7.2 Types of Garbage Collectors

← Back to Chapter 7: Garbage Collection & Serialization

Selecting the right GC collector depends on application priorities: throughput, pause latency, or memory footprint.

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1. Evolution of HotSpot GC Collectors

Collector	JVM Flag	Target Workload	STW Pause Duration
Serial GC	-XX:+UseSerialGC	Single-core micro-containers	High (Single-threaded STW)
Parallel GC	-XX:+UseParallelGC	Batch processing, high throughput	Medium-High (Multi-threaded STW)
G1 GC (Default)	-XX:+UseG1GC	General enterprise web apps	Low (Configurable, e.g. 200ms)
ZGC	-XX:+UseZGC	Ultra-low latency enterprise APIs	Sub-millisecond (< 1 ms)
Shenandoah	-XX:+UseShenandoahGC	Ultra-low latency concurrent	Sub-millisecond (< 1 ms)

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Serial Garbage Collector

Uses essentially one GC thread for collection work.

Good for:

small applications
small heaps
environments where simplicity matters

Not ideal for large, latency-sensitive applications.

Parallel Garbage Collector

Parallel Garbage Collector

Uses multiple threads for GC work.

GC
├── Thread 1
├── Thread 2
├── Thread 3
└── Thread 4

Goal: High throughput

Throughput means:

Spend more time doing application work and less time doing GC work.

G1 Garbage Collector

G1 = Garbage-First Garbage Collector

Instead of thinking only in terms of one large contiguous Young/Old area, G1 divides the heap into many regions.

Heap :

┌──┬──┬──┬──┐
│  │  │  │  │
├──┼──┼──┼──┤
│  │  │  │  │
├──┼──┼──┼──┤
│  │  │  │  │
└──┴──┴──┴──┘

Some of them are young object and some of them are old object. G1 tries to prioritize regions with lots of garbage.

Why is G1 useful?

G1 was designed for applications where you want:

relatively predictable pauses
large heaps
good overall performance
better control of pause targets

ZGC

ZGC is designed for very low pause times, including large heaps.

Its key idea is:

Perform much of the GC work concurrently with application threads.

So:

Application ────────────────>
              ↑
              │
             GC
              │
              └──── mostly concurrent

It is useful for applications where latency is extremely important.

Shenandoah GC

Shenandoah also focuses on: Very low GC pause times

It performs significant work concurrently with application execution.

Conceptually:

Application ────────────────>
GC          ────────────────>

They overlap heavily.

Ways of Garbage Collection :

finalize() (Deprecated)

finalize() was mainly used for cleanup of external resources such as files, connections, etc.

Try-with-resources — preferred

try (FileInputStream file = new FileInputStream("data.txt")) {
   // Use the file
} // File is automatically closed here

This is resource cleanup, not GC.

AutoCloseable

Your class can implement AutoCloseable:

class MyResource implements AutoCloseable {

    @Override
    public void close() {
        // Release the resource
    }
}

try (MyResource resource = new MyResource()) {
    // Use resource
}

Cleaner

Java also provides java.lang.ref.Cleaner.

It can be used for special cases where some cleanup action is needed as a safety net, but it should not be treated as a replacement for normal explicit resource management.