How Does Java Memory Management Work with Heap Generations?

Java Memory Management: Understanding Heap Generations

Java heap is a memory space where objects are allocated and managed during runtime. It is further divided into generations: young, old, and permanent, each serving specific purposes and interacting with each other.

Young Generation

The young generation is the first place where objects are allocated. It is further divided into:

• Eden Space: New objects are initially allocated here.
• Survivor Space: Objects that survive a garbage collection cycle (known as a "minor GC") are moved here.

Old Generation (Tenured Generation)

Objects that survive multiple minor GCs in the survivor space are promoted to the old generation. This is where long-lived objects reside, such as those representing static data or persistent entities.

Permanent Generation

Unlike the other generations, the permanent generation is not part of the heap. It holds non-heap memory for meta-information related to classes and methods. In Java 8 , the permanent generation was removed, and meta-information is now stored in a single space called the Metaspace.

Interactions Between Generations

The generations are connected through garbage collection cycles:

• Minor GC: Occurs frequently in the young generation to reclaim objects from the Eden space and move survivors to the survivor space.
• Major GC: Occurs less frequently in the old generation to reclaim objects that have aged and are no longer referenced.
• Full GC: A rare event that collects all generations and is used when the heap is severely fragmented.

In summary, the young generation is for short-lived objects, the old generation contains long-lived objects, and the permanent generation (or Metaspace in Java 8 ) stores non-heap data related to classes and methods. Garbage collection cycles move objects through these generations as they age and are no longer needed.

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