What is class loading? Class loading process?

What is class loading ？ The process of class loading ？
The virtual machine loads the data describing the class into memory , And verify the data 、 Parse and initialize , Finally, it can be used directly by virtual machine class object ;

The entire life cycle of a class includes ： load （Loading）、 verification （Verification）、 Get ready (Preparation)、 analysis (Resolution)、 initialization (Initialization)、 Use (Using) And uninstall (Unloading)7 Stages . And prepare for 、 verification 、 analysis 3 The parts are collectively called connections （Linking）.
load 、 verification 、 Get ready 、 Initialize and uninstall this 5 The order of the stages is fixed , The loading process of a class must start step by step in this order , The parsing phase is not necessarily ： It can in some cases start after the initialization phase , This is to support Java Language runtime binding （ Also known as dynamic binding or late binding ）

Class loading process is as follows ：

load , There are three steps to loading ： 1、 Get the binary stream of a class by its fully qualified class name ; 2、 The static storage structure of the binary stream is transformed into the runtime data structure of the method area ; 3、 Generate a... For this class in the heap class object ;

verification ： Verify the class The byte stream information in the file meets the requirements of virtual machine , It doesn't threaten jvm The safety of the ;

Get ready ： by class Object to allocate memory , Initialize its initial value ;

analysis ： In this stage, the symbol reference is transformed into direct reference ;

initialization ： It's time to initialize , Just start executing the java Code ; The initialization phase is the process of calling the class constructor ;

What is a class loader , What are the common class loaders ？
Class loaders are ： Getting the binary byte stream of a class by its fully qualified class name is called class loader ; There are four types of class loaders ：

Start class loader （BootStrapClassLoader）： Used to load java The core library , Can't be java The program directly refers to ;

Extend the classloader （Extension ClassLoader）： Used to load java Extension library for ,java The virtual machine implementation of will provide an extension Library Directory , This kind of loader looks up and loads in the extension library directory java class ;

system class loader （AppClassLoader）： It is based on java Class path to load the class , Generally speaking ,java Application classes are loaded through it ;

Custom class loaders ： from java Language implementation , Inherited from ClassLoader;
What is the parent delegate model ？ Why do I need a parental delegation model ？
When a class loader receives a request to load a class , He doesn't try to load it himself in the first place , Instead, delegate the request to the parent loader to load it , Only when the parent class loader cannot find the given class in its own search scope class , The child loader will try to load the class by itself ;

To prevent multiple identical bytecodes in memory ; Because if there's no parental appointment , Users can define a java.lang.String class , Then there is no guarantee of the uniqueness of the class .

Add ： So how to break the parental delegation model ？

Custom class loaders , Inherit ClassLoader class , rewrite loadClass Methods and findClass Method .