Eclipse Public License 2.0 GNU General Public License, version 2 with the GNU Classpath Exception Eclipse OpenJ9 (previously known as IBM J9) is a high performance, scalable, Java virtual machine (JVM) implementation that is fully compliant with the Java Virtual Machine Specification.

[3] OpenJ9 can be built from source, or can be used with pre-built binaries available at the IBM Semeru Runtimes project for a number of platforms including Linux, Windows[4] and macOS.

[6] The J9 JVM became the runtime engine for many of IBM's enterprise middleware products, where it has built its reputation for high performance, scalability, and reliability.

Compared to Oracle's HotSpot VM, OpenJ9 touts higher start-up performance and lower memory consumption at a similar overall throughput.

At the OpenJ9 project, an extra layer of code adds the language semantics to provide a runtime environment for Java applications.

The hotter the optimization level, the better the expected performance, but the higher the cost in terms of CPU and memory.

Methods are dynamically compiled into AOT code at runtime, which enables the JVM to start an application faster.

OpenJ9 automatically chooses which methods to compile based on heuristics that identify the start-up phase of large applications.

By default, OpenJ9 uses the Generational Concurrent (-Xgcpolicy:gencon) policy, which is best suited to transactional applications that have many short lived objects.

Different types of diagnostic data are automatically produced by default when certain events occur, but can also be triggered from the command line.