What is the JVM memory model?

This article will give you a preliminary understanding of the JVM memory model. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

Computer memory model

When the program is running, the CPU obtains data by accessing the main memory, but with the rapid development of the CPU , the CPU access speed is getting higher and higher, and the hardware cannot meet the conditions of the CPU, most memories have added a cache mechanism. Different CPUs have corresponding multi-level (usually three) caches. When the CPU fetches data, it will first follow the first-level cache -The second-level cache-third-level cache-main memory is accessed in sequence, which improves the access speed, but at the same time, the data in the cache of different CPUs may be different, causing the CPU and main memory to be unable to achieve data synchronization

JAVA Memory Model

The JAVA memory model (Java Memory Model, JMM) complies with the computer memory model specification. By setting memory barriers, it shields the access differences of different hardware and operating systems. It is a mechanism and specification that ensures that Java programs can achieve consistent access to memory on different platforms.

Five major memory areas

1. Program counter

Each thread has a program counter, and the program counter is used to identify The line number of the program bytecode of the current thread. When the java method is running, the program counter saves the line number of the bytecode file. When the native method is running, the content of the program counter is empty. Different threads have their own exclusive program counters. , private to the thread

2. JAVA virtual machine stack

Each method will create a stack frame when running, including storing local variable tables and operating stacks. Dynamic links, method exports and other information. The process of each method being called corresponds to the process of a stack frame from being pushed into the stack to being popped out of the stack in the virtual machine stack.

The JAVA virtual machine stack is mainly used to store method parameters, local variables inside the method, data types loaded during compilation and return address types, which are private to the thread

3. Local The method stack

has a similar function to the JAVA virtual machine stack. The difference is that the latter serves the java methods (bytecodes) run by the virtual machine, while the local method stack serves the native calls called by the virtual machine. Methods, that is, underlying services, are generally written in C or C

4. Heap

The heap is the largest part of the memory area loaded by the virtual machine and is used to store object instances , can be stored at discontinuous memory addresses, as long as the logical addresses are continuous. It is divided into the new generation and the old generation. The ratio of the new generation to the old generation is 1:2 (can be specified through the parameter –XX:NewRatio)

5. Method area

Also known as the permanent generation, it is used to store class information, constants, and static variables loaded by the virtual machine. It is the shared area of ​​​​threads. It was abandoned after JDK8 and replaced by metaspace.

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