Stack space of JVM

JVM Stack space of

Java Virtual machine memory region model

** Yellow area ：** The area of memory shared by all threads , There will be garbage collection .

** Grey areas ：** Thread private does not generate garbage collection .

Stack space is part of the runtime data area , So how does stack space store data ？

Java What is the virtual machine stack

In the runtime data area JAVA Stack 、 Native Method Stack 、 All three program counters are private to threads , The life cycle of the stack is the same as that of the thread , The virtual stack describes Java Memory model for method execution , Each method creates a stack frame as it executes , Used to store local variables 、 The stack of operands 、 Dynamic links 、 Method exit information , The process of each method from call to execution , Corresponding to a stack frame in the virtual machine stack to stack out of the process .

It is important to note that in Java In the virtual machine specification , Two exceptions are specified for stack space

• If the stack depth of the thread request is greater than the depth allowed by the virtual machine , Will throw out StackOverflowError abnormal .

• If the virtual machine stack can be expanded dynamically （ Most of the current Java Virtual machines can be dynamically expanded , It's just Java The virtual machine specification also allows for a fixed length of virtual machine stacks ）. If you can't request enough memory while expanding , Will throw OutOfMemoryError abnormal

How to use stack frames

A method from call to end means a stack frame from stack to stack , So how to do it ？ Refer to the following code

public class App {
    private String name;
    private Object object = new Object();

    public int add() {
        int a = 2;
        int b = 3;
        int c = (a + b) * 8;
        return c;
    }
    
    /**
     *  Program entrance 
     */
    public static void main(String[] args) throws InterruptedException {
        App app = new App();
        int result = app.add();
        System.out.println(result);
    }
}

We can see App.class Compiled file ,javap -c App.class Get the results , Such as add The bytecode of the method is as follows

  public int add();
    Code:
       0: iconst_2
       1: istore_1
       2: iconst_3
       3: istore_2
       4: iload_1
       5: iload_2
       6: iadd
       7: bipush        8
       9: imul
      10: istore_3
      11: iload_3
      12: ireturn

iconst_2： Indicates that the value will be 2 Push into the operand stack

istore_1： Indicates that the stack of the operand stack is pushed out of the stack , Put the local variable table index as 1 The location of （ That is, give variables a assignment , Of course, first allocate memory in the local variable table ）

iconst_3 and istore_2 It's like , The execution result is as follows

iload_1： Index the local variable table to 1 Value （ That's the variable a Value ） Put it on the top of the operand stack ,iload_2 similar , The execution result is as follows

iadd： Add the top element of the operand stack and the next element at the top of the stack , Put it on the top of the operand stack after operation

bipush 8： Push an octet signed integer onto the operand stack

imul： Multiplication , Similar to the above addition, the top element of the operand stack and the next element at the top of the stack are pushed out of the stack , Then put the result on the operand stack

istore_3： Take the top element of the operand stack out of the stack , Put the local variable table index as 3 The location of （ Is a variable c assignment ）

iload_3： Index the local variable table to 3 Value （ That's the variable c Value ） Put it on the top of the operand stack

ireturn： Returns the top element of the operand stack, that is 40

Come here add Method ends and returns , Method exit records the location that should be returned , That is to say add Method stack frame out of stack , Then enter main Method stack frame execution logic , So the overall logic is as follows .

Be careful

In the active thread , Only the stack frame at the top of the stack is valid , Call it the current stack frame , The method associated with this stack frame is called the current method .

All bytecode instructions of the execution engine operate only on the current stack frame .