Brief introduction to memory structure of virtual machine

There are many versions of this virtual machine , The most common one is HotSpot, I don't know God's situation download is this version . The approximate memory model is shown in the figure below .

Thread sharing is the same area of memory used by each thread , All threads are freely accessible . Thread isolation is unique to threads , Threads cannot be accessed together .

• Method area
  The memory threads in this area share , Used to save class information , Constant , Is a logical part of the heap , So garbage collection will also be recycled to . Wait for variables and other data ,1.7 There were string constants before HotSpot1.7 Transferred to heap after , therefore String.intern() Query whether the string exists in the virtual machine , Virtual machines with different methods may behave differently . stay HotSpot It is often called the permanent generation , Because this part of memory is also added to the recycling mechanism, but the conditions are more stringent ,

• Pile up
  When I first saw it, I thought it had something to do with the data structure and the heap in the algorithm , However, heap memory management is not a heap data structure , Usually in the form of a linked list . This heap is just a heap of memory . It has nothing to do with the pile of hairs in the data structure . This memory stores the real columns of the object , Thread sharing . Generally, the object reference refers to this area .

• Local method stack and Java Virtual machine stack
  These two are almost the same , In some virtual machines, the two are directly integrated , This area is thread private , Each method creates a stack frame as it executes ( The stack frame will be introduced later ), This stack frame is used to store the local variable table 、 The stack of operands 、 Dynamic links 、 Method exit information , The process of each method from call to execution , It corresponds to a stack frame in the virtual machine stack stack stack to the stack process . The local method is Java Method service ,Java The virtual machine method is native Method service （ It may be the method of other languages ）

• Program counter
  Thread private , Save the location where the program runs （ Instruction address ）, Where it refers, it is executed , Change the value of the counter before executing the indicated code .