OpenJDK is a free, open-source version of the Java Development Kit for the Java Platform, Standard Edition (Java SE). OpenJDK, which stands for Open Java Development Kit, originated from an effort initiated by Sun Microsystems in 2006 and is now sponsored and led by Oracle. The project is licensed under the GNU General Public License (GNU GPL) version 2 with a linking exception. Without the linking exception, components that linked to the Java class library would be subject to the terms of the GPL license.
Since the release of Java SE version 7, OpenJDK has been the official reference implementation. A few notable components that fall under the OpenJDK project include the Java class library, the Java compiler, the Java Runtime Environment (JRE) and the Java virtual machine (JVM). Unlike other JDK release projects, which focused on releasing one feature at a time before terminating, OpenJDK is a long-term, ongoing project. OpenJDK follows a strict, time-based model that is split into development branches and will release new features every six months.
Companies such as Azul, Eclipse, IBM, Red Hat, Oracle and SAP offer implementations of Java SE while the OpenJDK Community includes academics, open-source operating system distribution developers and individual software developers. Linux distributions such as Ubuntu, Fedora or Red Hat Enterprise Linux (RHEL) offer OpenJDK as their default Java SE implementation.
OpenJDK and Oracle JDK
In 2017, Oracle merged its commercially licensed JDK with the OpenJDK initiative and passed previously closed-source features, such as Java Flight Recorder and Java Mission Control, on to the open-source community. Additionally, it was announced that Oracle JDK would be mainly for commercial and support customers once OpenJDK binaries are interchangeable with the Oracle JDK.
Oracle said it would provide public updates of Oracle JDK 8 through at least December 2020 for personal desktop use and January 2019 for commercial use. After that, users can go onto a paid support plan or use a Java SE 8/OpenJDK 8 binary distribution from another provider. Changes to Oracle JDK distribution and support raised concerns among some Java users over the rights to use Oracle JDK versus Oracle’s OpenJDK builds versus OpenJDK builds from other providers.
Among OpenJDK’s top contributors is Red Hat, which extended its long-term support for OpenJDK on Windows. Red Hat already supports OpenJDK on its Red Hat Enterprise Linux distribution, specifically for Java 11 and Java 8. Red Hat's long-term support for OpenJDK on Windows extends the company's commitment to the primary operating system and platforms for enterprise desktop, data center and cloud environments as well as to Java developers who run their applications on those frameworks.
Also committed to OpenJDK is Amazon Web Services (AWS), which in late 2018 introduced Amazon Corretto, its OpenJDK 8-based distribution, with long-term support at least through June 2023. Amazon's commitment to OpenJDK support, and to keeping it free, indicates Java's popularity among AWS developers. Amazon Corretto, which corresponds with OpenJDK 8, provides more than four years of long-term support beyond what Oracle currently provides for its distribution of OpenJDK 8. This ensures that OpenJDK support will remain free for at least that long.