Java GC Basics: Important Concepts Explained

Hello! Let’s quickly review some common terms related to the Garbage Collector (GC) in the JVM.
Understanding these concepts can help you better analyze memory usage and performance issues in Java applications.

• Stop The World (STW):
  A moment when the JVM pauses all application threads to perform garbage collection tasks. During this pause, your application is temporarily stopped while the Garbage Collector reclaims memory or reorganizes the heap.

• Latency:
  The time it takes for your application to respond to a request. GC pauses can increase latency because the application is temporarily stopped.

• Throughput:
  The amount of work the application completes over time. Some garbage collectors focus on maximizing throughput instead of minimizing pause time.

  Example: Parallel GC, which focuses on completing more work even if pauses are longer.

• Pause Time:
  The duration of a GC pause. Different collectors are designed to reduce pause times depending on the system’s needs.

  Example: ZGC, which is designed to keep pause times very small, usually in the microseconds range.

• Java Heap:
  The Java Heap is the area of memory where objects are allocated.

  This is the main memory space managed by the Garbage Collector. When objects are no longer referenced, the GC frees that memory.

• Young Generation:
  The part of the heap where new objects are first allocated. Most objects have a short life and are collected here. Objects that survive several collections may be promoted to the Old Generation.

• Old Generation:
  Objects that survive longer are moved here. Garbage collections in this area happen less often but are usually heavier.

• Metaspace:
  The memory area where class metadata is stored, such as class definitions and method information. It replaced PermGen in Java 8 and uses native memory.

Understanding these basic terms makes it easier to reason about JVM performance and garbage collection behavior.

If you’d like a deeper explanation of how different Garbage Collectors work, let me know in the comments 🚀