Java’s class loading mechanism refers to the process of loading Java class bytecode files into memory and creating corresponding Class objects at runtime. Class loading is one of the core functions of the Java Virtual Machine (JVM), which is responsible for finding, loading, validating, preparing, and parsing the bytecode of a class.
The class loading process is mainly divided into the following steps:
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Loading: Find and read the bytecode file of the class through the class loader (ClassLoader), convert it into a data structure that the JVM can understand, and A Class object representing the class is generated in the method area.
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Verification (Verification): Verify the loaded bytecode to ensure that it meets the requirements of the Java language specification and JVM specification, including syntax checking, semantic checking, and bytecode Verification etc.
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Preparation: Allocate memory space for static variables of the class and set default initial values. These static variables will be assigned the correct initial value during the class initialization phase.
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Resolution: Convert the symbol reference in the class into a direct reference, that is, replace the symbol reference in the constant pool with a pointer or handle directly pointing to the memory address.
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Initialization: Execute the initialization code of the class, including the assignment of static variables and the execution of static code blocks. At this stage, all parent classes of the class are also initialized in turn.
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Usage: After the class is loaded, you can use the class by creating objects, calling methods, etc.
It should be noted that the class loading process is performed on demand, that is, the loading and initialization of a class will only be triggered when it is used for the first time. Moreover, the class loader adopts the parental delegation model, that is, the parent class loader first tries to load the class, and only when When the parent class loader fails to load, the child class loader attempts to load.
The advantage of this class loading mechanism is to ensure the security and consistency to avoid repeated loading and conflicts. At the same time, it also provides Java with the ability to dynamically expand, and can achieve hot deployment, plug-in and other functions through custom class loaders.