Q: How does ClassLoading work in Java?

Answer:

Class loading is the process of finding, loading, and initializing .class files into the JVM.

The Three Phases

1. Loading — The ClassLoader reads the .class bytecode file and creates a Class<?> object in memory.

2. Linking

• Verification: Bytecode is checked for correctness and security.
• Preparation: Static fields are allocated and set to default values (0, null).
• Resolution: Symbolic references (class names in bytecode) are resolved to actual memory addresses.

3. Initialization — Static initializers and static blocks are executed. This happens only when the class is first used.

ClassLoader Hierarchy (Delegation Model)

Bootstrap ClassLoader (C/C++)
    ↑ delegates to parent first
Application ClassLoader
    ↑
Extension ClassLoader
    ↑
Custom ClassLoader (your code)

Parent-Delegation Model

When a class needs to be loaded:

1. The current ClassLoader delegates to its parent first.
2. If the parent can't find it, the current ClassLoader tries.
3. If no one can find it → ClassNotFoundException.

Why? Prevents duplicate class loading and ensures core classes (java.lang.String) are always loaded by the Bootstrap ClassLoader, preventing tampering.

Common Interview Scenarios

// These are loaded by DIFFERENT classloaders:
String.class.getClassLoader();    // null (Bootstrap — implemented in native code)
MyApp.class.getClassLoader();     // AppClassLoader

ClassNotFoundException vs NoClassDefFoundError:

[!NOTE] Understanding class loading is essential for debugging issues in application servers (Tomcat, Spring Boot), OSGi frameworks, and anywhere with multiple classloaders. "Class X cannot be cast to Class X" errors typically mean the same class was loaded by two different classloaders.