JVM_ Common [interview questions]

One 、JVM The concept of

 Learn and use the topic by yourself ,` It's still improving , At present, the first draft is not !! I haven't recited yet ^^`, Hey !!!

1、JVM The specific role of

jvm What is it? ？

jvm yes java virtual machine Java Virtual Machine Abbreviation .

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What is? java virtual machine ？

Virtual machine is an abstract computer , It is realized by simulating various computer functions on the actual computer .Java Virtual machine has its own perfect hardware framework , Such as processor 、 Stack 、 Register, etc , Also has the corresponding instruction system .Java Virtual machine shields information related to specific operating system platform , bring Java The program only needs to be generated in Java Target code running on virtual machine （ Bytecode ）, It can run unmodified on multiple platforms . Simply speaking JVM It's for parsing and running Java programmatic .

java The benefits of virtual machines ？

java The main reason for cross platform operation is that java virtual machine , meanwhile jvm You can shield system differences and invent some new languages to use jvm To complete . That's how it's done 《 A compilation runs everywhere 》 Characteristics .

2、JVM Partition of memory area in

Want to know JVM Partition of memory area in , First of all, you need to know JVM Where does the memory area in come from ,JVM The memory in is JVM Request a piece of memory from the operating system , Divide an area for this memory .

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Pile up （ Runtime constant pool ）	new The object will be placed in the heap
Method area	The loaded classes are placed in the method area , Static members
Stack （JVM Stack and local method stack ）	local variable
Program counter	The address stored describes where the next instruction to be executed by the current thread is

** Be careful ：** There are multiple threads in a process , Each thread has its own stack and program counters , A process shares a heap and a method area .

Determine whether a variable is on the heap or on the stack , It has nothing to do with whether this variable is a basic type or a reference type , And this variable is a local variable 、 Static variables or member variables .

3、 Common interview questions

1） How to understand basic data type or reference data type

The corresponding memory space in the basic data type stores specific values

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The memory space corresponding to the reference data type stores new Address of the object

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2） How to understand references and objects

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Reference or store the address ,new Objects are ontologies .

3） How to understand local variables, member variables and static variables

Member variables are on the heap

Local variables are on the stack

Static variables are in the method area

public class Test01 {
    



    // Member variables 



    Test01 t1=new Test01();



    // Static variables 



    static Test01 t2=new Test01();



    public static void main(String[] args) {
    



        // local variable 



        Test01 t3=new Test01();



    }



}

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4） Recursive execution

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Although these variables a, But they are variables in different stack frames .

5）static The difference between method and ordinary method

Common methods include this（ Instance related ）, There is no in the static method this（ Class related ） Common methods are also called instance methods , Static methods are also called class methods .

Two 、 Dynamic memory manager (GC)

1、GC The specific role of

GC It's the garbage collection mechanism ,java The memory requested in can be recycled by the garbage collection device ,GC It can avoid memory leakage to a certain extent , But there will be some extra overhead .

2、GC Characteristics of recycling

GC The main recyclables in Heap and method area Memory in , The memory in the stack is released until the thread ends or the stack frame is destroyed , The address stored in the program counter does not need to be released .

The basic unit of the recycling object ：

about GC The basic unit recycled in is not bytes but object

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GC The recycled memory is generally the memory in the unused location

The basic idea of recycling objects ：

1） Mark ： Find out whether this object needs recycling , And mark it

2） Recycling ： Recycle this object

3、 Mark

1） Reference counting

Each object will be assigned a special counter variable. When a new reference points to this variable, the counter will increase by one , When a reference does not point to this variable, the counter will be decremented by one , When the reference counter is 0 This object tag will be recycled .

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But this quotes Circular reference The problem cannot be solved ：

public class Test01 {
    



    public static void main(String[] args) {
    



        Test a=new Test();



        Test b=new Test();



        



        a.t=b;



        b.t=a;



        



        a=null;



        b=null;



    }



}



 



class Test{
    



    Test t=null;



}

When you want to use objects a, find a References to , This reference is in the object b in , Looking for someone b Citation in a in

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2） Accessibility analysis

stay java in GC Accessibility analysis is used to describe

There are certain associations between objects in the code , They constitute a “ Directed graph ”, Then we go through this “ Directed graph ” If the object traverses, it is not garbage , The opposite is rubbish .

Generally, where to start traversal ？

1、 Local variable table of stack frame in each stack

2、 Objects referenced in constant pools

3、 Objects referenced by static variables in the method area

When traversing, you don't start from one place like a binary tree , But traverse from multiple places , Such things are collectively referred to as GCRoot.

3） Recycling rules for class objects in the method area

1、 All instances of this class have been recycled

2、 Load the class ClassLoader It has also been recycled

3、 This kind of object is not used in code

4、 The type of citation

** Strong citation ：** You can find an object and decide its life and death

** Soft citation ：** You can find the object and decide the life and death of the object to a certain extent

** Weak reference ：** You can find the object, but you can't decide the life and death of the object

** Virtual reference ：** If you can't find an object, you can decide whether it's alive or dead

5、 Recycling

1） Mark clear

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Mark clearing refers to directly releasing the memory in the marked area

advantage ： A simple and efficient

shortcoming ： It is easy to cause memory fragmentation

2） Mark copy

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Divide the memory into two areas and directly copy objects that are not garbage into another area

advantage ： It can solve the problem of memory fragmentation , There will be no debris

shortcoming ： Need extra memory space （ If there are many living objects, it will be inefficient ）

3） Tag to sort out

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Similar to deleting data from an array

advantage ： Do not want to copy the same need for additional memory space, there is no memory fragment

shortcoming ： The efficiency of handling is low, which is not suitable for frequent use

6、 Generational recycling

We can use generational recycling to better clean up memory

We divide the memory in the heap into new generations ( Eden area and survival area ) And the older generation

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The object was born in Eden , The memory of new objects is the memory in the new era

After the first round GC The marked object will be destroyed during scanning , Those that have not been killed will be copied to the survival area 01

Objects entering the survival zone will also be scanned and copied to the survival zone 02（ The objects in the living area come from another living area and Eden ）

When an object is not recycled after several copies in the living area, it means that the object will survive for a long time and be copied to the old age

The objects in the old generation also have to go through GC scanning , Because the objects in the elderly generation survive for a long time , The scanning cycle in the elderly generation will be longer

7、 Introduction to garbage collector

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Garbage collectors do two things Marking and recycling , The application thread will stop working when recycling STW.

CMS and G1 The difference between ：

a） Initial marker （ Just look for GCRoot Directly associated objects ）

b） Concurrent Tags （ Concurrent execution with application thread , To traverse all objects ）

*cms It's going to be carried out all the time 、G1 After finding that there are no surviving objects in the old age, they will be recycled directly .*

c） Final marker （ To fix b Error generated ）

d） Screening and recovery

3、 ... and 、 Class loader

1、 The basic process of class loading

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We find the file according to the class name , And read file structure parsing , Read the contents into memory , And construct the corresponding class object , This process is called load .

If this class has other associated classes , Other dependent content will be introduced , This process is called link .

Initial static members and execute static code blocks , This process is called initialization .

2、 When to trigger class loading

1） Construct an instance of this class

2） Call the static properties and static methods of this class

3） Using a subclass will trigger the loading of the parent class

class Test{
    



    static {
    



        System.out.println("Test.static{}");



    }



    {
    



        System.out.println("Test{}");



    }



 



    public Test() {
    



        System.out.println("Test Construction method ");



    }



}



class Test0 extends Test{
    



    static {
    



        System.out.println("Test0.static{}");



    }



    {
    



        System.out.println("Test0{}");



    }



    public Test0() {
    



        System.out.println("Test0 Construction method ");



    }



}



public class Test01 extends Test0{
    



    public static void main(String[] args) {
    



        System.out.println(" Start ");



        new Test0();



        new Test0();



        System.out.println(" end ");



    }



}

Execute static code block —— Execute code block —— Execute construction

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3、 Common class loaders

Built in three loaders	Load content	Location
BootstrapClassLoader	Load the classes in the standard library （String ArrayList…）	jdk\jre\lib\rt.jar( The implementation of classes in the standard library is in this rt.jar in )
ExternsionClassLoader	Load a special class	jdk\jre\lib\ext All of the jar Find in the bag
ApplicationClassLoader	Load the classes written by the application itself	CLASS_PATH environment variable java -cp Current directory

4、 Parent delegation model

The essence of the parental delegation model is to find the class name in the process of class loading .class File search process , Content in the loading process .

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When a class starts loading , We'll start with AppClassLoader Start , But it won't find it immediately, and it will give it to its parent class first ,ExtClassLoader It will also be given to its own parent , then BootstrapClassLoader After you get the class name, go to rt.jar Start looking for , Find and return , If you don't find it, you'll be ExtClassLoader Medium ext Start looking in the directory , Find and return , If you don't find it, you'll be AppClassLoader Medium CLASS——PATH In the environment variables 、-cp Specify... In the directory 、 Find three places in the current directory , If found, load , Throw one if you don't find it ClassNotFoundException abnormal .

** Be careful ：** The classloader written by yourself can violate the parental delegation model （Tomcat The classloader in does not follow the parental delegation model ）