Java garbage collection reclaims heap space from objects the application can no longer reach, but it still affects pauses, CPU use, throughput and memory footprint. There is no universally fastest collector: choose a starting point based on your service’s priorities, confirm what the deployed JVM actually uses, and diagnose logs before changing flags. This guide reflects Oracle’s HotSpot Virtual Machine Garbage Collection Tuning Guide, Release 25 (July 2026); defaults and options can vary with JDK version, platform and configuration.
How does garbage collection work in Java?
The JVM reclaims heap memory occupied by objects that are no longer reachable by the application. This automatic memory management does not make collection cost-free: some work can pause application threads, while concurrent work consumes CPU that could otherwise serve requests. Collection behavior also affects throughput and the heap’s committed footprint.
Evaluate a collector against the workload and service goals that matter to you: response-time pauses, throughput, and memory footprint. A setting that improves one can worsen another, and a collector’s real behavior depends on heap size, live data, allocation patterns and available processor capacity.
Which Java garbage collector should you use?
Oracle’s JDK 25 guidance treats collector recommendations as starting points, not rankings established by comparative benchmarks. The right choice must be checked under representative load.
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| Collector | Oracle’s suggested starting case | Trade-off to consider |
|---|---|---|
| Serial | Small data set (about 100 MB or less), or one processor, when pauses are not a constraint. | A simple single-processor case; suitability still depends on the workload. |
| Parallel | Peak application performance is the priority and pauses of a second or longer are acceptable. | Throughput-first; longer pauses may be acceptable. |
| G1 | Response time matters and shorter pauses are desired while maintaining throughput. | Concurrent work uses application resources, and pause targets are not guarantees. |
| ZGC | Response time is a high priority. | Low-latency design depends on sufficient heap headroom and resources for concurrent collection. |
Oracle notes that these guidelines depend on heap size, the amount of live data and processor count and speed. If a recommendation misses the application’s goal, its guide suggests first adjusting heap and generation sizing, then considering a different collector.
Is G1 the default collector?
Often, but not everywhere. Oracle’s JDK 25 guide says G1 is the default on most hardware and operating-system configurations. Its ergonomics documentation describes G1 on “server-class” machines and Serial otherwise. In that documentation, server-class means at least two processors and at least 1792 MB of physical memory.
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The same guide lists default initial and maximum heap selections of 1/64 and 1/4 of physical memory, respectively. These are documented JVM defaults, not sizing recommendations for every service. Containers, JDK versions and explicit runtime settings can change what applies. Check the actual deployed runtime and its startup configuration rather than assuming a default.
How should you balance pause time and throughput?
GC tuning is a trade-off. Oracle documents -XX:MaxGCPauseMillis as a hint, not a promise. Asking for shorter pauses can lead to more frequent collection and lower throughput; the JVM may also be unable to meet the requested goal for a particular workload.
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-XX:GCTimeRatio expresses a throughput goal. Heap sizing and the application’s minimum live data set limit the JVM’s options, so neither goal can be optimized independently of the workload. Measure the application under representative conditions and change a small number of relevant settings at a time.
What should you know about G1 pauses?
G1 is generational and incremental. It does some expensive work concurrently and uses stop-the-world pauses for collection phases. It uses prior application and pause behavior to estimate how much work to do, and seeks regions with high reclaim efficiency. Concurrent GC work still competes with the application for resources, and G1 is not a real-time collector: it does not guarantee a maximum pause for every event.
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How do you diagnose long G1 pauses or Full GC?
Start with GC logs and identify the event and its cause before changing settings. In particular, distinguish a Full GC from a long pause during ordinary collection, then use the log’s phase and CPU details to narrow the investigation.
- Look for Full GC and preceding evacuation failures. Search for
Pause Full (G1 Compaction Pause)and check the events before it. Oracle identifies old-generation occupancy, marking that does not finish in time, and humongous allocations as possible contributors to Full GC. - Check humongous-region counts. Use
gc+heap=infologging to inspect them. Oracle lists a larger G1 region size or a larger heap as possible remedies, but the object-allocation pattern may need attention too. Avoid increasing the heap without establishing that it addresses the cause. - Find which pause phases take time. Use phase logging to locate the work responsible for a long pause. For mixed collections that take too long, Oracle describes increasing
G1MixedGCCountTargetto spread reclamation across more collections. This can reduce the space reclaimed in the current cycle and may complicate sustained operation. - Separate JVM work from operating-system time. Use
gc+cpu=infoto compare user time, system time and elapsed time. Memory operations, transparent huge pages and log I/O can affect observed pauses, so a long elapsed time does not necessarily point to collector work alone.
After identifying a plausible cause, make a focused change and compare results under representative load. Avoid applying a generic bundle of GC flags: a change can shift the balance among pause time, throughput and heap use.
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What changed in ZGC in JDK 24 and later?
Oracle’s JDK 25 tuning guide states that ZGC has been generational since JDK 24 and that the ZGenerational option was removed. ZGC is adaptive: it adjusts generations, GC thread counts and tenuring thresholds.
The main sizing task is setting the maximum heap with -Xmx so it can hold the live set and leave room for allocations while concurrent collection runs. Oracle’s guide describes ZGC heap sizes up to 16 TB; that documented upper range is not a promise of equivalent performance on every machine. The guide also describes -XX:SoftMaxHeapSize as a soft maximum; -Xmx remains the hard maximum.
Where can you learn more?
For current collector behavior, defaults and flags, consult Oracle’s HotSpot Virtual Machine Garbage Collection Tuning Guide, Release 25, and check documentation for the JDK version you actually deploy. Scott Oaks’s Java Performance: The Definitive Guide is optional conceptual background; O’Reilly lists chapters on garbage collection, collector selection, tuning and tools, but the book was published in April 2014 and should not replace current JDK documentation.
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