Introduction to JVM basic concepts

JVM What is it?

  JVM also called Java virtual machine , yes Java The runtime environment of the program . It is to use C Write a software , Like other software, it runs on the operating system （LInux,win,mac etc. ） Of . Where does the operating system run ？

   Before the birth of computer software , Our computer can only execute certain programs , To run a program by splicing circuits . If you need to run the second program, you need to splice a new set of circuits and run , This is the early computer hardware . Later, von Neumann led to the wave of computer software , It wants to **‘ preservation ’** get up , This is the beginning of computer software . So the operating system is running on the hardware .JVM, operating system , The relationship among computer hardware is ：

JVM Architecture of

  JVM The most common thing in is tuning , In fact, JVM Of the five areas of , The basic thing that involves tuning is heap memory tuning , This is determined by the mechanism of heap memory . For local method interfaces , Namely native Related methods of modification , These methods are C language-written . The local method information is stored in the local method interface , When called, the local method in the local method library will be called .

Class loader

species

• Start class loader ：Bootstrap ClassLoader

• Extend the classloader ：Extension ClassLoader

• Application class loader ：Application ClassLoader

  Do not discuss custom class loaders

A simple loading process of a class

Test code , You can understand at a glance

![1](1.gifpublic class ClassLoaderTest {
    
    public static void main(String[] args) {
    
        String str = new String("abc");
        System.out.println(str.getClass().getClassLoader());
// System.out.println(classLoader.getParent());
        System.out.println("====== I'm the divider ======");
        Person person = new Person();
        System.out.println(person.getClass().getClassLoader());
        System.out.println(person.getClass().getClassLoader().getParent());
        System.out.println(person.getClass().getClassLoader().getParent().getParent());
    }
}

   Ladies and gentlemen, just look at two moving pictures , We know String Class is JDK A self-contained class in , Is loaded by the boot class loader , Start class loader at Java You can't get it in . Therefore, the second dynamic graph generates a null pointer exception . Compare... In the first graph Peron Class is custom , So the loading is finally completed by the application loader . Why print three class loaders , Leave a suspense .

The role of the three class loaders

1. Start class loader ： load <JAVA_HOME>\lib In the directory , Or be -Xbootclasspath The class of the path specified by the , And is JVM Identified class libraries , Users can't get .

2. Extend the classloader ： load <JAVA_HOME>\lib\ext In the directory , Or be java.ext.dirs Class libraries in the path specified by the system variable . Users can get .

3. Application class loader ： This loader is also known as the system loader （System）, yes ClassLoader in getSystemClassLoader() Return value of method . Load user path （ClassPath） The specified class library . Users can use . Users can get

Parent delegate mechanism

1. There is a need for class loading

2. The subclass loader passes the request for class loading to the parent class , Layer by layer to start the class loader

3. If the parent class loader fails to load, it will feed back to the child class to load itself

   The so-called parents , Is to extend the class loader and start the class loader . The advantage of parental delegation is that it doesn't make a class reload twice , For example, you can customize java.lang.String It's illegal .

Sandbox security mechanism

   A sandbox can be understood as a closed box , Our program can only run in this box . We develop locally , The development is completed and deployed to the remote server , stay Java Local code in is trusted by default , It's safe . But remote code is not trusted , So we need to add some security mechanisms to ensure the operation of the program , For example, when the remote code operates the operating system and local resources, the credit granting steps should be added ,jdk6 Then the domain is proposed （domain） The concept of , Make sandbox safer .

Native

  native The delegate is a local method , take Thread Class, for example , First Thread Not an abstract class, not an interface , Why is there a way to realize without writing methods ？ The reason is that this method is native Modify the . stay Java in , All are native The modification is the local method .Java Only its mark is stored , Use it to call the local method library .

PC register

  PC The register area is also called the program counter , We know that in the case of multithreading ,CPU It's a high-speed wheel , Each thread is allocated a time slice , There is ** Half the program is executed to execute other threads , Next time, go back to this thread ,CPU How to know where the program has been executed ？** The answer is the program counter , This can be understood as a pointer , It will point to the memory address of the next instruction .PC Registers are thread private , Every thread has a PC register , This ensures that threads are independent of each other .

Stack

   Stack also known as Java Virtual machine stack , It is characterized by first in and last out , This is related to its data structure . Inside the stack are stack frames , Each stack frame actually corresponds to a method to be executed . After each method is executed, it will pop up and disappear , The sign that a program has finished executing is that there is no stack frame in the stack . Usually, what is stored in the stack is some references , So the program is finished , There is basically no garbage left in the stack , So it basically won't appear OOM. The possible exception is that the stack frame loaded into the stack exceeds the maximum stack frame that the stack can hold , A stack overflow exception will be reported , Another case is that the memory requested by the current stack frame exceeds the maximum memory , Will appear OOM The problem of .

Pile up

  Java in new All objects are stored in the heap , The so-called garbage collection is the collection object . So most of the tuning we talk about is actually heap tuning . The following is the structure diagram of heap memory .