dynamic-method-handler

It shows that OpenFeign how is work and how integrate with Spring Framework.

"Feign makes writing java http clients easier"

Feign, How makes it this?

There is a pattern when making http request. There is a pattern when making http request. That are Marshalling and request and response and unmarshalling.

Basically, Feing uses the java.lang.reflect.Proxy class for make http requests easier.

"Proxy provides static methods for creating objects that act like instances of interfaces but allow for customized method invocation. A proxy class is a class created at runtime that implements a specified list of interfaces, known as proxy interfaces. A proxy instance is an instance of a proxy class. Each proxy instance has an associated invocation handler object, which implements the interface InvocationHandler. "

I need an Interface;

public interface MaleGenderInterface {
    @Gender(name = "Bay")
    String hello(String name);
}

public interface FemaleGenderInterface {
    @Gender(name = "Bayan")
    String hello(String name);
}

@Gender provides parameterization to us.

Next one is InvocationHandler;

public class CustomInvocationHandler implements InvocationHandler {

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) {
        if (args != null) {
            for (Object arg : args)
                System.out.println("args = " + arg);
        }

        final var methodAnnotation = method.getAnnotation(Gender.class);

        System.out.println("Invoked method:" + method.getName());
        return "Merhaba " + methodAnnotation.name() + " " + args[0] + "!";
    }
}

CustomInvocationHandler handles method invocation of Interfaces. We can get additional data from method annotations. For example with @Gender.

Tester

import java.lang.reflect.Proxy;

/**
 * @author dilaverdemirel
 * @since 28/07/2022
 */
public class Tester {
    public static void main(String[] args) {
        MaleGenderInterface maleGenderInterface = (MaleGenderInterface) Proxy.newProxyInstance(
                MaleGenderInterface.class.getClassLoader(),
                new Class[]{MaleGenderInterface.class},
                new CustomInvocationHandler());
        System.out.println("result = " + maleGenderInterface.hello("Dilaver"));

        FemaleGenderInterface femaleGenderInterface = (FemaleGenderInterface) Proxy.newProxyInstance(
                FemaleGenderInterface.class.getClassLoader(),
                new Class[]{FemaleGenderInterface.class},
                new CustomInvocationHandler());
        System.out.println("result = " + femaleGenderInterface.hello("Derya"));
    }
}

I am binding Interface and InvocationHandler, above and invoke hello methods.

Output:

args = Dilaver
Invoked method:hello
result = Merhaba Bay Dilaver!
args = Derya
Invoked method:hello
result = Merhaba Bayan Derya!

Spring Integration

@Configuration
public class Config {

    @Autowired
    private ApplicationContext applicationContext;

    @Bean
    public MaleGenderInterface maleGenderInterface() {
        return (MaleGenderInterface) Proxy.newProxyInstance(
                MaleGenderInterface.class.getClassLoader(),
                new Class[]{MaleGenderInterface.class},
                new CustomInvocationHandler());
    }

    @Bean
    public FemaleGenderInterface femaleGenderInterface() {
        return (FemaleGenderInterface) Proxy.newProxyInstance(
                FemaleGenderInterface.class.getClassLoader(),
                new Class[]{FemaleGenderInterface.class},
                new CustomInvocationHandler());
    }
}

@Component
public class DemoBean {
    @Autowired
    private MaleGenderInterface maleGenderInterface;

    @Autowired
    private FemaleGenderInterface femaleGenderInterface;

    public void test() {
        System.out.println(maleGenderInterface.hello("Dilaver"));
        System.out.println(femaleGenderInterface.hello("Derya"));
    }
}

Output:

args = Dilaver
Invoked method:hello
Merhaba Bay Dilaver!
args = Derya
Invoked method:hello
Merhaba Bayan Derya!