Spring boot,Spring Securityを使用したform認証とGoogle OAuth2認証を併用したい

私が無知の為、大変ご面倒おかけしますが、力を貸して下さい。

実現したいこと

SpringSecurityを使用して、同じログイン画面でform認証とGoogle OAuth2認証を併用したいです。

起こっている問題

Google OAuth2認証はログイン出来、画面遷移するが、form認証を行うと、以下のエラーメッセージが現れてログインが出来ない。

ErrorMessage
java.lang.IllegalStateException: Current user principal is not of type [org.springframework.security.oauth2.client.authentication.OAuth2AuthenticationToken]: UsernamePasswordAuthenticationToken [Principal=com.test.main.AuthUserDetails@62f854e2, Credentials=[PROTECTED], Authenticated=true, Details=WebAuthenticationDetails [RemoteIpAddress=0:0:0:0:0:0:0:1, SessionId=9793C4B6FBC9D3FA656F567410ED74E3], Granted Authorities=[USER]]
	at org.springframework.web.servlet.mvc.method.annotation.ServletRequestMethodArgumentResolver.resolveArgument(ServletRequestMethodArgumentResolver.java:169) ~[spring-webmvc-5.3.13.jar:5.3.13]
	at org.springframework.web.servlet.mvc.method.annotation.ServletRequestMethodArgumentResolver.resolveArgument(ServletRequestMethodArgumentResolver.java:124) ~[spring-webmvc-5.3.13.jar:5.3.13]
	at org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:121) ~[spring-web-5.3.13.jar:5.3.13]

エラーが出ている該当のソースコード

Java:AuthUserDetails.java
package com.test.main;

import java.util.Collection;

import org.springframework.security.core.GrantedAuthority;
import org.springframework.security.core.userdetails.UserDetails;

public class AuthUserDetails implements UserDetails {
	
	private AuthEntity auth;
	private Collection<GrantedAuthority> authorities;
	
	public AuthUserDetails() {}
	
	public AuthUserDetails(AuthEntity auth, Collection<GrantedAuthority> authorities) {
		this.auth = auth;
		this.authorities = authorities;
	}　←ここでエラー発生
	
	@Override
	public Collection<? extends GrantedAuthority> getAuthorities() {
		return this.authorities;
	}

	@Override
	public String getPassword() {
		return auth.getPassword();
	}

	@Override
	public String getUsername() {
		return auth.getUsername();
	}

	@Override
	public boolean isAccountNonExpired() {
		return true;
	}

	@Override
	public boolean isAccountNonLocked() {
		return true;
	}

	@Override
	public boolean isCredentialsNonExpired() {
		return true;
	}

	@Override
	public boolean isEnabled() {
		return true;
	}
}

関係があると思っているソースコード

HTML:login.html
略
<body>
	<div>
		<div>
			<form action="#" th:action="@{/login}" method="post" th:object="${loginForm}" id="loginF">
				<input type="text" id="username" name="username" placeholder="メールアドレス" /> 
               <input type="password" id="password" name="password" placeholder="パスワード" />
				<button type = "submit">
			</form>
			<div>
				<a th:href="@{/oauth2/authorization/google}" class="anchor"><span>Googleアカウントでログイン</span></a>
			</div>	
		</div>
	</div>
</body>
</html>

Java:DbSecurityConfig.java
package com.test.main;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.security.config.annotation.authentication.builders.AuthenticationManagerBuilder;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.config.annotation.web.builders.WebSecurity;
import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity;
import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;
import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
import org.springframework.security.crypto.password.PasswordEncoder;
import org.springframework.security.web.firewall.DefaultHttpFirewall;

import com.komesuku.main.model.service.AuthAccountService;

@EnableWebSecurity
public class DbSecurityConfig extends WebSecurityConfigurerAdapter {

	@Autowired
	private AuthAccountService userDetailsService;
	
    @Bean
    public PasswordEncoder passwordEncoder() {
        return new BCryptPasswordEncoder();
    }

    @Override
	public void configure(WebSecurity web) {
        DefaultHttpFirewall firewall = new DefaultHttpFirewall();
        web.httpFirewall(firewall);
    }
    
    @Override
    protected void configure(HttpSecurity http) throws Exception {
    	http.authorizeRequests().antMatchers("/css/**", "/png/**", "/js/**", "/regist", "/forgot", "/message", "/registPass", "/registPassDone", "/reYesterday").permitAll()
    			.anyRequest().authenticated()
    			.and()
    			.formLogin()
    			.loginPage("/login").permitAll()
    			.defaultSuccessUrl("/success",true)
    			.and()
    			.oauth2Login()
    			.loginPage("/login").permitAll()
    			.defaultSuccessUrl("/success",true)
    			.and()
    			.logout().logoutUrl("/logout").logoutSuccessUrl("/login").permitAll();
    	http.requiresChannel().anyRequest().requiresSecure();
    }
    
    @Override
    protected void configure(AuthenticationManagerBuilder auth) throws Exception {
    	auth.userDetailsService(userDetailsService).passwordEncoder(passwordEncoder());
    }
}

Java:AuthAccountService.java
package com.test.main.model.service;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.security.core.authority.AuthorityUtils;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.core.userdetails.UserDetailsService;
import org.springframework.security.core.userdetails.UsernameNotFoundException;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;
import org.springframework.util.ObjectUtils;

import com.komesuku.main.AuthEntity;
import com.komesuku.main.AuthMapper;
import com.komesuku.main.AuthUserDetails;

@Service
public class AuthAccountService implements UserDetailsService {

	@Autowired
	private AuthMapper mapper;
	
	@Autowired
    JdbcTemplate jdbcTemplate;
	
	@Override
	public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
		if(ObjectUtils.isEmpty(username)) {
			throw new UsernameNotFoundException("ユーザー名を入力してください");
		}
		AuthEntity auth = mapper.findByUsername(username);
		if(auth == null) {
			throw new UsernameNotFoundException("ユーザーが見つかりません");
		}
		
		return new AuthUserDetails(auth, AuthorityUtils.createAuthorityList("USER"));
		
	}
}

Java:AuthEntity.java
package com.test.main;

import javax.validation.constraints.NotBlank;
import javax.validation.constraints.Size;

import lombok.Data;

@Data
public class AuthEntity {
	
	public AuthEntity(String username, String password) {
		this.username = username;
		this.password = password;
	}
	
	@NotBlank(message="必須項目です")
	@Size(max=50,message="必須項目です")
	private String username;
	
	@NotBlank(message="必須項目です")
	@Size(max=100,message="必須項目です")
	private String password;

}