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JDK 16 begins to take shape

Although not due to arrive until March 2021, Java Development Kit (JDK) 16 has begun to take shape, with proposed features including support for C++ 14 language features in JDK C++ source code, a vector API, and an “elastic metaspace” capability to more quickly return unused class metadata memory to the OS.

JDK 16 will be the reference implementation of the version of standard Java set to follow JDK 15, which arrived September 15. The six-month release cadence for standard Java would have JDK 16 arriving next March.

As of September 18, five proposals target JDK 16. The most recent, the elastic metaspace capability, was added in mid-September. The new capabilities coming to Java 16:

  • An elastic metaspace capability, which returns unused HotSpot VM class metadata (metaspace) memory more promptly to the OS, reduces metaspace footprint and simplifies metaspace code to reduce maintenance costs. Metaspace has had issues with high off-heap memory use. The plan calls for replacing the existing memory allocator with a buddy-based allocation scheme, providing an algorithm to divide memory into partitions to satisfy memory requests. This approach has been used in places such as the Linux kernel and will make it practical to allocate memory in smaller chunks to reduce class-loader overhead. Fragmentation also will be reduced. In addition, the commitment of memory from the OS to memory management arenas will be done lazily, on demand, to reduce the footprint for loaders that start out with large arenas but do not use them immediately or might not use them to their full extent. To fully exploit the elasticity offered by buddy allocation, metaspace memory will be arranged into uniformly sized granules that can be committed and uncommitted independently of each other.
  • Enablement of C++ 14 language features, to allow the use of C++ 14 capabilities in JDK C++ source code and give specific guidance about which of these features may be used in HotSpot VM code. Through JDK 15, language features used by C++ code in the JDK have been limited to the C++98/03 language standards. With JDK 11, the source code was updated to support building with newer versions of the C++ standard. This includes being able to build with recent versions of compilers that support C++ 11/14 language features. This proposal does not propose any style or usage changes for C++ code that is used outside of HotSpot. But to take advantage of C++ language features, some build-time changes are required, depending on the platform compiler.
  • A vector API in an incubator stage, in which the JDK would be fitted with an incubator module, jdk.incubator.vector, to express vector computations that compile to optimal vector hardware instructions on supported CPU architectures, to achieve superior performance to equivalent scalar computations. The vector API provides a mechanism to write complex vector algorithms in Java, using pre-existing support in the HotSpot VM for vectorization but with a user model that makes vectorization more predictable and robust. Goals of the proposal include providing a clear and concise API to express a range of vector computations, being platform-agnostic by supporting multiple CPU architectures, and offering reliable runtime compilation and performance on x64 and AArch64 architectures. Graceful degradation also is a goal, in which a vector computation would degrade gracefully and still function if it cannot be fully expressed at runtime as a sequence of hardware vector instructions, either because an architecture does not support some instructions or another CPU architecture is not supported.
  • Migration of OpenJDK source code repositories from Mercurial to Git. Driving this effort are advantages in version control system metadata size and available tools and hosting.
  • Migration to GitHub, related to the Mercurial-to-Git migration, with JDK 16 source code repositories to be on the popular code-sharing site.

Early-access builds of JDK 16 for Linux, Windows, and MacOS can be found at jdk.java.net. Like JDK 15, JDK 16 will be a short-term release, supported for six months. JDK 17, due in September 2021, will be a long-term support (LTS) release that will receive several years of support. The current LTS release, JDK 11, was released in September 2018.

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