Is it possible to implement an interface at runtime in Java?

Question

I am working on a project where there are a lot of objects that are created by a library, and I have no access to the creation process of these objects.

The following sn

Answer 1

You can use the java instrumentation API to (forcefully) adapt the class to the interface. This technique is usually used by APM, AOP frameworks, and profilers to inject logging and metrics measurement code into target classes at runtime. It is very unusual for applications to directly use this technique. It would be a big red flag in the least if I see this in production code.

Nonetheless,

Given these Clazz:

package com.sabertiger.example;

public class Clazz {
    public void purr(){
        System.out.println("Hello world");
    }

}

Interface

package com.sabertiger.example;

public interface ExampleInterface {
    void run();
}

Executor

package com.sabertiger.example;

public class ExampleExecutor {  
    public static void main(String[] args) {
        Clazz c=new Clazz();
        // Normally a ClassCastException
        ExampleInterface i=(ExampleInterface)(Object)(Clazz) c;
        i.run();
    }
}

A Normal run produces this error:

Exception in thread "main" java.lang.ClassCastException:
  com.sabertiger.example.Clazz cannot be cast to 
  com.sabertiger.example.ExampleInterface
    at com.sabertiger.example.ExampleExecutor.main(ExampleExecutor.java:7)

You can make it work by supplying the missing interface and implementation by transforming the class:

package com.sabertiger.instrumentation;

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.lang.instrument.Instrumentation;
import java.security.ProtectionDomain;

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.CtNewMethod;

public class ExampleInterfaceAdapter implements ClassFileTransformer {

    public static void premain(String agentArgument, Instrumentation instrumentation) {
        // Add self to list of runtime transformations
        instrumentation.addTransformer(new ExampleInterfaceAdapter());
    }

    @Override
    // Modify only com.sabertiger.example.Clazz, return all other unmodified
    public byte[] transform(ClassLoader loader, String className,
            Class classBeingRedefined, ProtectionDomain protectionDomain,
            byte[] classfileBuffer) throws IllegalClassFormatException {

        if(className.matches("com/sabertiger/example/Clazz")) {
            return addExampleInterface(className, classfileBuffer );            
        } else {
            return classfileBuffer;
        }
    }

    // Uses javassist framework to add interface and new methods to target class
    protected byte[] addExampleInterface(String className, byte[] classBytecode) {
        CtClass clazz= null;
        try {
            ClassPool pool = ClassPool.getDefault();
            clazz = pool.makeClass(new java.io.ByteArrayInputStream(classBytecode));

            String src=
              "{         "+
              "  purr(); "+
              "}         ";

            //Add interface
            CtClass anInterface = pool.getCtClass("com.sabertiger.example.ExampleInterface");
            clazz.addInterface(anInterface);

            //Add implementation for run method
            CtMethod implementation = CtNewMethod.make(
                    CtClass.voidType,
                    "run",
                    new CtClass[0],
                    new CtClass[0],
                    src,
                    clazz);
            clazz.addMethod(implementation);

            classBytecode=clazz.toBytecode();
        } catch(Throwable e) {
            throw new Error("Failed to instrument class " + className, e);
        }
        return classBytecode;
    }

}

and the required MANIFEST.MF

Manifest-Version: 1.0
Premain-Class: com.sabertiger.instrumentation.ExampleInterfaceAdapter
Boot-Class-Path: javassist.jar

Pack everything into a jar to make it work:

jar -tf agent.jar
META-INF/MANIFEST.MF
com/sabertiger/instrumentation/ExampleInterfaceAdapter.class

Now we are able to pass Clazz to ExampleExecutor

java -javaagent:agent.jar -classpath ..\instrumentation\bin com.sabertiger.example.ExampleExecutor
Hello world