What is the working mechanism of the Java classloader?

The ClassLoader in Java is a crucial component of the Java Virtual Machine (JVM), responsible for loading Java class files and converting byte code files into runtime Class objects.

The working mechanism of ClassLoader can be briefly described as follows:

1. The ClassLoader searches for the corresponding class file based on the fully qualified name of the class (including package name) in locations such as the file system, network, and database.
2. Loading class files: Once a class file is located, the ClassLoader will read the bytecode data of this class file and convert it into a Class object in memory.
3. When Loader turns the bytecode data in class files into Class objects, the process of loading the class is complete. This Class object contains information about the class structure, such as methods and fields.
4. During the process of loading classes, the ClassLoader also resolves dependencies between classes by searching for and loading other classes referenced within a class.
5. Upon completion of loading a class, the ClassLoader will also execute the class’s static code block to initialize the class.

The mechanism of ClassLoader is an important foundation for Java to dynamically load classes, supporting the loading and unloading of classes, and enabling flexible class loading mechanisms such as hot deployment using custom ClassLoader. ClassLoader also serves as the basis for implementing class loading cache and security policies in Java. ClassLoader is widely used in Java, for example in Servlet containers in JavaEE, and in loading Beans in the Spring framework.