How the JVM Stores Static and Instance Variables

The Java Virtual Machine (JVM) manages memory for variables in different ways, depending on whether they are static or instance members. Understanding where and how these variables are stored helps you write more efficient and reliable code.

Static Variables: Stored in the Method Area

• Static variables are declared with the static keyword inside a class;
• The JVM loads static variables once, when the class is first loaded;
• All instances of the class share the same static variable; changes to the variable are visible to all instances;
• Static variables are stored in a special memory region called the method area (sometimes referred to as the "meta space" in modern JVMs);
• The method area holds class-level data such as static fields, method definitions, and constant pool information.

Memory and Reference Management:

• Static variables remain in the method area for as long as the class is loaded by the JVM;
• The JVM does not garbage collect static variables until the class is unloaded, which usually happens only when the JVM shuts down or the class loader is garbage collected.

Instance Variables: Stored on the Heap

• Instance variables are declared without the static keyword and belong to individual objects;
• Each object created from a class has its own copy of the instance variables;
• Instance variables are stored in the heap memory, which is dedicated to object storage at runtime.

Memory and Reference Management:

• Each time you create a new object with new, the JVM allocates memory for its instance variables on the heap;
• The JVM's garbage collector automatically frees heap memory for objects (and their instance variables) when there are no more references to them in your program;
• Instance variables exist for as long as their containing object exists in memory.

Understanding these details helps you make better decisions about when to use static or instance variables, and how your code will behave in memory.