Solving Runtime Class Conflicts with a Custom ClassLoader

Tejas Parmar

September 5, 2025

Introduction

When building SDKs that integrate into diverse applications, one of the biggest challenges is avoiding runtime class conflicts. In this post, we’ll explore how we solved this problem using a Custom ClassLoader.

Problem Statement

When integrating our SDK with target applications, we faced runtime conflicts between SDK classes and application classes.
Typical issues included:

ClassCastException
NoClassDefFoundError
Instrumentation interference

These conflicts broke both the SDK and the target application execution.

Solution: Custom ClassLoader

We solved this by implementing a Custom ClassLoader that:

Loads SDK classes in a separate namespace
Delegates application classes to the parent system ClassLoader
Ensures complete class isolation

This prevented runtime conflicts between the instrumentation framework and the target application.

Before: Problem Example

java

// Application tries to cast SDK class to its own version
Object obj = someMethodFromSDK();

// Throws ClassCastException because
// AppClass and SDKClass loaded from different sources conflict
MyAppClass myClass = (MyAppClass) obj;

Error message:

code

java.lang.ClassCastException: com.app.MyAppClass cannot be cast to com.app.MyAppClass

After: Custom ClassLoader Implementation

java

public class SDKClassLoader extends ClassLoader {
    private final String sdkPath;

    public SDKClassLoader(String sdkPath, ClassLoader parent) {
        super(parent);
        this.sdkPath = sdkPath;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        if (name.startsWith("com.sdk")) {
            byte[] classData = loadClassFromFile(name);
            return defineClass(name, classData, 0, classData.length);
        }
        return super.findClass(name);
    }

    private byte[] loadClassFromFile(String className) {
        // load class bytes from JAR in sdkPath
        // simplified for demo
        return new byte[0];
    }
}

Usage Example:

java

ClassLoader parent = ClassLoader.getSystemClassLoader();
SDKClassLoader sdkLoader = new SDKClassLoader("/path/to/sdk.jar", parent);

// Load SDK class in isolation
Class<?> sdkMain = sdkLoader.loadClass("com.sdk.Main");

// Invoke method reflectively
Object sdkInstance = sdkMain.getDeclaredConstructor().newInstance();
sdkMain.getMethod("start").invoke(sdkInstance);

Key Takeaway

By using a Custom ClassLoader, we achieved clean isolation between SDK and application code. This solution eliminated class conflicts and enabled smooth execution of both components.

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