What is the difference between static and non-static in java

First of all, let’s talk about the jvm memory model:

JVM memory is divided into: method area, heap, stack and program counter.

Let’s briefly talk about the method area, stack and heap:

Method area: It is an area shared by each thread, storing class information/constants/static variables;

Stack: In fact, each thread will be allocated a stack, and its life cycle is the same as that of the thread. When executing a method, a stack frame will be generated. The stack frame contains the local variables in the method.

Heap: All class instantiations are stored in the heap.

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1. Analyze the difference between static and non-static from the perspective of memory

Static modifies member variables and member methods. The address where their values ​​are stored is the constant pool in the method area. The order of execution is to store the statically modified variable names or methods in the constant pool when loading the class.

Non-statically modified variables can be member variables and local variables.

Member variables are the same as the address where the object is stored, because it is part of the class and its address is in the heap.

Local variables are generally declared in methods, and their storage addresses are on the stack.

2. Analysis from the running mechanism

When the source code is compiled into a .class bytecode file by the compiler, once the class is loaded, it is modified with static The blocks/variables/methods will be stored in the method area immediately. At the same time, the non-static blocks/variables/methods have not been compiled yet. Once the non-static variables are stored in the static methods, the program counter will indicate the local variables. An error will be reported because the local variables have not been compiled at this time.

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