Demystifying the JVM memory model: in-depth analysis

In-depth analysis of the JVM memory model: To explore its secrets, specific code examples are needed

1. Introduction

Java Virtual Machine (JVM) is the Java language The core is responsible for core functions such as program running and memory management. The JVM memory model means that during the running process of the JVM, the memory is divided into different areas for storing different types of data. Understanding the working principle of the JVM memory model can help developers better optimize program performance and avoid problems such as memory leaks. This article will provide an in-depth analysis of the JVM memory model from beginning to end, and help readers gain a deeper understanding of its working principles through specific code examples.

2. Components of the JVM memory model

The JVM memory model consists of the following parts:

1. Program Counter Register: used to indicate The bytecode line number executed by the current thread. Each thread has an independent program counter. When a thread executes a Java method, the program counter records the address of the instruction being executed; when the thread executes a Native method, the value of the program counter is Undefined.
2. Java Virtual Machine Stack: When each thread is created, a stack frame (Stack Frame) will be allocated in the virtual machine stack. Stack frames are used to store local variables, operand stacks, dynamic links, method exits and other information. When each method is called, a stack frame is created and pushed onto the virtual machine stack. After the method is executed, it is popped out of the stack. When the virtual machine stack cannot be dynamically expanded, a StackOverflowError will be thrown; when the stack depth requested by the thread is greater than the depth allowed by the virtual machine, an OutOfMemoryError will be thrown.
3. Native Method Stack: used to support the execution of Native methods.
4. Java Heap: The memory area where object instances are stored. The Java heap is the largest memory area in the JVM memory model and is shared by all threads. OutOfMemoryError is thrown when the Java heap cannot allocate memory.
5. Method Area: used to store constants, static variables, class information, runtime constant pool and other data. The method area is also shared by all threads. When the method area cannot meet the memory requirements of the class loader, an OutOfMemoryError will be thrown.
6. Runtime Constant Pool: Each Class file will have a constant pool used to store various literals and symbol references generated by the compiler. The runtime constant pool is part of the method area.
7. Direct Memory: When the JVM uses the NIO library, it will also involve the use of direct memory.

3. Specific implementation of the JVM memory model

The following is a specific code example that demonstrates the implementation process of the JVM memory model:

public class MemoryDemo {
    public static void main(String[] args) {
        int a = 1;
        int b = 2;
        int c = add(a, b);
        int d = multiply(a, b);

        System.out.println("c = " + c);
        System.out.println("d = " + d);
    }

    public static int add(int x, int y) {
        return x + y;
    }

    public static int multiply(int x, int y) {
        return x * y;
    }
}

In the above code , we define a MemoryDemo class and define four variables in the main method: a, b, c, d. We called two methods, add and multiply, to implement addition and multiplication operations respectively, and assigned the results to c and d respectively. Finally, output via System.out.println.

First of all, when the program starts, the JVM will automatically allocate a piece of heap memory to the instance object of the MemoryDemo class, which contains the main method and The bytecode information of the two methods add and multiply.

When the main method is called, the JVM will create a stack frame and push the stack frame into the virtual machine stack. The stack frame contains the local variable table, operand stack, dynamic link, method exit and other information of the main method.

In the main method, we assign values ​​to a and b respectively, and call add and multiplymethod. At this time, the JVM will create two stack frames respectively and push the stack frame into the virtual machine stack. In the stack frame of the

add method, memory will be allocated for x and y, and a and will be allocated The value of b is passed to these two variables. After the add method is executed, the return value will be saved in the stack frame and passed to c. In the stack frame of the

multiply method, memory will also be allocated for x and y, and a and ## The value of #b is passed to these two variables. multiplyAfter the method is executed, the return value will be saved in the stack frame and passed to d.

Finally, output the values ​​of

c and d through System.out.println.

4. Summary

Through the above code examples, we can see the specific implementation process of the JVM memory model. When the program is running, the JVM will create an independent virtual machine stack for each thread and create a stack frame for each method call.

A precise understanding of the JVM memory model is very important for developers. We need to understand the functions and limitations of each memory area, make reasonable use of memory resources when writing code, and avoid problems such as memory leaks. Only by deeply understanding the working principle of the JVM memory model can we better leverage the advantages of the Java language and write efficient and stable programs.

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