How does garbage collection work in Java, and what are the different types of garbage collectors available?

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Garbage Collection in Java

  • Garbage Collection (GC) in Java is an automatic memory management process that frees up memory occupied by objects that are no longer in use.
  • This helps prevent memory leaks and ensures efficient use of memory resources.

❓ How Garbage Collection Works

Garbage collection works by identifying and removing objects that are no longer referenced by any part of a program. Here's the typical process:

  1. Object Creation: When an object is created, it is stored in the heap memory.
  2. Object Referencing: As long as an object is referenced by variables or other objects, it is considered "in use."
  3. Mark and Sweep: The garbage collector periodically looks for objects that are no longer referenced:
  4. Mark Phase: It marks all reachable objects starting from root references (like static fields and active thread stacks).
  5. Sweep Phase: It removes unmarked objects, freeing up memory.
  6. Compaction: After collecting, it may compact memory by moving objects to make memory allocation more efficient.

Types of Garbage Collectors in Java

Java provides several types of garbage collectors, each optimized for different use cases:

1. Serial Garbage Collector (GC):

  • How it works: It uses a single thread to handle garbage collection. Both the mark-and-sweep and compaction phases are handled in a serial manner.
  • Best for: Applications with small heaps and single-threaded environments.
  • Pros: Simple and low memory overhead.
  • Cons: Causes noticeable pauses (stop-the-world) during garbage collection.

2. Parallel Garbage Collector (Parallel GC):

  • How it works: Also known as the throughput collector, it uses multiple threads to speed up the garbage collection process. Both minor and major garbage collections are parallelized.
  • Best for: Applications that can afford longer pauses but need to maximize throughput.
  • Pros: Higher throughput due to parallel processing.
  • Cons: Still experiences stop-the-world pauses.

3. G1 Garbage Collector (Garbage-First):

  • How it works: Divides the heap into regions and performs garbage collection incrementally, prioritizing areas that are mostly garbage.
  • Best for: Applications requiring low-pause times with large heaps.
  • Pros: Reduces long pauses, provides predictable GC behavior.
  • Cons: Slightly higher memory overhead due to regionalized memory.

4. CMS Garbage Collector (Concurrent Mark-Sweep):

  • How it works: Focuses on minimizing pause times by performing most garbage collection concurrently with the application.
  • Best for: Low-latency applications that can't afford long GC pauses.
  • Pros: Lower pause times because the mark phase runs concurrently with the application.
  • Cons: Can cause memory fragmentation and is being replaced by G1 GC in recent Java versions.

5. Z Garbage Collector (ZGC):

  • How it works: A low-latency collector that works concurrently with the application, like CMS, but with better scaling capabilities.
  • Best for: Applications requiring very large heaps (multi-terabyte) and minimal pause times.
  • Pros: Pause times are consistently low, even with large heaps.
  • Cons: Still relatively new and might require more tuning for specific use cases.

6. Shenandoah Garbage Collector:

  • How it works: Similar to ZGC, Shenandoah is also a low-latency collector designed for low-pause times with large heaps.
  • Best for: Real-time systems requiring minimal pauses, and scalability in large heap environments.
  • Pros: Pause times don't scale with heap size.
  • Cons: Still evolving, and tuning may be necessary for optimal performance.