Java is a memory-managed language, and much of that memory management is handled automatically by the Java Virtual Machine. One of the most crucial components of this automation is garbage collection, which ensures that memory no longer in use is efficiently reclaimed.
The Basics of Java Memory Management
Java programs allocate memory primarily on the heap, where objects are created. Over time, some of these objects are no longer needed. If they are not removed, the application may run out of memory. To prevent this, Java uses a garbage collector that identifies and removes unused objects.
Objects in memory are categorized as:
- Live: These objects are currently in use and are being referenced by other objects or threads.
- Dead: These objects are no longer referenced and can be safely deleted.
All objects are connected in a graph starting from a special root node called the Garbage Root Object. The garbage collector traverses this graph to identify which objects can be discarded.
Phases of Garbage Collection
Garbage collection typically occurs in three phases:
- Mark: The collector identifies which objects are still in use.
- Sweep: Unused objects are deleted.
- Compact: Remaining objects are moved to form a contiguous memory block, reducing fragmentation.
Although garbage collection happens automatically, developers can attempt to trigger it manually using the System.gc() method. However, this is only a suggestion to the JVM and is not guaranteed to execute immediately.
Generational Garbage Collection
Modern JVMs use a generational approach to manage memory more efficiently. The heap is divided into several regions based on object lifespan:
Young Generation
Newly created objects start here. This area is further divided into:
- Eden Space: Where all new objects are allocated.
- Survivor Spaces: Objects that survive one or more garbage collections are moved here.
When memory is reclaimed from this space, it is called a minor garbage collection.
Old Generation
Objects that have survived multiple garbage collection cycles in the Young Generation are moved to the Old Generation. When memory is reclaimed from this region, it is called a major garbage collection. Major GCs typically involve a pause in the application, known as a stop-the-world event.
Permanent Generation and Metaspace
In earlier versions of Java, the Permanent Generation (PermGen) held metadata about classes and methods. Starting with Java 8, this space was replaced with Metaspace, which automatically resizes and avoids memory errors due to class metadata.
Types of Garbage Collection Algorithms
Mark-Copy (Young Generation)
- Mark all live objects.
- Copy them from Eden to a Survivor Space (S1 or S2).
- Clear the Eden space.
One survivor space is always empty to enable efficient copying.
Mark-Sweep-Compact (Old Generation)
- Mark all live objects.
- Sweep or remove all dead objects.
- Compact memory by moving live objects together.
Garbage Collector Types and Use Cases
Here are common garbage collectors and when to use them:
- UseSerialGC
Suitable for small applications or low-resource environments. Single-threaded collection. - UseParallelGC
Ideal for high-throughput systems where long pauses are acceptable. Multi-threaded minor GC. - UseConcMarkSweepGC
Deprecated since Java 9. Concurrent marking and sweeping reduce pause times. CPU-intensive. - UseG1GC
Splits the heap into regions and collects the most garbage-heavy regions first. Predictable pause times. - UseEpsilonGC
A no-op collector that does nothing and shuts down the JVM when the heap is full. Good for performance testing. - UseShenandoahGC
Low-pause, concurrent collector. Designed for minimal latency applications. - UseZGC
Ultra-low pause collector, suitable for large heaps. Pause times are typically under two milliseconds. - ZGenerational
An enhancement of ZGC that adds generational awareness, improving efficiency for short-lived objects.
How to Choose the Right Garbage Collector
| Collector | Best Use Case |
|---|---|
| Serial GC | Small apps, single-core CPUs, low memory footprint |
| Parallel GC | Batch jobs, high throughput, long pauses acceptable |
| G1 GC | Balanced latency and throughput, large heaps |
| Shenandoah GC | Predictable latency, low-pause systems |
| ZGC | Large heaps, ultra-low pause requirements |
| Epsilon GC | Benchmarking and performance testing |
Tuning JVM Memory
You can configure the JVM memory regions using the following options:
- Set the size of the Young Generation with
Xmn - Set the initial heap size with
Xms - Set the maximum heap size with
Xmx
Final Thoughts
Garbage collection is a vital part of Java’s memory management. Understanding how it works, and how different collectors behave, helps developers choose the right configuration for their use case. As systems grow in complexity and scale, selecting the appropriate garbage collector becomes increasingly important for ensuring performance and stability.
Let your architecture and service-level expectations guide your GC and memory tuning decisions.
Leave a Reply