Case explanation JVM memory space (recommended collection)

Today, let’s talk with you about the topic of JVM memory space. This is also a question that is often asked in interviews at first-tier Internet companies. It is recommended that friends collect it and take it out often. Read, focus on understanding. Okay, no more talk, let’s get to the main topic today.

The JVM will divide the memory into different data areas, so where are the loaded classes allocated?

The following figure shows various areas of memory, including: method area, heap, virtual machine stack, local method stack, and program counter.

Method area

The method area is used to store the data that has been used by the virtual machine Loaded class information, constants, static variables, code compiled by the just-in-time compiler and other data. The five stages of class loading are mentioned in Class Loading. In the loading phase, the static storage structure represented by the byte stream will be converted into the runtime data structure of the method area. In the preparation phase, all the memory used by the variables will be allocated in the method area.

Program Counter

Come on a simple code, calculate (1 2)*3 and return

public int cal() {
    int a = 1;
    int b = 2;
    int c = 3;
    return (a + b) * c;
}

When this code is loaded into the virtual machine, it becomes the following bytecode. When the virtual machine executes it, it will be executed line by line.

#Java is multi-threaded. After the thread switches back, it needs to know where the original execution position is. The program counter is used to record this execution position. In order to ensure that the counters between threads do not affect each other, this memory area is private to the thread.

虚拟机栈

虚拟机栈也是线程私有的，生命周期与线程相同。每个线程都有自己的虚拟机栈，如果这个线程执行了一个方法，就会创建一个栈帧，方法从调用直至执行完成的过程，就对应着一个栈帧在虚拟机栈中入栈到出栈的过程。比如下面的例子，fun1调用fun2，fun2调用fun3，fun3创建Hello对象。

public void fun1() {
    fun2();
}

public void fun2() {
    fun3();
}

public void fun3() {
    Hello hello = new Hello();
}

调用的时候，流程图如下：

执行完成的时候，流程图如下：

每一个栈帧都包括了局部变量表、操作数栈、动态连接、方法返回地址和一些额外的附加信息。局部变量主要是存放方法参数以及方法内部定义的局部变量，操作数栈是一个后入先出栈，当方法刚刚开始执行的时候，这个方法的操作数栈是空的，在方法的执行过程中，会有各种字节码指令往操作数栈中写入和提取内容，也就是出栈/入栈操作。

我们通过上面(1+2)*3的例子，把方法区、程序计数器、虚拟机栈的协同工作理一下。首先通过javap查看它的字节码，经过类加载器加载后，此时这个字节码存在方法区中。stack表示栈深度是2，locals是本地变量的slot个数，args_size是入参的个数，默认是this。栈的深度、本地变量个数，入参个数，都是在编译器决定的。

如下图，指令的位置是方法区，局部变量和操作数栈的位置是虚拟机栈，程序计数器就在程序计数器（这个下面的图就不再重复）。当执行偏地址为0的指令的时候，程序计数器为0，局部变量第一个值是this，当前的指令就是方法区0:iconst_1,指令iconst_1就是把int常量值1进栈，这个1就到了虚拟机栈的操作数栈中。

当执行偏地址为1的指令的时候，程序计数器为1，把操作数栈的值赋值到局部变量，此时操作数栈清空了，局部变量多了一个1，这条指令执行完，就是对应上面int a=1的语句。

In addition, the assignments of the two statements b and c correspond to instructions 2, 3, 4, and 5, which will not be repeated here. After executing 5, as shown in the following figure:

When executing 6, iload_1 is executed, which is to push the second int-type local variable onto the top of the stack. Here The variable is 1.

When executing 7, iload_2 is executed, which means pushing the third int-type local variable onto the top of the stack. The variable here is 2.

When executing 8, it is the iadd statement, which means that the two int-type elements on the top of the stack are popped out of the stack, and then pushed onto the top of the stack after the result is obtained.

When executing 9, assign the element 3 on the top of the stack to the fifth local variable.

When execution reaches 11, the fifth local variable value is pushed onto the top of the stack. When execution reaches 13, the fourth local variable value is pushed onto the top of the stack. , when executing 14, the two int-type elements on the top of the stack are popped off the stack, and the multiplied result is pushed onto the stack. When executing 15, the current int-type element on the top of the stack is returned from the current method. These are almost the same as the above, so I won’t go into details.

Heap

The only purpose of the heap memory area is to store object instances, almost all object instances are here Allocate memory. For example, fun1 above calls fun2, fun2 calls fun3, and fun3 creates a Hello object. When the object is created in the fun3 method, it is created in the heap, and the address is assigned to the local variable of fun3. The Java heap can also be subdivided into: new generation and old generation; the new generation is also subdivided into Eden space, From Survivor space, and To Survivor space.

Summary

The overall process is as follows, first compile the java file into a class file, Loaded into the method area through the class loader. When a thread calls a method, it will create a stack frame, read the bytecode in the method area and execute the instruction. When the instruction is executed, the execution position will be recorded in the program counter. If an object is created, it will be created in the heap memory. After the method is executed, the stack frame will be popped.

Related parameters

-XX:PermSize: Permanent generation memory capacity .

-XX:MaxPermSize: The maximum memory capacity of the permanent generation.

-XX:MetaspaceSize: The size of the initial value of the metaspace

-XX:MaxMetaspaceSize: The maximum size of the metaspace

-XX:CompressedClassSpaceSize: Store Klass classes in the metaspace The space size of the metadata part

-Xss: stack memory capacity.

-Xms: Heap memory capacity.

-Xmx: The maximum memory capacity of the heap, usually the same as the -Xms setting, to prevent the impact of runtime expansion.

-Xmn: New generation memory capacity, the old generation is the heap memory capacity - New generation memory capacity

-XX: SurvivorRatio=8: The new generation is also subdivided into Eden space and From Survivor space , To Survivor space, set to 8 to represent Eden space: From Survivor space: To Survivor space = 8:1:1. For example, the new generation has 10M, then the Eden space occupies 8M, and the From Survivor space and To Survivor space each occupy 1M.

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