The first word of JVM -- detailed introduction to JVM and analysis of runtime data area

1.JVM What is it?

JVM(Java Virtual Machine) yes Jvm A specification for virtual machines .

C:\Users\pc>java -version
java version "1.8.0_151"
Java(TM) SE Runtime Environment (build 1.8.0_151-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.151-b12, mixed mode)

When installed JDK in the future , You can see that the default is installed HotSpot The virtual machine ,Java The program needs to run on a virtual machine , Different platforms have their own virtual machines , therefore Java Language is a cross platform language .

This brings out Java Characteristics of language ： A compilation , Multiple execution , Cross platform , Security (1. Discard pointer 2、GC Garbage collection mechanism ：( Automatically release and recycle the memory of objects that are not used for a long time ))

structure ：JDK & JRE & JVM Structure diagram of the three

• jvm Distribution chart
  Input class File to virtual machine , Virtual machine output of different platforms CPU Instructions . Memory 、 controller 、 Arithmetic unit by JVM Run time data area
  

2. One java The compilation and running process of the file

Tools ：javac compile 、javap Decompile

Java Source file -> Javac Translate it into class file -> JVM Virtual machine running Class The file is actually the machine code converted to the corresponding operating system -> Major operating systems

2.1 javac

Javac Convert the source file to bytecode （ Lexical analysis , Syntax analysis , Semantic analysis , Bytecode generation code ）
class file A storage binary file , Deposit 16 Hexadecimal byte

2.2 javap

take class Decompile file to cpu Instructions

// Original code 
static int num = 1;
static final int a1 = 2;
int a2 = 2; 
   
// adopt  javap -p -v xxx.class  Decompiled results 
// Will open a in the heap memory num=0 Space 
static int num;	
 	descriptor: I
 // jurisdiction 
  flags: ACC_STATIC
static final int a1;
 	descriptor: I
  flags: ACC_STATIC, ACC_FINAL
  // Inform the virtual machine to assign values to static variables   It means that the value has been initialized in the preparation stage  
  ConstantValue: int 2
  // Not static The instance variable of the variable will be assigned to java Pile up   In the initialization phase, it is initiated by the class constructor 
int a2;
 	descriptor: I

3. Class loading mechanism

Purpose ： take class Class is loaded into the memory space of the virtual machine

3.1 load Loading

find class The full path of the file , Then load it into memory . The specific implementation is implemented by the class loader ClassLoader

Overview of class loader types

Class loader	function
Bootstrap ClassLoader Start class loader	load $JAVA_HOME in jre/lib/rt.jar Everything in it class perhaps Xbootclasspath Option specified jar package
ExClassLoader Extend the classloader	load java Some of the extensions in the platform jar package , Include $JAVA_HOME in jre/lib/*.jar perhaps -Djava.ext-dirs Specify the package under the directory
AppClassLoader system class loader	load classpath Specified in the jar Package and Djava.class.path Specify the classes under the directory and jar package
Custom ClassLoader Custom class loaders	adopt ClassLoader Custom loading of subclasses class, It belongs to the application customized according to its own needs ClassLoader, Such as Tomcat,Jboss Will be implemented according to the specification ClassLoader

Class loading mode

• Overall responsible for
  • As long as a loader is responsible for a certain class , Then the class loader is responsible for the parent and child classes of this class
• Parent delegation
  • Determine whether the top-level class has been loaded
• Caching mechanisms
  • A class with the same name will only be loaded once All must be used to cache

Custom class loaders java Realization

Realization ClassLoader, Loading classes cannot be placed under the classpath ( Otherwise it will be AppClassLoader load )

3.2 link Linking

verification Verify： Verify the correctness of the class
Get ready Prepare： Assign default values to static fields of a class or interface

private static int a =10; At this time, the default value will be assigned first 0

analysis Resolve： Dynamically convert symbolic references in the running constant pool to direct references ( Physical memory address )

3.3 initialization Initalization

Attach the real value to the variable , For example, for the above static int a The assignment is 10

What triggers initialization

• new
• Accessing static variables of a class or interface
• Calling static methods of a class
• Reflection
• Initializing a subclass of a class
• Virtual machine specifies the startup class

4.JVM Run time data area

According to the preceding jvm Distribution chart , Divided into 5 Regions , Stack 、 Pile up 、 Program counter 、 Method area 、 Native Method Stack

41. Method area Method Area

• java Thread shared area , Life cycle and process binding
• Store metadata information of the class , Template information
• When memory is insufficient OutOfMeoryError: Metespace
• The method area stores content that will not be easily changed , stay jdk8 Is also called meta space Metaspace
• Runtime constant pool Run-Time Constant pool
  • String str = “hello” hello In the constant pool , Can be quoted more
  • String str = new String(“hello”) 1.7 before hello In heap memory ,1.8 after String Objects are stored in heap memory ,hello Stored in the string constant pool

4.2 Pile up Heap

• The largest thread sharing area of the virtual machine , Life cycle and process binding
• When memory is insufficient OutOfMeoryError: Java heap space, adopt -Xms20M -Xmx20M Set heap memory size
• Instance object of class or member variable , for example new String(“hello”) String Object in heap memory
• JDK1.8 Have already put String The constant pool is moved from the method area to the heap , that new String(“hello”) Two objects will be generated ,String Objects are stored in heap memory ,hello Stored in the string constant pool , If the constant pool exists, it will not be created

4.3 Stack Stacks

• Thread private , By preemption CPU Time slice to execute stack frame
• Stack frame is the smallest working unit , The way is to press the stack 、 Out of the stack , Stack structure first, then out 、 Last in, first out
• Recursion leads to insufficient stack depth and will report StackOverfolwError, The default size of stack frame depth 1M, Can pass -Xss 128k modify
• Stack frame press stack stack thread mian() -> a() ->b() -> c(),mian() It must be the last execution , It must be first in and last out
  

4.4 Composition of stack frame

public static int calc(int num1,num2){
    
    num1 = 2;
    int res = num1 + num2;
    return res;
}
calc(1,8);
// adopt javap Decompile content   Look at the calc Stack frame 
public static int calc(int, int);
    descriptor: (II)I
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
	 // The stack of operands 2 individual   local variable 3 individual   Parameter values 2 individual 
      stack=2, locals=3, args_size=2
         0: iconst_2	//push One int Value is pushed into the operation stack  2
         1: istore_0	// Assign the value of the operand stack out of the stack to the... Of the local variable 0 individual  num1=2
         2: iload_0		// The first 0 The values of local variables are pushed into the operand stack  2
         3: iload_1		// The first 1 The values of local variables are pushed into the operand stack  8
         4: iadd		// Perform an addition operation  10
         5: istore_2	// Assign the value of the operand stack out of the stack to the... Of the local variable 2 individual  res = 10
         6: iload_2		// The value of the second local variable is pressed against the operand stack  res
         7: ireturn		// The second local variable comes out of the operand stack   return 10  Then return the address to main Stack frame 

• Local variable table , Storing local variables
• The stack of operands , A temporary stack used to store operations
• Dynamic links , Convert a symbolic reference to a direct reference
• Method return , Continue with the following method

java One line of code in , It may be multiple assembly instructions

4.5 Program calculator PC Register

• Just like the registers in the operating system , When the thread is preempted by other threads during execution, the time slice is executed first , You need to record the method and location being executed
• Thread private

4.6 Native Method Stack Native Method Stacks

• When a thread executes a method, it needs to call local Native Method , There is no such method JVM There is no maintenance , So every thread has a local method stack , The place called is linked

5. What does it do to create an object in memory ？

1. First set the specified class File loaded into memory
2. perform main When the method is used , In the stack memory, it is opened up to belong to main Stack space of thread , Assign to a variable p
3. new Keyword to apply for an instance object space from heap memory , Allocate a memory address value
4. Space allocation of attributes in the instance object space , And default initialization
5. Dynamically convert symbolic references to direct references for attributes in space
6. Initialize the construction code block of the instance , Call the constructor of the instance class , Initialize the constructor
7. Assign the first address to p,p Variable refers to this instance , Pointed to the object

That's all for this chapter .

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