Azul produces two distinct Java platforms that serve fundamentally different use cases. Azul Zing and Azul Zulu are the underlying JVM runtimes; Azul Prime and Azul Core are the commercial products built on top of them, respectively. Both runtimes come from the same company, but the products they power target different performance requirements, operational contexts, and budget realities.
What Is Azul Prime?
Azul Zing is Azul’s enterprise Java runtime, purpose-built for latency-sensitive, high-throughput workloads. It is the runtime underneath Azul Prime, Azul’s commercial product for running Zing in production with enterprise support and licensing. If you encounter references to Zing in documentation, blog posts, or benchmark reports, they refer to this same runtime that powers Azul Prime today.
Azul Prime is a commercial enterprise JVM engineered specifically for latency-sensitive, high-throughput production workloads. Its defining technical characteristics are:
- C4 GC (Continuously Concurrent Compacting Collector): a garbage collector designed to be pauseless, avoiding stop-the-world pauses. This is the core technical differentiator: it is designed to avoid pauses during marking, during relocation, and during reference healing. Pause times remain consistently sub-millisecond regardless of heap size, including on heaps of hundreds of gigabytes or multiple terabytes
- Falcon JIT Compiler: an LLVM-based Just-In-Time compiler that applies more aggressive optimization than HotSpot’s C2 compiler. Falcon uses deeper inlining, better loop vectorization, and improved use of modern CPU instruction sets to achieve higher steady-state throughput for compute-intensive workloads
- ReadyNow! Technology: eliminates JIT compilation warmup by persisting compilation profiles across JVM restarts. Applications running on Prime start with near-peak performance from the first request rather than degrading during a warmup period after each deployment or restart
- Self-Tuning: C4 adapts heap sizing and GC behavior automatically to current allocation patterns, reducing the need for extensive GC flag tuning that G1GC-based deployments often require
Azul ZingAzul Prime is a drop-in JVM replacement. Most applications require no code changes. The JVM is a compatible OpenJDK distribution with additional capabilities layered on top.
Azul Prime is a commercial product with enterprise licensing. It targets organizations where JVM performance directly affects business outcomes: trading firms, real-time data platforms, large-scale API services, and any application where GC pause unpredictability is an architectural problem.
What Is Azul Zulu?
Azul Zulu is a free, open-source distribution of OpenJDK maintained by Azul. It is a rigorously tested, production-quality OpenJDK build — binary-compatible with the OpenJDK reference implementation — with several key differences from raw OpenJDK builds:
- Long-term support: Azul provides extended support timelines for LTS releases, including Java 8, 11, 17, and 21, beyond what the OpenJDK community officially maintains
- Broad platform coverage: Zulu builds are available for Linux, Windows, macOS, and a wide range of CPU architectures including x86-64, AArch64, ARM, and more
- Free to use: Zulu Community builds are available at no cost for any use, including production deployment
Azul Core is the commercial support tier for Zulu. It adds enterprise support contracts, security patch SLAs, and contractual protections. Zulu Community and Azul Core both use the standard HotSpot JVM from OpenJDK. They do not include C4 GC, Falcon JIT, or ReadyNow!.
Zulu is the right choice for organizations that need a reliable, supported OpenJDK distribution without the complexity or cost of an enterprise JVM. It serves the same workloads that any OpenJDK distribution would serve: web applications, microservices, batch jobs, and general-purpose Java workloads where standard G1GC pause times are acceptable.
Azul Prime vs. Azul Zulu: Feature Comparison
| Feature | Azul Prime | Azul Zulu (Core) |
|---|---|---|
| JVM Base | OpenJDK-compatible (Azul builds) | OpenJDK (HotSpot) |
| Garbage Collector | C4 (Continuously Concurrent Compacting Collector) – pauseless | G1GC, ZGC, Shenandoah, Serial, Parallel (standard OpenJDK collectors) |
| GC Pause Behavior | Near-zero stop-the-world pauses by design | Pauses depend on collector choice. G1GC: varies by heap |
| JIT Compiler | Falcon (LLVM-based, more aggressive than C2) | HotSpot C2 (standard OpenJDK JIT) |
| Warmup Behavior | ReadyNow! eliminates warmup via persistent compilation profiles | Standard tiered compilation warmup. AppCDS available for class loading |
| Self-Tuning GC | Yes: C4 adapts heap and GC automatically | No: manual tuning of GC parameters required for optimal configuration |
| Max Heap Size | Multi-terabyte with consistent latency | Limited in practice by GC pause times on G1GC; ZGC handles large heaps better |
| Azul Mission Control | Bundled (enhanced for C4 and Falcon) | Bundled |
| Azul Intelligence Cloud | Available as add-on | Available as add-on |
| Azul Vulnerability Detection | Available as add-on | Available as add-on |
| Target Workloads | Latency-sensitive, high-throughput, large heap, mission-critical | General-purpose Java: web apps, microservices, batch processing |
| Licensing | Commercial (enterprise subscription) | Free (Community) or commercial support (Core subscription) |
| Code Changes Required | None (drop-in JVM replacement) | None (standard OpenJDK binary compatibility) |
| More Information | azul.com/products/prime/ | azul.com/downloads/ |
When to Use Azul Zing vs. Azul Zulu
Choose Azul Prime when:
- GC pause times are causing latency SLA violations or business impact, and ZGC tuning has not resolved them
- Your application runs on large heaps (50 GB or more) and G1GC pause times scale to unacceptable levels
- Your workload is latency-sensitive at P99 or P99.9, particularly financial trading, real-time bidding, or telecommunications
- Application restarts or deployments cause performance degradation periods that affect users and ReadyNow! warmup elimination would address this
- Your team spends significant engineering time tuning GC parameters across environments and self-tuning would free those resources
- You are running large-scale in-memory data platforms (Cassandra, Kafka, Hazelcast) with multi-hundred-gigabyte heaps
Choose Azul Zulu (Core) when:
- Your workload is general-purpose Java without strict sub-10ms latency requirements
- Standard G1GC or ZGC pause times are acceptable for your SLA
- You need a free, reliable, well-supported OpenJDK distribution with extended LTS support
- You need platform coverage beyond what other distributions provide (ARM, embedded, older OS versions)
- You want commercial support for OpenJDK without the cost of an enterprise JVM
- You are migrating from Oracle JDK and need a drop-in replacement at no cost
Frequently Asked Questions
Is Azul Prime the Same Product as Zing?
Not quite — they’re related but distinct. Zing is the JVM runtime; Prime is the commercial product built on Zing, with enterprise support and licensing. Similarly, Zulu is the runtime that Azul Core is built on (Zulu itself remains available as a free, open-source build). If a benchmark or case study references Zing, the results apply to the runtime inside Azul Prime.Yes. Azul Prime is the current name for the product formerly marketed as Zing. The underlying technology, including the C4 garbage collector, ReadyNow!, and the Falcon JIT compiler, is unchanged — only the product name has been updated.
Is Zulu free?
Azul Zulu Community builds are free to download and use in production without licensing fees. There are no restrictions on commercial use. Azul Core adds commercial support contracts, security patch SLAs, and contractual protections on top of the same Zulu binary. Azul Prime is a separate commercial product and is not free.
Can I upgrade from Zulu to Prime?
Yes, and this is one of the most common adoption paths. Because both Zulu and Prime are OpenJDK-compatible, the migration typically requires only swapping the JVM binary. Most applications require no code changes. Teams often start with Zulu, encounter GC-related performance problems as heap sizes or latency requirements grow, and migrate to Prime specifically to address those problems. Azul’s sales and technical teams provide migration assistance for organizations considering this path.
Does Prime include Zulu features?
Both products include Azul Mission Control. Both are available across standard server platforms. Prime adds C4 GC, Falcon JIT, and ReadyNow! on top of a baseline that is functionally comparable to Zulu for standard Java compatibility. Prime is not based on Zulu’s HotSpot JVM; it uses a distinct JVM implementation with its own compilation and GC infrastructure.
Which Azul product replaces Oracle JDK?
For most organizations migrating from Oracle JDK due to licensing changes, Azul Zulu is the appropriate replacement. It is binary-compatible with Oracle JDK builds, provides the same HotSpot JVM, includes extended LTS support, and is free. Azul Prime is an appropriate replacement for Oracle JDK in scenarios where the migration is also an opportunity to address latency or GC limitations in the application.