Understanding Heap Memory
Java Memory Model – Method Area
Method Area is part of space in the Perm Gen and used to store class structure (runtime constants and static variables) and code for methods and constructors.
Java Memory Model – Memory Pool
Memory Pools are created by JVM memory managers to create a pool of immutable objects if the implementation supports it. String Pool is a good example of this kind of memory pool. Memory Pool can belong to Heap or Perm Gen, depending on the JVM memory manager implementation.
Java Memory Model – Runtime Constant Pool
Runtime constant pool is per-class runtime representation of constant pool in a class. It contains class runtime constants and static methods. Runtime constant pool is part of the method area.
Java Memory Model – Java Stack Memory
Java Stack memory is used for execution of a thread. They contain method specific values that are short-lived and references to other objects in the heap that is getting referred from the method.
Memory Management in Java – Java Heap Memory Switches
Java provides a lot of memory switches that we can use to set the memory sizes and their ratios. Some of the commonly used memory switches are:
| VM Switch | VM Switch Description |
|---|---|
| -Xms | For setting the initial heap size when JVM starts |
| -Xmx | For setting the maximum heap size. |
| -Xmn | For setting the size of the Young Generation, rest of the space goes for Old Generation. |
| -XX:PermGen | For setting the initial size of the Permanent Generation memory |
| -XX:MaxPermGen | For setting the maximum size of Perm Gen |
| -XX:SurvivorRatio | For providing ratio of Eden space and Survivor Space, for example if Young Generation size is 10m and VM switch is -XX:SurvivorRatio=2 then 5m will be reserved for Eden Space and 2.5m each for both the Survivor spaces. The default value is 8. |
| -XX:NewRatio | For providing ratio of old/new generation sizes. The default value is 2. |
Java Garbage Collection
Java Garbage Collection is the process to identify and remove the unused objects from the memory and free space to be allocated to objects created in future processing. One of the best features of Java programming language is the automatic garbage collection, unlike other programming languages such as C where memory allocation and deallocation is a manual process.
Garbage Collector is the program running in the background that looks into all the objects in the memory and find out objects that are not referenced by any part of the program. All these unreferenced objects are deleted and space is reclaimed for allocation to other objects.
One of the basic ways of garbage collection involves three steps:
- Marking: This is the first step where garbage collector identifies which objects are in use and which ones are not in use.
- Normal Deletion: Garbage Collector removes the unused objects and reclaim the free space to be allocated to other objects.
- Deletion with Compacting: For better performance, after deleting unused objects, all the survived objects can be moved to be together. This will increase the performance of allocation of memory to newer objects.
There are two problems with a simple mark and delete approach.
- First one is that it’s not efficient because most of the newly created objects will become unused
- Secondly objects that are in-use for multiple garbage collection cycle are most likely to be in-use for future cycles too.
The above shortcomings with the simple approach is the reason that Java Garbage Collection is Generational and we have Young Generation and Old Generation spaces in the heap memory. I have already explained above how objects are scanned and moved from one generational space to another based on the Minor GC and Major GC.