Java heap and stack (Memory Allocation Policy)

Memory Allocation Policy
According to the compilation principle, there are three policies for memory allocation during program running: static, stack, and heap.
Static Storage Allocation refers to the ability to determine the storage space requirements of each data target at runtime during compilation. Therefore, a fixed memory space can be allocated to each data target during compilation. this allocation policy requires that the program code does not allow the existence of a variable data structure (such as a variable array) or nested or recursive structure, because they both cause compilation programs to fail to calculate accurate storage space requirements.

Stack-based storage allocation, also known as dynamic storage allocation, is implemented by a stack-like running stack. in contrast to static storage allocation, in stack-based storage solutions, the program's requirements for data areas are completely unknown during compilation and can only be known at runtime, however, when entering a program module during running, you must know the size of the Data zone required by the program module to allocate memory for it. like the stack we are familiar with in the data structure, stack-based storage allocation is distributed based on the principle of first-in-first-out.

Static storage allocation requires that the storage requirements of all variables be known during compilation, and stack-based storage allocation requires that all storage requirements be known at the entrance of the process, heap Storage allocation is specifically responsible for the memory allocation of data structures that cannot be determined at the module entrance during compilation or runtime, such as variable length strings and object instances. the heap consists of a large part of available blocks or idle blocks. The memory in the heap can be allocated and released in any order.

Comparison Between Stack and stack
The above definition is summarized in the textbook on compilation principles. In addition to static storage allocation, all of them seem dull and hard to understand. The following describes static storage allocation and how to compare stacks and stacks in a centralized manner:
Compared with the functions and functions of stacks, stacks are mainly used to store objects and stacks are mainly used to execute programs. this difference is mainly determined by the features of the stack and stack:
In programming, for example, in C/C ++, all method calls are performed through stacks, and all local variables and formal parameters are allocated memory space from stacks. In fact, there is no allocation, just use it from the top of the stack, just like a conveyor belt in the factory (Conveyor Belt), stack pointer will automatically guide you to the place where you put things, all you have to do is put things down. when you exit the function, you can modify the stack pointer to destroy the stack content. this mode is the fastest, of course, used to run the program. it should be noted that, during the allocation, for example, when assigning a data zone to a program module to be called, you should know the size of the Data zone in advance, that is, although the allocation is
Run in sequence, but the size of the allocation is determined and remains unchanged. The "size" is determined during compilation, not during runtime.
Heap requests the operating system to allocate memory when the application is running. Because the memory allocated is managed by the operating system, it takes time to allocate and destroy the heap, therefore, the efficiency of using the heap is very low. however, the advantage of heap is that the compiler does not have to know how much storage space is allocated from the heap or how long the stored data will stay in the heap. Therefore, storing data in a heap gives you more flexibility. In fact, the heap memory allocation is essential for the polymorphism of the face object, because the storage space required for the polymorphism variable can be determined only after the object is created at run time. in C ++, when creating an object, you only need to use the new command to compile the relevant code. When executing the code, the data will be automatically saved in the heap. Of course, to achieve this flexibility, it will inevitably pay a certain price: It will be spent in allocating storage space in the heap.
Take longer! This is exactly the reason why we have just said that the efficiency is low. It seems that comrade Lenin is good at saying that the advantages of people are also the disadvantages of people, the disadvantage of a person is often the advantage of a person (dizzy ~).

Heap and stack in JVM
JVM is a stack-based Virtual Machine. JVM allocates a stack for each newly created thread. That is to say, for a Java program, its operation is done through stack operations. The stack stores the thread status in frames. JVM only performs two types of operations on the stack: frame-based stack pressure and outbound stack operations.

We know that the method being executed by a thread is called the current method of this thread. We may not know that the frame used by the current method is called the current frame. When a thread activates a Java method, JVM will press a new frame into the thread's java stack. This frame naturally becomes the current frame. during the execution of this method, this frame will be used to save parameters, local variables, intermediate calculation processes, and other data. this frame is similar to the activity record concept in the compilation principle.

From the perspective of Java's allocation mechanism, the stack can be understood as follows: a stack is a process or thread created by the operating system (a thread in an operating system that supports multithreading) the storage area created for this thread has the advanced and later features.

Each Java application corresponds to only one JVM instance, and each instance corresponds to only one heap. All the class instances or arrays created by the application during running are stored in this heap and shared by all the threads of the application. unlike C/C ++, heap memory allocation in Java is automatically initialized. In Java, the storage space of all objects is allocated in the heap, but the reference of this object is allocated in the stack, that is, the memory is allocated from both places when an object is created, the memory allocated in the heap actually creates this object, and the memory allocated in the stack is just a pointer (reference) pointing to this heap object.

Specifically:
Both stacks and stacks are places where Java is used to store data in Ram. Unlike C ++, Java automatically manages stacks and stacks, and programmers cannot directly set stacks or stacks.
The Java heap is a runtime data zone, from which the class objects allocate space. These objects are created through new, newarray, anewarray, and multianewarray. They do not need program code to be explicitly released. The heap is responsible for garbage collection. The advantage of the heap is that the memory size can be dynamically allocated, and the lifetime does not have to be told in advance, because it allocates memory dynamically at runtime, the Java Garbage Collector automatically collects the unused data. However, the slow access speed is due to the need to dynamically allocate memory during runtime. Objects and arrays in Java are stored in the heap.

The advantage of stack is that the access speed is faster than that of stack, second only to register, and stack data can be shared. However, the disadvantage is that the data size and lifetime in the stack must be fixed, and there is a lack of flexibility. The stack mainly stores some basic types of variables (, Int, short, long, byte, float, double, Boolean, char) and object references.

A very important feature of stacks is that data in stacks can be shared. Suppose we define both:
Int A = 3;
Int B = 3;
The compiler first processes int A = 3. First, it creates a reference with the variable A in the stack, and then finds whether the value 3 in the stack exists. If no value is found, store 3 and point A to 3. Then process int B = 3. After the referenced variable of B is created, B is directed to 3 because there is already 3 in the stack. In this way, both A and B point to 3 at the same time. At this time, if A is set to 4 again, the compiler will re-search whether there are 4 values in the stack. If not, it will store 4 and make a point to 4; if yes, direct a to this address. Therefore, changing the value of A does not affect the value of B. Note that the sharing of the data is different from the sharing of the two objects pointing to one object at the same time, because the modification of a does not affect B,
It is completed by the compiler, which helps save space. A variable referenced by an object modifies the internal state of the object, which affects the variable referenced by another object.

PS: Memory Allocation for C ++

The memory occupied by a C/C ++ compiled program is divided into the following parts:
1. STACK: the stack zone is automatically allocated and released by the compiler, and stores function parameter values and local variable values. The operation method is similar to the stack in the data structure.
2. Heap-generally assigned and released by the programmer. If the programmer does not release the heap, it may be recycled by the OS at the end of the program. Note that it is different from the heap in the data structure. The allocation method is similar to the linked list.
3. Global (static)-the storage of global variables and static variables is put together, and the initialized global variables and static variables are in one area, uninitialized global variables and uninitialized static variables are in another adjacent area. -The system is released after the program ends.

4. Text Constant Area-constant strings are placed here. The program is released by the System
5. program code area-stores the binary code of the function body.
Ii. Example Program
This is written by a senior. It is very detailed.
// Main. cpp
Int A = 0; global initialization Zone
Char * P1; uninitialized globally
Main ()
{
Int B; stack
Char s [] = "ABC"; stack
Char * P2; stack
Char * P3 = "123456"; 123456 \ 0 is in the constant zone, and P3 is in the stack.
Static int C = 0; Global (static) initialization Zone
P1 = (char *) malloc (10 );
P2 = (char *) malloc (20 );
The allocated 10-byte and 20-byte areas are in the heap area.
Strcpy (P1, "123456"); 123456 \ 0 is placed in the constant area, and the compiler may optimize it into a place with the "123456" that P3 points.
}