OpenJDK

OpenJDK began in 2006 when Sun Microsystems announced its intention to release the Java platform as free software, a move culminating in the first open-source build in 2007.¹ The project was established under the governance of the Java Community Process, with Sun (later Oracle) as the lead steward. Since Java 7 (2011), OpenJDK has been the official reference implementation of Java SE, replacing the prior proprietary JDK. Development is organized into projects with specific focus areas, such as HotSpot, the virtual machine, and the java.net repository hosting. The community includes individual developers, companies like Red Hat, IBM, and Amazon, and the Adoptium working group, which produces certified builds. A formal process, the JDK Enhancement Proposal (JEP), guides major changes, ensuring transparent design and review.

OpenJDK comprises several interlocking components that together form a complete Java development environment. The centerpiece is the HotSpot virtual machine, which includes a just-in-time (JIT) compiler that dynamically optimizes bytecode into native machine code. The class libraries implement the Java API, ranging from core java.lang and java.util packages to enterprise and networking APIs. The reference implementation of the Java compiler, javac, translates Java source into bytecode. Other tools include the Java debugger (jdb), the documentation generator (javadoc), and packaging utilities. Since Java 9, OpenJDK has embraced a modular structure using the Java Platform Module System (JPMS), enabling developers to create custom runtime images with the jlink tool. The source tree also includes testing frameworks and the JCK (Java Compatibility Kit) conformance tests, which are crucial for ensuring the implementation remains true to the Java SE specification.

OpenJDK follows a strict release cadence, with a new feature release every six months since Java 10, and long-term support (LTS) releases every three years. The latest LTS is Java 21 (September 2023), preceded by LTS versions 17 and 11, while Java 22 and 23 are feature releases. Each release includes enhancements to performance, security, and language features; for instance, Java 21 introduced virtual threads (Project Loom) and pattern matching for switch expressions. Distribution models vary: Oracle provides a commercial JDK built from OpenJDK, while the Eclipse Adoptium project (formerly AdoptOpenJDK) offers free, certified builds called

Beyond the mainstream, OpenJDK harbors several fascinating and less-known facets. The project's codebase includes experimental features such as the Foreign Function & Memory API, which enables direct interoperation with native code and memory, and the Vector API, which aims to accelerate scientific computing. The GraalVM project, originally incubated at Oracle Labs, is built partly on OpenJDK and offers a high-performance JIT compiler and the ability to compile Java applications into native executables. Another intriguing component is the Flight Recorder, a low-overhead profiling tool that was proprietary until Java 11, when it was open-sourced. The project also hosts a surprising variety of platforms: beyond Linux and Windows, OpenJDK supports AIX, macOS on Apple silicon, and even experimental builds for RISC-V. Historically, the project has served as an experimental ground for ideas that later became standard, such as the Shenandoah garbage collector, which reduces pause times.

HotSpot

The virtual machine implementation within OpenJDK that includes a JIT compiler and garbage collectors.

JEP

JDK Enhancement Proposal; a document that proposes and tracks a major change to the JDK.

LTS

Long-Term Support; a release that receives updates for an extended period, typically several years.

JPMS

Java Platform Module System; a modular system that allows developers to define and use modules in Java applications.