were terminally deprecated in JDK 23 and have been superseded by standard APIs. The sun.misc.Unsafe class was created to provide a mechanism for Java classes to perform low-level operations. Most of its methods are for accessing memory, either in the JVM’s garbage-collected heap or in off-heap memory, which is not controlled by the JVM. As the class name suggests, these memory-access methods are unsafe.

Structured concurrency, back for another preview, is intended to simplify concurrent programming by introducing an API for structured concurrency. With the structured concurrency concept, groups of related tasks running in different threads are treated as a single unit of work, thereby streamlining error handling and cancellation, improving reliability, and enhancing observability. The goal is to promote a style of concurrent programming that can eliminate common tasks arising from cancellation and shutdown, such as thread leaks and cancellation delays. Improving the observability of concurrent code also is a goal.

Deprecating the 32-bit x86 port for removal, which follows a proposal to deprecate the Windows 32-bit x86 port (see below), will deprecate the Linux 32-bit x86 port, which is the only 32-bit x86 port remaining in the JDK. It will also effectively deprecate any remaining downstream 32-bit x86 ports. After the 32-bit x86 port is removed, the architecture-agnostic Zero port will be the only way to run Java programs on 32-bit x86 processors. Deprecating the 32-bit x86 port in JDK 24 will allow for its removal in JDK 25.

The two features proposed for improving Java security through quantum-resistance include a quantum-resistant module-lattice-based key encapsulation mechanism (ML-KEM) and a quantum-resistant module-lattice-based digital signature algorithm (ML-DSA). ML-DSA would secure against future quantum computing attacks by using digital signatures to detect unauthorized modifications to data and to authenticate the identity of signatories. Key encapsulation mechanisms (KEMs) are used to secure symmetric keys over insecure communication channels using public key cryptography. Both features are designed to secure against future quantum computing attacks.

Flexible constructor bodies are in a third preview after being featured in JDK 22 and JDK 23, albeit with a different name in JDK 22, when the feature was called statements before super(...). The feature is intended to reimagine the role of constructors in the process of object initialization, letting developers more naturally place logic that they currently must factor into auxiliary static methods, auxiliary intermediate constructors, or constructor arguments. The proposal introduces two distinct phases in a constructor body: The prologue contains code that executes before the superclass constructor is invoked, and the epilogue executes after the superclass constructor has been invoked. The feature also would preserve the existing guarantee that code in a subclass constructor cannot interfere with superclass instantiation.

Ahead-of-time class loading and linking aims at improving startup times by making classes of an application instantly available in a loaded and linked state, when the HotSpot Java virtual machine starts. This would be achieved by monitoring the application during one run and storing the loaded and linked forms of all classes in a cache for use in subsequent runs.

The Windows 32-bit x86 port was deprecated for removal in JDK 21 with the intent to remove it in a future release. Plans call for removing the source code and build support for the Windows 32-bit x86 port. Goals include removing all code paths that apply only to Windows 32-bit x86, ceasing all testing and development efforts targeting the Windows 32-bit x86 platform, and simplifying the JDK’s build and test infrastructure. The proposal states that Windows 10, the last Windows operating system to support 32-bit operation, will reach its end of life in October 2025.

Synchronizing virtual threads without pinning involves improving the scalability of Java code that uses synchronized methods and statements by arranging for virtual threads that block in such constructs to release their underlying platform for use by other threads. This would eliminate almost all cases of virtual threads being pinned to platform threads, which severely restricts the number of virtual threads available to handle an application workload.

A fourth preview of simple source files and instance main methods would evolve the Java language so beginners can write their first programs without needing to understand language features designed for large programs. The feature was previously previewed in JDK 21, JDK 22, and JDK 23. The goal is to allow beginning Java programmers to write streamlined declarations for single-class programs and then seamlessly expand their programs to use more advanced features as their skills grow.

Permanently disabling the security manager involves revising the Java platform specification so developers cannot enable the security manager, while other platform classes do not refer to it. The security manager has not been the primary means of securing client-side Java code for many years, has rarely been used to secure server-side code, and has been costly to maintain, the proposal states. The security manager was deprecated for removal in Java 17.

Module import declarations, previously previewed in JDK 23, enhance the Java programming language with the ability to succinctly import all of the packages exported by a module. This simplifies the reuse of modular libraries but does not require the importing of code to be a module itself.

Compact object headers would reduce the size of object headers in the HotSpot VM from between 96 and 128 bits down to 64 bits on 64-bit architectures. The goal of the proposed feature is to reduce heap size, improve deployment density, and increase data locality.

A second preview of primitive types in patterns, instanceof, and switch in JDK 24 would enhance pattern matching by allowing primitive types in all patterns and contexts. The feature also would extend instanceof and switch to work with all primitive types. The goals of the feature include enabling uniform data exploration by allowing type patterns for all types, whether primitive or reference; aligning types with instanceof and aligning instanceof with safe casting; and allowing pattern matching to use primitive types in both nested and top-level pattern contexts. Other goals include providing easy-to-use constructs that eliminate the risk of losing information due to unsafe casts, following the enhancements to switch in Java 5 and Java 7, and allowing switch to process values of any primitive type. This feature was previously previewed in JDK 23.

With linking runtime images without JMODs, the plan is to reduce the size of the JDK by roughly 25% by enabling the jlink tool to create custom runtime images without JDK JMOD files. This feature must be enabled by default and some JDK vendors may choose not to enable it. Goals include allowing users to link a runtime image from modules regardless of whether those modules are standalone JMOD files, modular JAR files, or part of a runtime image linked previously. Motivating this proposal is the notion that the installed size of the JDK on the file system is important in cloud environments, where container images that include an installed JDK are automatically and frequently copied over the network from container registries. Reducing the size of the JDK would improve the efficiency of these operations.

Generational Shenandoah would enhance the garbage collector with experimental generational collection capabilities to improve sustainable throughput, load-spike resistance, and memory utilization. The main goal is to provide an experimental generational mode, without breaking non-generational Shenandoah. The generational mode is intended to become the default mode in a future release.

Scoped values enable a method to share immutable data both with its callees within a thread and with child threads. Scoped values are easier to reason about than local-thread variables. They also have lower space and time costs, particularly when used together with virtual threads and structured concurrency. The scoped values API was proposed for incubation in JDK 20, proposed for preview in JDK 21, and improved and refined for JDK 22 and JDK 23. Scoped values will be previewed in JDK 24.

With the key derivation function (KDF) API, an API would be introduced for key derivation functions, which are cryptographic algorithms for deriving additional keys from a secret key and other data. A goal of this proposal is allowing security providers to implement KDF algorithms in either Java code or native code. Another goal is enabling applications to use KDF algorithms such as the HMAC (hash message authentication code)-based extract-and-expand key derivation function (RFC 5869) and Argon2 (RFC 9106).

Removing the non-generational mode of the Z Garbage Collector (ZGC) is a proposal aimed at reducing the maintenance cost of supporting two different modes. Maintaining non-generational ZGC slows the development of new features, and generational ZGC should be a better solution for most use cases than non-generational ZGC, the proposal states. The latter eventually should be replaced with the former to reduce long-term maintenance costs. The plan calls for removing the non-generational mode by obsoleting the ZGenerational option and removing the non-generational ZGC code and its tests. The non-generational mode will expire in a future release, at which point it will not be recognized by the HotSpot JVM, which will refuse to start.

Stream gatherers would enhance the stream API to support custom intermediate operations. Stream gatherers allow stream pipelines to transform data in ways that are not easily achievable with the existing built-in intermediate operations. This feature was proposed as a preview in JDK 22 and JDK 23. The API would be finalized in JDK 24. Goals include making stream pipelines more flexible and expressive and allowing custom intermediate operations to manipulate streams of infinite size.

The vector API is designed to express vector communications that reliably compile at runtime to optimal vector instructions on supported CPU architectures, thus achieving performance superior to equivalent scalar computations. The vector API previously was incubated in JDK 16 through JDK 23. It would be re-incubated in JDK 24 with no API changes and no substantial implementations relative to JDK 23. Goals of the proposal include clearly and concisely expressing a wide range of vector computations in an API that is platform-agnostic, that offers reliable runtime compilation and performance on x64 and AArch54 architectures, that degrades gracefully and still functions when a vector computation cannot be expressed at runtime, and that aligns with Project Valhalla, leveraging enhancements to the Java object model.

The class-file API, previously previewed in JDK 22 and JDK 23, would be finalized in JDK 24, with minor changes. This API provides a standard API for parsing, generating, and transforming Java class files. It aims to provide an API for processing class files that tracks the class file format defined by the Java Virtual Machine specification. A second goal is to enable JDK components to migrate to the standard API, and eventually remove the JDK’s internal copy of the third-party ASM library. Changes since the second preview include a renaming of enum values, removal of some fields, the addition of methods and method overloads, methods renamed, and removal of interfaces and methods deemed unnecessary.

Late barrier expansion for the G1 garbage collector is intended to simplify the implementation of G1’s barriers. The G1 garbage collector’s barriers record information about application memory accesses, by shifting their expansion from early in the C2 compilation pipeline to later. Goals include reducing the execution time of C2 compilation when using the G1 collector, making G1 barriers comprehensible to HotSpot developers who lack a deep understanding of C2, and guaranteeing that C2 preserves invariants about the relative ordering of memory accesses, safepoints, and barriers. A fourth feature is preserving the quality of C2-generated JIT (just-in-time)-compiled code, in terms of speed and size.

The first JDK 24-targeted feature, officially called “Prepare to Restrict the Use of JNI,” calls for issuing warnings about uses of JNI and adjusting the foreign function and memory (FFM) API, featured in JDK 22, to issue warnings in a consistent manner. These warnings are intended to prepare for a future release that ensures integrity by default by uniformly restricting JNI and the FFM API. Goals of the plan include preserving JNI as a standard way to interoperate with native code, preparing the Java ecosystem for future releases that disallow interoperation with native code by default, and aligning the use of JNI and the FFM API so library maintainers can migrate from one to the other without requiring developers to change command-line options.

The most recent LTS release, JDK 21, arrived in September 2023 and is due to get at least five years of Premier support from Oracle. The next LTS version, JDK 25, is due in September 2025. LTS releases have dominated Java adoption, which means adoption of JDK 23 and JDK 24 could be on the low end as users await JDK 25.