Hotspot vs. Openj9 (Expert Tips)

How does HotSpot fit into the JVM and OpenJDK ecosystems? A library, perhaps? What does it actually do? What distinguishes HotSpot from OpenJDK?

Key Points

HotSpot and OpenJ9 are both implementations of the JVM concept.

The choice between the two depends on what you want to achieve. Each is best suited for different scenarios.
While HotSpot is the more widespread implementation, OpenJ9 is slowly becoming the most preferred JVM implementation.

We’re going to contrast Hotspot and OpenJ9 in this article. We’ll find out which is better and learn if OpenJ9 is a HotSpot.

In addition, this post looks at the HotSpot technology and all the critical features of OpenJ9 and HotSpot.

Important Definitions

Before looking at the difference between the two, it would help to define what they are:

Java Virtual Machine (JVM)

By definition, the JVM is a virtual machine or a computer program that mimics the operation of a physical machine. It has a virtual computer architecture, an execution paradigm, and an instruction set like a real machine.

It can execute code created using this virtual instruction set as a real machine would.

What is HotSpot technology in Java?

HotSpot is an implementation of the JVM idea. Sun originally developed it, but Oracle currently owns it. There are other implementations of JVM, such as IBM OpenJ9 and JRockit.

HotSpot is a dynamic compiler. It combines the best features of an interpreter and Just In Time (JIT) compiler and integrates them into a single feature. This combination brings several benefits in dynamic compilation, garbage collection, and synchronization.

Just In Time compilations means compiling on the fly. Therefore, HotSpot is the concept where only the code in use is the one compiled, i.e., the “hot” pieces of code used frequently.

Does OpenJDK use HotSpot?

An open-source version of HotSpot is being developed as part of the OpenJDK project. The OpenJDK is an open source implementation of the Java Platform, Standard Edition (Java SE). Additionally, it is the official reference for the Java SE (since version 7) implementation.

The Java Specifications, JSRs, and JEPs that outline the Java platform are implemented by the open-source effort known as OpenJDK. To get Java running on your PC, this project does not supply installers or binaries; only source code.

In short, HotSpot is a component within OpenJDK and Oracle JDK. In addition, today’s HotSpot also includes parts of JRockit.

Is OpenJDK a HotSpot?

“OpenJDK is a place, not a thing,” – said Donald Smith, Senior Director of Java Product Management at Oracle.

HotSpot is the JVM included in all OpenJDK Java versions. In this sense, people sometimes think that HotSpot and OpenJDK are ‌the same. While Oracle JDK includes the HotSpot VM, it’s only because it is part of OpenJDK. Therefore, all unmodified builds of OpenJDK use HotSpot.

What is OpenJ9 technology in Java?

OpenJ9 is another JVM implementation created by IBM. It is currently out-sourced through the Eclipse Foundation. In addition, it is available for free. Some JVM distributions based on OpenJDK can replace HotSpot with OpenJ9 and continue using the rest of the OpenJDK.

IMB was created to run on mobile devices having limited memory, making it ideal for use on the cloud. In addition, applications deployed using OpenJ9 were more economical, especially when deployed on the cloud.

Is OpenJ9 better than HotSpot?

Several factors make OpenJ9 better than Hotspot. As mentioned earlier, they are both JVM implementations and are suitable for different situations. For instance, experts believe ‌OpenJ9 is best suited for cloud-based solutions.

Some users have revealed in their reviews that they significantly reduced their memory consumption when they switched their JVM from HotSpot to OpenJ9.

Advantages of OpenJ9 include:

High optimization for a fast startup;

Lower memory footprint;
Quick ramp-up;
Excellent throughput performance;

It is free to use;
It is very easy to switch to OpenJ9 from other VM implementations.

Is OpenJ9 good for Minecraft?

Increasing performance can be good if running Minecraft with low frame rates. Low FPS can occasionally occur if you have a low-end machine, are using too many mods, or have an expensive shader.

Tests have confirmed that Open9J runs Minecraft faster than HotSpot. The JVM plays a significant role in running Minecraft. HotSpot is the default one, meaning it is crucial to run tests.

Follow these steps to use OpenJ9:

Download OpenJ9 from Adopt OpenJDK. However, the version varies depending on your Minecraft version. Ensure you check the OpenJ9 option before downloading.

Open the Minecraft Launcher.
Click the Installations tab.
Hover your mouse on the Minecraft installation you want to change to OpenJ9. Click the three dots button on the right and edit.

The Minecraft installation settings will open, on the bottom. Click to Expand More Options.
By default it will say use Java bundled runtime where it says Java Executable. Navigate to the Open9J folder. For OpenJ9 8, the default path is C:\Program Files\Semeru\jdk-8.0.302.8-openj9\; your path could also be similar.
Once you find the OpenJ9 folder, open the jre folder followed by the bin folder.

Select javaw.exe and open it to set it as your Java Executable.
Note that, several files have similar names, but javaw.exe has an icon.
Save the installation settings.

Expert Tips

Suppose you’re running different Minecraft versions and want to set OpenJ9 for all of them. It would help to download versions 8 and 16 and change the installation path accordingly.
You might get the Java mismatch error after Open9J installation and setup. When that happens, it means you selected the bin folder javaw.exe instead of the one in the JRE, then the bin folder. To rectify this error, simply change the path in the installation settings and try again.
On the other hand, getting the Java runtime incompatible error implies you are using the wrong version. Note that you use OpenJ9 16 for Minecraft 1.17 or higher and Openj9 8 in other versions.

What is the distinction between HotSpot and OpenJ9?

Features	HotSpot	OpenJ9
Best for	Best suited for servers and desktops.	Best for cloud-based solutions.
Workloads	HotSpot works well for all workloads.	OpenJ9 is an enterprise-grade VM suited for low memory footprint and fast start-up.
Resource usage	Has a relatively high resource usage.	OpenJ9 offers significantly reduced resource usage.

Besides that, several tests have confirmed that switching from HotSpot to OpenJ9 makes ojAlgo faster and ACM slower. On the other hand, the switch also causes EJML fluctuations.

Moreover, the tests also revealed that OpenJ9 is less stable in its results than HotSpot. It also makes the gap between the fastest and slowest code wider – it makes fast faster and slows slower.

Final thoughts

For many Java developers, JVM is used to run Java applications. However, if you take a keen interest, you’ll discover that there is particular reasoning behind every JVM implementation.

For instance, they develop HotSpot for desktop and server use, while OpenJ9 is best for cloud-based solutions. In this regard, the best JVM implementation depends on what you want to use it for. Nonetheless, I believe OpenJ9 seems to offer better performance than HotSpot.