Where are static methods and static variables stored in Java?
For example:
class A {
static int i=0;
static int j;
static void method() {
// static k=0; can't use static for local variables only final is permitted
// static int L;
}
}
Where will these variables be stored in Java, in heap or in stack memory? How are they stored?
Static methods (in fact all methods) as well as static variables are stored in the PermGen section of the heap, since they are part of the reflection data (class related data, not instance related).
Update for clarification:
Note that only the variables and their technical values (primitives or references) are stored in PermGen space.
If your static variable is a reference to an object that object itself is stored in the normal sections of the heap (young/old generation or survivor space). Those objects (unless they are interal objects like classes etc.) are not stored in PermGen space.
Example:
static int i = 1; //the value 1 is stored in the permgen section
static Object o = new SomeObject(); //the reference(pointer/memory address) is stored in the permgen section, the object itself is not.
A word on garbage collection:
Do not rely on finalize() as it's not guaranteed to run. It is totally up to the JVM to decide when to run the garbage collector and what to collect, even if an object is elligible for garbage collection.
Of course you can set a static variable to null and thus remove the reference to the object on the heap but that doesn't mean the garbage collector will collect it (even if there are no more references).
Additionally finalize() is run only once, so you have to make sure it doesn't throw exceptions or otherwise prevent the object to be collected. If you halt finalization through some exception, finalize() won't be invoked on the same object a second time.
A final note: how code, runtime data etc. are stored depends on the JVM which is used, ie HotSpot might do it differently than JRockit and this might even differ between versions of the same JVM. The above is based on HotSpot for Java 5 and 6 (those are basically the same) since at the time of answering I'd say that most people used those JVMs. Due to major changes in the memory model as of Java 8, the statements above might not be true for Java 8 HotSpot - and I didn't check the changes of Java 7 HotSpot, so I guess the above is still true for that version, but I'm not sure here.
Class variables(Static variables) are stored as part of the Class object associated with that class. This Class object can only be created by JVM and is stored in permanent generation .
Also some have answered that it is stored in non heap area which is called Method Area. Even this answer is not wrong. It is just a debatable topic whether Permgen Area is a part of heap or not. Obviously perceptions differ from person to person. In my opinion we provide heap space and permgen space differently in JVM arguments. So it is a good assumption to treat them differently.
Another way to see it
Memory pools are created by JVM memory managers during runtime. Memory pool may belong to either heap or non-heap memory.A run time constant pool is a per-class or per-interface run time representation of the constant_pool table in a class file. Each runtime constant pool is allocated from the Java virtual machine's method area and Static Variables are stored in this Method Area. Also this non-heap is nothing but perm gen area.Actually Method area is part of perm gen.(Reference)
This is a question with a simple answer and a long-winded answer.
The simple answer is the heap. Classes and all of the data applying to classes (not instance data) is stored in the Permanent Generation section of the heap.
The long answer is already on stack overflow:
There is a thorough description of memory and garbage collection in the JVM as well as an answer that talks more concisely about it.
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