JVM virtual machine

1.JVM Class loading process

java Class loading process ： load –> verification –> Get ready –> analysis –> initialization , Then the class can be used . In most cases, the whole process of class loading is completed in this order . But there are exceptions , Parsing can also be performed after initialization , This is to support java Runtime binding for , And in the process of one stage, the latter stage may also be activated , Instead of waiting for the end of one phase before proceeding to the next .

• When a class is loaded by reference , Class will only be loaded once

  • Active reference to class （ Class initialization will happen ）

    • new An object of a class
    • Calling static members of a class （ except final Constant ） And static methods
    • Use java.lang.reflect Package's method makes a reflection call on the class
    • When the virtual machine starts ,java Demo01, It must be initialized Demo01 class , To put it bluntly, start first main The class of the method
    • When initializing a class , If its parent class is not initialized , Then initialize its parent class first

  • Class （ Class initialization will not occur ）

    • When accessing a static domain , Only classes with real names in this domain will be initialized
      • Reference the static variables of the parent class through the child class , Does not cause subclass initialization
    • Define a reference to a class through an array , Such initialization will not be triggered
    • Reference constants do not trigger initialization of this class （ Constants are stored in the constant pool of the calling class at the compile stage ）

2. Let's talk about the parental delegation model

Why the parental delegation model

1. Orderly
2. Security （ Only the class loader and the class itself can determine a class （instanceof））： For example, write a package with the same name String class , Because class loaders are different, they can determine and jdk Is not a class

• Start class loader ： The load is stored in <JAVA_HOME>\lib Catalog , Or be -Xbootclasspath Stored in the path specified by the , And it's Java Virtual machines can recognize （ Identify by file name , Such as rt.jar、tools.jar, The name doesn't match Even if the combined class libraries are placed in lib The directory will not be loaded ） The class library is loaded into the memory of the virtual machine .
• Extend the classloader ： This class loader is in class sun.misc.Launcher$ExtClassLoader China and Israel Java In the form of code . It's responsible for loading <JAVA_HOME>\lib\ext Directory , Or be java.ext.dirs In the path specified by the system variable Some class libraries .
• Application class loader ： This class loader consists of sun.misc.Launcher$AppClassLoader To achieve . Because the application class loader is ClassLoader Class getSystem-ClassLoader() Return value of method , So on some occasions Call it “ system class loader ”. It is responsible for loading the user classpath （ClassPath） All the class libraries on , Developers can also use this classloader directly in the code . If there is no customization in the application Use your own classloader , In general, this is the default classloader in the program .

3. Run time data area

1. Virtual machine stack （ Thread stack ）： Method call on and off the stack , Store the local variable table 【 There are also object reference addresses （ Point to the pile ）】, The stack of operands （ Additive and subtractive … The values and results of such operations are temporarily stored ）, Dynamic links （ Find the method area （ Meta space ） Address ）, Methods the export （ Returns the location where this method was called ）
2. Program counter ： Where to store program execution （ Address ）（ Multithreading Program execution may be suspended , You need to record where the execution has gone ）
3. Native Method Stack ：native Method
4. Method area （ Meta space ）： Constant , Static variables , Class information
5. Pile up ：new Out object

Bytecode engine modify Program counter Value