ServletRequestMethodArgumentResolver issue

ResolveArgumentTest.java

package com.example;

import java.lang.reflect.Method;
import java.security.Principal;

import org.junit.Assert;
import org.junit.Test;
import org.springframework.core.MethodParameter;
import org.springframework.security.core.Authentication;

public class ResolveArgumentTest {
    // The Principal.class.isAssignableFrom(...) check in ServletRequestMethodArgumentResolver.resolveArgument(...) can result in
    // it returning a value that clearly cannot be assigned to the parameter in question.
    // This test boils things down to the minimum - we have:
    // * a method that wants an Authentication instance
    // * a request whose getUserPrincipal() something that isn't an instance of Authentication.
    // * a cut down version of resolveArgument(...)
    // The test fails because the resolveArgument(...) logic thinks we can satisfy our need for an Authentication instance with
    // the result of calling getUserPrincipal().
    @Test
    public void testResolveArgument() throws NoSuchMethodException, SecurityException {
        DummyRequest request = new DummyRequest();
        Method method = getClass().getMethod("myRequest", Authentication.class);
        MethodParameter parameter = new MethodParameter(method, 0);
        
        Object arg = resolveArgument(parameter, request);
        
        try {
            method.invoke(this, arg);
        } catch (Exception e) {
            Assert.fail(e.getMessage());
        }
    }
    
    private Object resolveArgument(MethodParameter parameter, DummyRequest request) {
        Class<?> paramType = parameter.getParameterType();
        
        if (Principal.class.isAssignableFrom(paramType)) {
            return request.getUserPrincipal();
        } else {
            throw new AssertionError("paramType of unexpected type");
        }
    }
    
    public void myRequest(Authentication authentication) {
        System.out.println("success");
    }
    
    private static class DummyRequest {
        private final Principal userPrincipal = new AccountPrincipal();
        
        public Principal getUserPrincipal() {
            return userPrincipal;
        }
    }
    
    private static class AccountPrincipal implements Principal {
        @Override
        public String getName() {
            return null;
        }
    }
}