The open-source reference implementation of Java is OpenJDK.
OpenJDK is the community-developed source-code project that implements the Java Platform, Standard Edition, including the Java compiler, core libraries, and the HotSpot virtual machine. It is released under the GNU General Public License with the Classpath Exception, allowing broad use and redistribution. Oracle and other vendors build downloadable JDK distributions from OpenJDK source.
The name is often confused with the individual pieces around it. HotSpot is a JVM implementation within many OpenJDK builds, not the name of the complete reference implementation. The JRE is a runtime concept rather than the project that implements Java, while GraalVM is a separate distribution and technology platform that can include or build on OpenJDK components.
OpenJDK became the official Java SE reference implementation beginning with Java SE 7. “Reference implementation” means an implementation used to demonstrate the specification in working software; it does not mean every vendor’s Java distribution is identical. Builds such as Eclipse Temurin, Amazon Corretto, and Microsoft Build of OpenJDK may differ in packaging, support, and patches while remaining compatible with the Java platform.