Preliminary foundation JVM

jvm

1、jvm Architecture of – General flow chart

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1.1 Class loader and parental delegation mechanism

​ Class loader ：

​ 1. Start class loader ：Bootstrap classLoader: Mainly responsible for loading the core class library (java.lang.* etc. ), structure ExtClassLoader and APPClassLoader

​ 2. Extend the classloader ： Mainly responsible for loading jre/lib/ext Some of the extended jar.

​ 3. Apply the class loader ： It is mainly responsible for loading the main function class of the application

Parent delegate mechanism ：

​ When class When the file is about to execute , Not considering that we define class loaders , Then we will keep looking for the parent class until we find our final parent class, that is, the startup class loader . Generally, getting the final parent class will return null, Roughly because of oil c/c++ We don't have permission to access

1.3 Sandbox security mechanism

The default local code is safe by default , And the remote code may be infected by viruses , So we need to give some permissions

1.4 Method area -----native

Method area is shared by all threads , All bytecodes

Primary storage ： static const 、 Constant 、 Class information

static、final、class、 Constant pool

-----————————

Everything is added. native Method statement java The effect of , Going to call C The library of languages

public class jjk{
    
  public static void main(String[] args){
    
      
  }
    // No mistake. 
    private native void hello();
}

effect ： Expand java Use , Integrate different programming languages into java Use

*** Not commonly used , Unless there is a problem with the calling hardware

2 Stack

data structure

Program = data structure + Algorithm

Program = frame + Business logic

2.1 characteristic

First in, then out 、 Last in, first out

----> contrast queue fifo (FIFO)

2.2 Stack memory

In charge of the running of the program , Life cycle and thread synchronization , When the stack ends, the stack memory is released , There will be no garbage

8 The big basic type + Object reference + Example method

Operation principle ： Stack frame

3 in jvm

java -version Check what you use jvm

What we often use is Sun The company's HotStop

4 Pile up

One jvm There's only one heap memory , The size of heap memory can be adjusted ( There is only one pile, so there is a lot of garbage )

General storage ： class 、 Method 、 Constant 、 Variable , Reference the real object of the class

• New Area
• Retirement area
• The permanent zone

gc: Garbage recycling is mainly in Yidian district and pension area

When the memory is full oom, There's not enough heap memory

4.1 New Area

• Where classes are born and die
• Eden ： All objects in this go inside new Coming out

4.2 The permanent zone

• jdk1.6： Forever , The constant pool is in the method area
• jdk1.7: Forever , Slowly degenerated , To the eternal generation , The constant pool is in the heap
• jdk1.8： There is no permanent generation , Constant pool in cloud space

tuning :-Xms1024m -Xmx1024m -Xx:PrintGCDetails

-Xms1m -Xmx8m -Xx:+HeapDumpOn( Add abnormal error )

Tools ：JProfiler

• analysis dump Memory files , Quickly locate memory leaks
• Get the data in the heap memory
• Get big objects

5 Garbage collection mechanism

Garbage is generated in the method area and heap --GC

GC It is divided into ： heavy GC( overall situation ） And light GC

5.1 Reference counting

Basically useless , When new An object is +1 Reduce when not in use 1 When it comes to 0 It's clear when

5.2 Mark clearly

Mark the surviving object

When it is used up, it will be scanned again ： The unmarked ones will be cleared

advantage ： It doesn't take up space

Take some ： Two scans waste time , Memory fragmentation will occur

5.3 Copy algorithm

— Who is empty and who is to

When survival passes enda Enter the area from and to District It will from Copy it to to District , The of this space-time is to The one in the district is from District ; Generally, when the object experiences 15 After that, you will enter the elderly area