How is the Java classloader used?

The Java ClassLoader is a crucial component of the Java Virtual Machine (JVM) that is responsible for loading Java class files. It loads compiled Java class files into memory and instantiates these classes at runtime. It enables dynamic class loading, class isolation, and isolation of class loaders.

The usage method of ClassLoader is as follows:

1. Customize ClassLoader: You can create a custom ClassLoader by either extending the ClassLoader class or implementing the ClassLoader interface. A custom ClassLoader can be used to implement specific loading logic and enable hot deployment functionality.
2. Load class file: Load the specified class file by calling the loadClass() or findClass() method of ClassLoader. Classes can be loaded by their fully qualified name (package name + class name).
3. Instantiate a class object: After loading the class with a ClassLoader, instantiate the class object using reflection, and then call the class’s methods and fields.
4. ClassLoader chain: ClassLoader utilizes the Parent Delegation Model to load class files, which means it first delegates the parent class loader to load the class, and if the parent class loader is unable to load it, then it loads the class itself. The logic of class loading can be altered by customizing the ClassLoader.
5. The role of class loaders: using ClassLoader can achieve the isolation of classes, allowing the same-named classes loaded by different ClassLoaders to coexist without interfering with each other’s class instances.

Overall, ClassLoader is a crucial component in Java as it enables many flexible features such as dynamic loading of classes.