静态方法的Java实例变量

Question

when does instance variable of a static method gets memory in java? it is perfectly right to create instance variable in static method (either in parameter or declaring inside the function), but i want to know when does the instance variable of static method is allocated memory, if no object of the class is created?

Actually i meant to say the local variable inside static method

Answer 1

Firstly, the term instance variable is not accurate, I am assuming that you are asking about local variable such as a and b in the example below:

void method(int a) {
    int b = 3;
}

In Java, only primitives and references are stored on stack, objects are stored in heap when they are constructed. Stack will be cleaned as soon as the scope ends, heap is cleaned by garbage collector.

Here is an example:

public class Main  {

    static A a = new A();

    static void method() {
        int b = 2;
        C c = new C();
    }

}

The first time your code refers to Main , class loader will load the class and initialize all its static fields - object new A() will go into heap. Nothing more happens here, method method could as well not exist.

When you call method method , it will add value 2 on the stack, then it will add reference c on the stack which will be pointing to the object new C() in the heap.

When method exists, 2 and c are removed from the stack (there is actually no removal, but the top of the stack is changed to two positions below so these two values will be overriden whenever something else comes onto stack), while new C() will remain in heap until garbage collector triggers. It's likely that it will be garbage collected immediately as GC may detect that there are no more references to this object.

Answer 2

If you declare a variable or use parameter inside a static method, its not at all instance variable. Its a local variable and it will be initialized once the method gets invoked

static void methodTest(int i) {

String s = "Hello";

}

Here s is a local variable to that method. and i is the function parameter which is also a local variable to the method

Answer 3

There is not such thing as an instance variable in a static method.

If you mean parameters and local variables, those are allocated on stack (any created objects being allocated on heap as usual).

Static fields of the class will be initialized when the class is loaded.

Instance variables of a class will be allocated (and possibly initialized) when an instance is constructed.

Answer 4

The variable defined by static method is still local variable in scope of the method, just as if non-static method created local variable. However, problem is that the variable never gets garbage collected, as long as the class loader (usually main class, which gets "unloaded" only at runtime termination, which is like scratching your left ear with your right hand) is not unloaded and garbage collected. And objects get GC'd only if they are not referenced anymore, and in case of static fields, they have tendency to be referenced even when nothing uses them because of the behavior I described. So in large applications, where memory and resources are issue, it is pretty bad idea to declare anything inside static methods since you pretty soon pollute the memory with LOCAL VARIABLES that cannot be collected.