Deep understanding of JVM (I) - memory structure (I)

In depth understanding of JVM（ One ）—— Memory structure （ One ）

The overall architecture

1、 Program counter

effect

Used to hold JVM The address of the next instruction to be executed in

characteristic

• Thread private
  • CPU Time slices are allocated to each thread , When the time slice of the current thread is used up ,CPU Will execute the code in another thread
  • The program counter is Every thread the private Of , When the time slice of another thread runs out , When you return to executing the code of the current thread , Through the program counter, you can know which instruction should be executed
• There will be no memory overflow

2、 Virtual machine stack

Definition

• Every Threads Memory space required for operation , be called Virtual machine stack
• Each stack consists of multiple Stack frame form , Corresponds to the memory occupied each time the method is called
• Each thread can only have An active stack frame , Corresponding Currently executing method

demonstration

Code

public class Main {
	public static void main(String[] args) {
		method1();
	}

	private static void method1() {
		method2(1, 2);
	}

	private static int method2(int a, int b) {
		int c = a + b;
		return c;
	}
}

You can see in the console , When the method in the main class enters the virtual machine stack , Conform to the characteristics of stack

Problem Formulation

• Does garbage collection involve stack memory ？
  • Unwanted . Because the virtual machine stack is composed of stack frames , After the method is executed , The corresponding stack frame will be popped out of the stack . So there is no need to recycle memory through garbage collection mechanism .
• The larger the stack memory allocation, the better ？
  • No . because Physical memory is certain , The larger the stack memory , Can support more recursive calls , But the fewer threads you can execute .
• Whether local variables in methods are thread safe ？
  • If within the method Local variables do not escape the scope of the method , It is Thread safety Of
  • If Local variables refer to objects , and Escaped the scope of the method , You need to consider thread safety

out of memory

Java.lang.stackOverflowError Stack memory overflow

Cause of occurrence

• In the virtual machine stack , Too many stack frames （ Infinite recursion ）
• Every stack frame Occupied too much

Thread running diagnostics

CPU Occupy too much

• Linux When running some programs in the environment , May lead to CPU The occupation is too high , At this time, it is necessary to locate the occupied area CPU Too high thread
  • top command , Check which one is process Occupy CPU Too high
  • ps H -eo pid, tid（ Threads id）, %cpu | grep Just passed top The process number found adopt ps Command to further see which thread is occupying CPU Too high
  • jstack process id By looking at the number of threads in the process nid, Just passed ps Command to see tid Come on Contrast positioning , Be careful jstack Found thread id yes 16 It's binary , Need to transform

3、 Native Method Stack

Some with native keyword The best way is to need JAVA To call the local C perhaps C++ Method , because JAVA Sometimes you can't directly interact with the underlying operating system , So you need to use local methods