In depth study of JVM bottom layer (V): class loading mechanism

Class loading mechanism

Concept

The virtual machine takes the data describing the class from Class File loaded into memory , And verify the data 、 Replace resolution and initialization , Finally, it can be directly used by virtual machine Java type

Class loading time

Pictured ： Load in 、 verification 、 Get ready 、 The order of initialization and uninstallation is determined . The parsing phase can start after initialization ( Runtime binding or dynamic binding or late binding ).

The following five cases must initialize the class ( And load 、 verification 、 Preparation naturally needs to be completed before )：

1. encounter new、getstatic、putstatic or invokestatic this 4 Trigger initialization is not initialized when byte code instruction is set . for example ： Use new Keyword instantiation object 、 Read a static field of a class ( By final modification 、 Except for static fields that have been put into the constant pool at compile time )、 Call a static method of a class
2. Use java.lang.reflect When a package's method makes a reflection call to a class
3. When initializing a class , If it is found that its parent class has not been initialized , First trigger the initialization of its parent class
4. When the virtual machine starts , The user needs to specify a main class to load ( contain main() Method type ), Virtual opportunity initializes the main class first
5. When using JDK 1.7 Dynamic language support for , If one java.lang.invoke.MethodHandle The final parsing result of the example REF_getStatic、REF_putStatic、REF_invokeStatic Method handle for , And the class corresponding to this method handle has not been initialized , It needs to trigger its initialization first

Class loading process

load

1. Get the binary stream that defines the subclass through the fully qualified name of a class (ZIP package 、 The Internet 、 Operation generation 、JSP Generate 、 Database read )
2. Convert the static storage structure represented by the byte stream into the runtime data structure of the method area
3. Generate a representation of this class in memory java.lang.Class object , As a method to access all kinds of data in this class

verification

This is the first step of connection , Make sure Class The information contained in the byte stream of the file meets the requirements of the current virtual machine

• File format validation ： The first stage is to Verify that the byte stream matches Class Specification of document format , And can be processed by the current version of virtual machine . Verification points mainly include ： Whether to count by magic 0xCAFEBABE start ; Lord 、 Is the minor version number within the current virtual machine processing range ; Is there an unsupported constant type in the constant of the constant pool ;Class Whether each part of the file and the file itself has been deleted or attached with other information, etc .
• Metadata validation ： The second stage is right The information described by bytecode is analyzed semantically , In order to ensure that the information described in it conforms to Java The requirements of language norms , Verification points for this phase include ： Does this class have a parent class ; Whether the parent class of this class inherits the class that is not allowed to be inherited ; If this class is not an abstract class , Whether it implements all the methods required to be implemented in its parent class or interface ; Fields in class 、 Whether the method conflicts with the parent class and so on .
• Bytecode verification ： The third stage is the most complicated stage in the whole verification process , The main purpose is to analyze the data flow and control flow , Make sure the program semantics are legal 、 Logical .
• Symbol reference validation ： The last phase of verification occurs when the virtual machine converts the symbol reference to a direct reference , This transformation will take place in the third phase of the connection – Occurs during the parsing phase . Symbol reference validation can be seen as Beyond the class itself （ Various symbol references in the constant pool ） The image of matching is checked .

Get ready

This stage formally allocates memory for the class and sets the initial value of the class variable , Memory is allocated in the method ( contain static Decorated variables do not contain instance variables )

analysis

This stage is the process of virtual machine replacing symbol reference in constant pool with direct reference .

initialization

Start executing in class Java Code .

Class loader

The virtual machine design team loads the classes in the **「 Get the binary byte stream that describes a class by its fully qualified name 」 This action is put in Java Virtual machine external to achieve , So that the application itself can decide how to get the required classes . The code module that implements this action is called 「 Class loader 」**.

Class loader Responsible for loading types in the program （ Classes and interfaces ）, And give a unique name to identify .

Class and class loader

For any class , It needs to be established by the class loader that loads it and the class itself Java The uniqueness of virtual machines , Each class loader has a separate class namespace . in other words ： Compare two classes whether or not 「 equal 」, It only makes sense if the two classes are loaded by the same class loader , otherwise , Even if the two classes come from the same Class file , Loaded by the same virtual machine , As long as the classloaders that load them are different , Then these two classes must not be equal .

Parent delegation model

from Java From a developer's point of view , Class loaders can be divided into ：

• Start class loader （Bootstrap ClassLoader）： This class loader is responsible for storing the <JAVA_HOME>\lib The class library in the directory is loaded into the virtual machine memory . The boot class loader cannot be Java The program directly refers to , When you write a custom class loader , If you need to delegate the load request to the boot class loader , Use Zener null Instead of just ;
• Extend the classloader （Extension ClassLoader）： This class loader consists of sun.misc.Launcher$ExtClassLoader Realization , It's responsible for loading <JAVA_HOME>\lib\ext Directory , Or be java.ext.dirs All class libraries in the path specified by the system variable , Developers can use the extension class loader directly ;
• Application class loader （Application ClassLoader）： This class loader consists of sun.misc.Launcher$App-ClassLoader Realization .getSystemClassLoader() Method returns the class loader , Therefore, it is also called system class loader . It is responsible for loading the user classpath （ClassPath） Class library specified on . Developers can use this classloader directly , If there is no custom classloader in the application , In general, this is the default classloader in the program .

Our applications are all made up of 3 The type loaders cooperate with each other to load , If necessary, you can also define your own class loader . Their relationship is shown in the figure below ：

Class loader relationship

From the top down , Step by step

1. Bootstrap Classloader Is in Java Initialized after the virtual machine starts .
2. Bootstrap Classloader Responsible for loading ExtClassLoader, And will ExtClassLoader The parent loader of is set to Bootstrap Classloader.
3. Bootstrap Classloader Finished loading ExtClassLoader after , Will be loaded AppClassLoader, And will AppClassLoader The parent loader of is specified as ExtClassLoader.

The function of classloader

Class loader features

• Hierarchical structure ：Java The class loaders in are organized into a parent-child hierarchy .Bootstrap Class loaders are the father of all loaders .
• The proxy pattern ： Based on hierarchy , Class can proxy between loaders . When the loader loads a class , First, it checks whether it is loaded in the parent loader . If the upper loader has already loaded this class , This class will be used directly . conversely , The class loader requests the class to be loaded
• Visibility limits ： A child loader can find classes in the parent loader , But a parent loader cannot find classes in child loaders .
• Uninstall is not allowed ： Class loader can load a class, but it can't unload it , However, you can delete the current class loader , Then create a new class loader to load .

Break the parental delegation model

To break the parental appointment mechanism, we should not only inherit ClassLoader class , And rewrite loadClass and findClass Method .