Can Two Classes Have the Same Fully Qualified Name in Java?

What Is a Fully Qualified Name (FQN)?

In Java, a class’s fully qualified name is its package name + class name. For example:

package com.example;
public class MyService {}

FQN = com.example.MyService

But What Really Identifies a Class in the JVM?

Contrary to common belief, in the JVM, a class is not just identified by its name, but by:

(classloader + class name)

This means:

textCopyEditClass A loaded by ClassLoader1 ≠ Class A loaded by ClassLoader2

Even if their bytecode is identical and they come from the same .class file, the JVM treats them as completely unrelated types if their classloaders differ.

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✅ Use Cases Where This Happens Intentionally

1. Servlet Containers (Tomcat, Jetty)

Each deployed WAR file is loaded with its own classloader. So:

• com.myapp.Controller in WAR 1 is not the same as the one in WAR 2
• Allows deploying multiple apps with overlapping class names

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2. OSGi & Module Systems

Frameworks like OSGi create isolated classloader namespaces for each module (called a “bundle”).

• Each module can define its own version of com.example.Logger
• Prevents version conflicts

What Happens If You Try to Use Both?

Let’s say you try to pass an object of class com.example.MyService (loaded by LoaderA) into a method that expects the same class — but from LoaderB.

Example:

javaCopyEditClassLoaderA → com.example.MyService
ClassLoaderB → com.example.MyService

MyService obj = new MyService();  // Loaded via A
someComponent.setService(obj);    // Expects B's MyService

🔴 Result:

textCopyEditjava.lang.ClassCastException: com.example.MyService (loaded by LoaderA)
  cannot be cast to com.example.MyService (loaded by LoaderB)

Even though they look identical in name and structure, the JVM treats them as completely unrelated.