JVM memory model: virtual machine stack

java Instruction design based on stack

Due to the cross platform design ,Java The instructions of are designed according to the stack . Different platforms CPU Different architectures , So it can't be designed as register based .

advantage ： Cross platform , Instruction set is small , The compiler is easy to implement

shortcoming ： Performance degradation , More instructions are needed to implement the same function .

java Virtual machine stack ：

Each thread creates a virtual machine stack when it is created , Its internal storage of a stack frame , One after another java Method call .

The life cycle of virtual machine stack is consistent with that of threads .

effect ： executive director java Program operation , Local variables that hold methods 、 Part of the result , And participate in the method call and return .

The local variables stored in the virtual machine stack can be basic data types , It can also be a reference data type （ Save the address ）

The characteristics of the stack ：

Stack is a fast and efficient way to allocate storage , Access speed is second only to the program counter .

jvm There are only two operations directly on the virtual machine stack ： Each method executes , Along with the stack ; After the method is executed, it is out of the stack .

For the virtual machine stack , No, GC

Possible exceptions in the virtual machine stack ：

java The virtual machine specification allows java The stack size is dynamic or fixed .

（1） If you use a fixed size Java Virtual machine stack , And every thread Java The virtual machine stack capacity can be selected independently when the thread is created .

If the stack capacity allocated by the thread request exceeds Java The maximum capacity allowed by the virtual machine ,Java The virtual machine will throw a StatckOverflowError abnormal .

（2） If the virtual machine stack can be dynamically expanded , And trying to expand Cannot apply for enough memory when , Or when creating a new thread, there is not enough memory to create the corresponding virtual machine stack , that java The virtual machine will throw a OutOfMemoryError abnormal .

Set stack memory size ：

have access to -Xss Option to set the maximum stack space for the thread , The size of stack directly determines the maximum depth of function call .

The storage unit of a stack ： Stack frame

* Each thread has its own stack , All the data in the stack is in the form of Stack frame （Stack Frame） The format of exists

Each method being executed on this thread corresponds to a Stack frame (StackFrame)

* Stack frame is a memory block , Is a dataset , It maintains all kinds of data information in the process of method execution .

* In an active thread , At one point in time , There will only be one active stack frame , That is, only the stack frame of the currently executing method （ Stack top stack frame ） Is effective , This stack frame is called Current stack frame , The method corresponding to the current stack frame is The current method , The class that defines this method is The current class . All bytecode instructions run by the execution engine only operate on the current stack frame .

* If you call other methods in this method, , The corresponding new stack frame will be created , At the top of the stack , Become the new current frame .

* Stack frames contained in different threads are not allowed to refer to each other , That is, it is impossible to reference the stack frame of another thread in one stack frame .

* If the current method calls another method , Method return , The current stack frame will return the execution result of this method to the previous stack frame , next , The virtual opportunity discards the current stack frame , Make the previous stack frame become the current stack frame again .

*java Method has two ways to return a function , One is a normal function return , Use return Instructions ;

The other is to throw an exception . Either way , Will cause the stack frame to pop up .

Stack frame structure ：