Explain the concept of classloader in Java

💡 Concept: Java Classloader

Classloader is a subsystem of JVM that loads classes dynamically into the Java runtime environment.

📘 Quick Intro

Classloaders enable dynamic loading, linking, and initialization of classes at runtime, supporting modularity and flexibility.

🧠 Analogy

Think of a classloader as a librarian fetching and organizing books (classes) only when needed rather than having all books upfront.

🔧 Technical Explanation

Bootstrap Classloader loads core Java API classes (rt.jar).
Extension Classloader loads JDK extensions.
System/Application Classloader loads application classes from classpath.
Supports hierarchical delegation model.
Allows custom classloaders for advanced use cases.

🎯 Use Cases

✅ Loading classes at runtime without restarting JVM.
✅ Supporting modular applications and plugins.
✅ Enabling dynamic class reloading and hot deployment.

💻 Example: Using ClassLoader to Load Class


public class CustomLoader {
    public static void main(String[] args) throws Exception {
        ClassLoader classLoader = CustomLoader.class.getClassLoader();
        Class clazz = classLoader.loadClass("java.lang.String");
        System.out.println("Loaded class: " + clazz.getName());
    }
}

❓ Interview Q&A

Q1: What is a classloader?
A: JVM component that loads classes dynamically.

Q2: What types of classloaders are there?
A: Bootstrap, Extension, System.

Q3: What is the delegation model?
A: Classloaders delegate loading to parent classloader first.

Q4: Can you create a custom classloader?
A: Yes, by extending ClassLoader.

Q5: What does bootstrap classloader load?
A: Core Java classes.

Q6: Why is classloader important?
A: Enables dynamic and modular applications.

Q7: Can classes be unloaded?
A: Only when classloader is garbage collected.

Q8: How to get classloader of a class?
A: Using Class.getClassLoader() method.

Q9: What is system classloader?
A: Loads application classes from classpath.

Q10: Can classloaders be chained?
A: Yes, parent delegation model.

📝 MCQs

Q1. What is a classloader in Java?

A memory manager
JVM component that loads classes dynamically
A compiler
A debugger

Q2. Which classloader loads core Java classes?

System Classloader
Extension Classloader
Bootstrap Classloader
Custom Classloader

Q3. What is the delegation model?

Child classloader loads first
Parent classloader first tries to load class
Random loading
None

Q4. Can you create custom classloader?

No
Yes
Sometimes
Never

Q5. What does system classloader load?

Core classes
Application classes
Extension classes
Native code

Q6. Why are classloaders important?

For debugging
Support dynamic and modular apps
For memory
For performance only

Q7. Can classes be unloaded?

Always
Never
Only when classloader is GCed
Sometimes

Q8. How to get classloader of a class?

Class.getClassLoader()
ClassLoader.get()
Class.load()
ClassLoader.find()

Q9. What is system classloader?

Loads core classes
Loads app classes from classpath
Loads extension classes
Loads native code

Q10. Can classloaders be chained?

No
Yes
Sometimes
Never

💡 Bonus Insight

Classloaders are a key feature enabling Java's dynamic, modular, and flexible runtime environment.

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