Virtual machine class loading mechanism

1. Class load time

Class must be initialized 5 In this case ：

1. encounter new、getstatic、putstatic、invokestatic this 4 Bytecode instructions , If the class is not initialized , You need to trigger its initialization
2. Use java.lang.reflect When a package's method makes a reflection call to a class , If the class is not initialized , You need to trigger its initialization
3. When initializing a class , Its parent class has not been initialized , You need to trigger the initialization of its parent class first
4. When the virtual machine starts , The user needs to specify a main class to execute （ contain main Class of method ), Virtual opportunity initializes the main class first
5. When using JDK1.7 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 , You need to trigger its initialization first

Be careful ：

1. For static fields , Only classes that directly define this field will be initialized , Reference the static fields defined in the parent class through its subclasses , Only the initialization of the parent class will be triggered , Does not trigger subclass initialization
2. Define reference classes through arrays , Initialization of this class will not be triggered
3. Constants are stored in the constant pool of the unused class during compilation , In essence, there is no direct reference to the class defining constant , So it doesn't trigger the initialization of the class that defines the constant
4. When a class is initialized, its parent class must have been initialized , But when an interface is initialized , It is not required that all its parent classes have been initialized , It is initialized only when the parent interface is actually used

2. The process of class loading

2.1 load

• load yes Class loading A stage in the process

During the loading phase, the virtual machine needs to complete 3 thing ：

• Get the binary byte stream that defines this class through the fully qualified class name of a class
• Convert the static storage structure represented by the byte stream into the runtime data structure of the method area
• Generate a representation of this class in memory java.lang.Class object , As the access to all kinds of data of this class in method area

2.2 verification

• Verification is the first step in the connection phase , The purpose is to make sure that Class The information contained in the byte stream of the file meets the requirements of the current virtual machine
• File format validation
• Metadata validation
• Bytecode verification
• Compliance with reference validation

2.3 Get ready

• The preparation stage is officially for Class variables The stage of allocating memory and setting the initial value of class variables , The memory used by these variables will be allocated in the method area

2.4 analysis

• The parsing phase is the process in which the virtual machine replaces the symbolic reference in the constant pool with a direct reference

2.5 initialization

• The class initialization phase is the last step in the class loading phase

3. Class and class loader

• Even if 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 the two classes must not be equal （ Representative class of equality Class Object's equals Method 、isInstance Method , It also includes the use of instanceOf Keyword is used to judge the relationship of the object ）

4. Parent delegation model

• from Java From a virtual machine perspective, there are only two different class loaders ： Start class loader and Other class loaders

To subdivide ：

1. Start class loader
2. Extended class loader
3. Application class loader

• The two parent delegation model requires in addition to the top-level bootloader , The rest of the class loaders should have their own parent class loaders

The working process of the parent delegation model ：

If a class loader receives a class load request , It doesn't try to load the class itself first , Instead, delegate the request to the parent loader to complete , This is true of class loaders at every level , Therefore, all load requests should eventually be sent to the top-level startup class loader , Only when the parent loader reports that it is unable to complete the request , Sub loader will try to load by itself

benefits ：

Java Class has a hierarchical relationship with priority along with its classloader , such as Object class , No matter which class the loader is , Finally, it is delegated to the top boot class loader to load , therefore Object Class is a class in the various classloader environments of a program ; contrary , If you don't use the parental delegation model , The user writes his own class loader to load Object class , There will be many different Object class