How to invoke a method inside a external jar using reflection

Question

I want to invoke a method which inside a external .jar by using reflection.

This is a sample code which inside the jar.

public class HelloFunction implements Runnable, RequestHandler<Request> {

    @Override
    public void handle(final Request request) {
        System.out.println("handle : " + request);
    }

    @Override
    public void run() {
        System.out.println("run......");
    }
}

Then i loaded this jar in a separate java program and tried to invoke HelloFunction.handle() method. This is the sample code for that part.

public class App {

    public static void main(String[] args) {
        test();
    }

    public static void test(){

        try{

            final Class<?> functionClass = getClassFromFunctionJar("com.hello.HelloFunction");

            final Object functionClassObject = functionClass.newInstance();

            if(functionClassObject instanceof RequestHandler){

                @SuppressWarnings("unchecked")
                final RequestHandler<Object> handler = RequestHandler.class.cast(functionClassObject);

                final Object inputObject = getRequestClass(functionClass).newInstance();

                handler.handle(inputObject);


            }

        }catch(final Exception e){

            System.err.println(e.getMessage());

        }
    }

    public static Class<?> getRequestClass(final Class<?> cls) throws FunctionInvokerException{

        try{
            final Type[] types = cls.getGenericInterfaces();

            for(Type type : types){

                //check RequestHandler
                if(type.getTypeName().contains(RequestHandler.class.getName())){

                    final ParameterizedType parameterizedType = (ParameterizedType) type;

                    // [0]=> Request Type
                    final String inputClassName = parameterizedType.getActualTypeArguments()[0].getTypeName();
                    return getClassFromFunctionJar(inputClassName);

                }

            }

            throw new Exception("UNABLE_TO_FIND_REQUEST_TYPE");

        }catch(final Exception e){
            throw new FunctionInvokerException(e);
        }

    }

    private static Class<?> getClassFromFunctionJar(final String className) throws ClassNotFoundException, MalformedURLException{
        final ClassLoader classLoader = URLClassLoader.newInstance(new URL[]{new URL("file:" + "/jar-location/hello.jar")}, App.class.getClassLoader());
        return Class.forName(className, true, classLoader);
    }

}

You can see here i used getClassFromFunctionJar() method to load Class from a jar. And getRequestClass() method is used to find class type of the HelloFunction.handle() method's input parameter. Everything is fine until invoking handle() method.

Finally i got an error which says "com.hello.Request cannot be cast to com.hello.Request". Could you please help me to solve this issue.

Answer 1

The same class definition loaded by different class loaders is seen as two distinct classes by the JVM.

your code "URLClassLoader.newInstance" while get different class loaders

the first(handler#parameter) is: URLClassLoader#1 & com.hello.Request

the second(inputObject) is: URLClassLoader#2 & com.hello.Request

"com.hello.Request cannot be cast to com.hello.Request".

The actual error is

"URLClassLoader#2 com.hello.Request cannot be cast to URLClassLoader#1 com.hello.Request"

and suggest this:

private static final ClassLoader classLoader = URLClassLoader.newInstance(new URL[]{new URL("file:" + "/jar-location/hello.jar")}, App.class.getClassLoader());

private static Class<?> getClassFromFunctionJar(final String className) throws ClassNotFoundException, MalformedURLException{
    return Class.forName(className, true, classLoader);
}