Class loading mechanism (detailed explanation of the whole process)

One : background

The class loading mechanism is in our real java One of the operation phases of .

Two : What is a happy planet ( Class loading mechanism )

We wrote Java All the documents are based on .java A file with a suffix , The compiler will write .java The files are compiled into .class file , In short, the class loading mechanism is jvm From the file system, a series of class file z Convert to binary stream loading JVM In memory and generate a of this class Class object , The act of providing resources for subsequent program operations .

3、 ... and : Class loading process

1: Overall flow chart

2: Analysis of each stage

(1): load

a: The bytecode file of the loaded class and the source of the binary file

Find such bytecode files through the full path of a class （class Files are binary files ）. Convert the static storage structure of binary file to the runtime data structure of method area , And use binary stream file to create a Class object , Stored in Java The entry in the heap for the data structure reference to the method area ;
class The source of the document ： It should be noted that the class loading mechanism can not only be read from the file system class file , It can also be obtained through the Internet , other jar Package or other program generation , Such as JSP application .

b: Class loader

• Class loader ： When it comes to class loading, we have to talk about the order of class loading and class loader .Java There are about four types of loaders , Namely ： Start class loader （Bootstrap ClassLoader）, Extend the classloader （Extension ClassLoader）, system class loader （System ClassLoader）, Custom class loaders （Custom ClassLoader）, In turn, it belongs to the inheritance relationship （ Note that inheritance here is not Java The inside of the class extends）
  Start class loader （Bootstrap ClassLoader）： Mainly responsible for loading and storage in Java_Home/jre/lib Next , Or be -Xbootclasspath Under the path specified by the parameter , And can be recognized by the virtual machine class library （ Such as rt.jar, be-all java.* The first classes are all Bootstrap ClassLoader load ）, Boot class loader is unable to be Java The program directly refers to .

• Extend the classloader （Extension ClassLoader）： Mainly responsible for the loader by sun.misc.Launcher$ExtClassLoader Realization , It's responsible for loading Java_Home/jre/lib/ext Directory , Or by java.ext.dirs All class libraries in the path specified by the system variable （ Such as javax.* Initial class ）, Developers can use the extension class loader directly .

• system class loader （System ClassLoader）： Mainly responsible for the loader by sun.misc.Launcher$AppClassLoader To achieve , It is responsible for loading the user classpath （ClassPath） The specified class , Developers can use this kind of loader directly , If there is no custom classloader in the application , In general, this is the default classloader in the program .

• Custom class loaders （Custom ClassLoader： Self developed classloader

c: Parents delegate

If a class loader needs to load classes , First of all, it will delegate the class loading request to the parent class loader , If the parent class still has a parent class, then delegate , Every layer is like this .
Recursion all the way to the top level , When the parent loader cannot complete the request , Subclasses will try to load .

(2): verification

As long as the verification process is to guarantee class Security and correctness of documents , Make sure that the class Documents do not lead to JVM There are any exceptions , Will not harm JVM Our own safety . Validation includes validation of file formats , Metadata and bytecode validation .

(3): Get ready

The preparation phase is the process of allocating memory and initializing zero values for class variables . Note that the memory allocated at this time is the class variable , This memory is allocated in the method area . Memory for instance variables will not be allocated at this time , Because instance variables are created together when an object is instantiated Java Heap . And the class variable is assigned to zero , namely int The initial value of type is 0, The initial value of reference type is null, Instead of displaying the assigned value in the code .

(4): analysis

Convert the symbolic reference of constant pool to direct reference . A symbolic reference can be understood as just an alternative label , For example, you need to make a plan at this time , There are no candidates yet , You set up a A To do this . And then when the plan really comes to fruition, we must find a definite candidate , At that time, Xiao Ming will do one thing .
Analysis is to put A( Symbol reference ) Replace it with Xiao Ming ( Direct reference ). A symbolic reference is a literal quantity , It doesn't make any sense , Just a representative . Direct reference refers to a real reference , You can find the target through this reference in memory .

(5): initialization

The initialization phase is the last step in class loading , This stage is mainly the implementation of java Code , Perform the relevant initialization actions ; At this point, some static code blocks are executed , Assign values to static variables , The assignment here is the assignment in the code , The preparation stage is just to set the initial value to occupy a pit .