Java JVM – Different Types of Heap in Java

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I was faced with this question recently for the different types of heap memory available in Java.

I couldn't find much information online. Are there different types of heap memory available in Java ?

Best Answer

Heap is divided Young Generation, Old or Tenured Generation, and Permanent Generation.

The Young Generation is where all new objects are allocated and aged. When the young generation fills up, this causes a minor garbage collection. Minor collections can be optimized assuming a high object mortality rate. A young generation full of dead objects is collected very quickly. Some surviving objects are aged and eventually move to the old generation.

The Old Generation is used to store long surviving objects. Typically, a threshold is set for young generation object and when that age is met, the object gets moved to the old generation. Eventually, the old generation needs to be collected. This event is called a major garbage collection.

The Permanent generation contains metadata required by the JVM to describe the classes and methods used in the application. The permanent generation is populated by the JVM at runtime based on classes in use by the application. In addition, Java SE library classes and methods may be stored here.

Refer Oracle Java documentation for more details:

"Browse to JVM Generations"

Related Solutions

Stack vs Heap – Why Use a Heap if Stack is More Efficient?

The problem with stacks is that you can't "free" memory unless it is on top of the stack. For instance, say you allocated 3 things of varying sizes:

a = allocate(2000000); // 2000000 bytes
b = allocate(1);
c = allocate(5000000);

The stack would have a on the bottom, b in the middle, and c on top. This becomes problematic if we want to free b:

free(b); // b is not on top! We have to wait until c is freed!

The workaround is to move all the data after b and shift if so that it comes after a. This works, but will require 5000000 copies in this case - something that will be much slower than a heap.

This is why we have a heap. While allocation may be slower than a stack (O(log n) vs O(1)), heaps allow freeing memory at an arbitrary location to be fast - O(log n), compared to a stack's O(n)

Java Heap Space – Understanding and Management

The heap as in the memory available for dynamic memory allocation has nothing to do with the heap as in any data structure or the heap invariant (which is in fact related to the data structure). "Heap" is just a common word, and was chosen for both concepts independently (Wikipedia notes that the term was used for data structure first, but no mention of influence).

While it is probably possible to construct a memory management system which uses a heap data structure (or rather, a priority queue) in some places, I am not aware of any existing algorithm that does it, and I doubt it would be useful, outside perhaps some very specific niche.

Best Answer

Related Solutions

Stack vs Heap – Why Use a Heap if Stack is More Efficient?

Java Heap Space – Understanding and Management

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