Conditions to Trigger Garbage Collection:
- The Java Virtual Machine (JVM) triggers garbage collection when it detects that the Java heap memory is running low and needs space to allocate new objects.
- Specifically, the JVM uses a variety of strategies and algorithms to determine when to start garbage collection, depending on the memory management settings and the garbage collector being used.
Here’s a general overview of how the JVM decides to trigger garbage collection:
1. Memory Usage Threshold
The JVM monitors the memory in use, particularly in the heap, which is divided into the Young Generation (Eden and Survivor spaces) and the Old Generation. When memory usage in these areas reaches a certain threshold, the JVM triggers garbage collection:
- Young Generation GC (Minor GC): The JVM frequently triggers this when the Eden space becomes full. Objects that survive this process may be moved to the Old Generation.
- Old Generation GC (Major GC or Full GC): When the Old Generation fills up (after several Minor GCs or from the creation of long-lived objects), the JVM triggers a Full GC, which is a more comprehensive and time-consuming process.
2. Explicit Garbage Collection
You can explicitly request garbage collection in the application by calling System.gc(). However, this is merely a suggestion to the JVM, and it may or may not initiate garbage collection immediately. The JVM manages the actual timing based on its internal heuristics.
3. Heap Usage Patterns and Garbage Collection Algorithms
The garbage collection strategy varies depending on the garbage collector in use:
- Serial GC: It triggers based on heap usage reaching a certain threshold.
- Parallel GC: It collects garbage in parallel and uses CPU and memory efficiently, monitoring the usage patterns of different generations.
- G1 (Garbage First) GC: It divides the heap into regions and triggers garbage collection based on heuristics to meet a pause-time goal. It keeps track of which regions to collect based on their garbage-to-live-object ratio.
- ZGC/Shenandoah: These are low-latency collectors that aim to perform garbage collection concurrently with the application threads. They trigger based on thresholds but strive to minimize pause times.
4. Out of Memory Condition
If the JVM cannot allocate space for new objects (due to insufficient memory or the heap being full), and if garbage collection is not able to free up enough memory, it will trigger a Full GC. If even after this, memory cannot be reclaimed, the JVM will throw an OutOfMemoryError.
5. Adaptive Tuning and Ergonomics
The JVM has adaptive features that monitor application behavior and tune garbage collection automatically. These features adjust thresholds and decide when to trigger garbage collection based on:
- Application workload: The JVM tries to balance memory allocation and garbage collection frequency.
- User-specified goals: Goals like maximum pause time or throughput requirements can influence when garbage collection is triggered.
In summary, garbage collection is triggered when memory pressure increases due to a full heap, depending on the type of collector in use, heap usage thresholds, and sometimes, explicit suggestions from the application.