12.Java’s class loading mechanism
Java's class loading mechanism refers to loading the data describing the class from the Class file into the memory, verifying, converting, parsing and initializing the data, and finally forming a Java type that can be directly used by the virtual machine. This process is called a virtual machine. class loading mechanism.
The class loading process is divided into the following five stages:
- Loading : Read the Class file from disk into memory and place it in the method area.
- Verification : Verify the Class file to ensure that it complies with the Java language specification.
- Preparation : Allocate memory space for the static variables of the class and initialize their default values.
- Parsing : Convert symbol references to direct references.
- Initialization : Assign initial values to static variables of the class and execute the initialization method of the class.
The loading phase mainly does three things:
- Get the binary stream of the .class file;
- Put the contents of the .class file such as class information, static variables, bytecodes, and constants into the method area;
- Generate a java.lang.Class object representing this .class file in memory, which serves as the access point for various data of this class in the method area.
The verification phase mainly does the following work:
- File format verification: Check whether the format of the Class file complies with the Java language specification.
- Semantic verification: Check whether the contents of the Class file comply with the Java language specification.
- Security verification: Check whether the Class file contains any security risks.
The preparation phase mainly involved the following work:
- Allocate memory space for static variables of the class.
- Assign to static variables of the class