在Spring Security身份验证中避免使用字符串，可以改为使用RestTemplate进行调用。下面是一个示例解决方案：

首先，创建一个类来处理身份验证调用，例如AuthenticationClient：

import org.springframework.http.*;
import org.springframework.stereotype.Component;
import org.springframework.web.client.RestTemplate;

@Component
public class AuthenticationClient {

    private final RestTemplate restTemplate;

    public AuthenticationClient(RestTemplate restTemplate) {
        this.restTemplate = restTemplate;
    }

    public ResponseEntity authenticate(String username, String password) {
        String url = "http://your-authentication-service-url";
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_JSON);

        // 创建身份验证请求对象
        AuthenticationRequest request = new AuthenticationRequest(username, password);

        // 构建请求体
        HttpEntity entity = new HttpEntity<>(request, headers);

        // 发送POST请求进行身份验证
        return restTemplate.exchange(url, HttpMethod.POST, entity, String.class);
    }
}

然后，在Spring Security的配置类中注入AuthenticationClient，并使用它来进行身份验证：

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Configuration;
import org.springframework.security.authentication.AuthenticationProvider;
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.configuration.EnableWebSecurity;
import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;

@Configuration
@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {

    private final AuthenticationClient authenticationClient;

    @Autowired
    public SecurityConfig(AuthenticationClient authenticationClient) {
        this.authenticationClient = authenticationClient;
    }

    @Override
    protected void configure(AuthenticationManagerBuilder auth) throws Exception {
        auth.authenticationProvider(authenticationProvider());
    }

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.authorizeRequests()
                .anyRequest().authenticated()
                .and()
                .formLogin();
    }

    private AuthenticationProvider authenticationProvider() {
        return (authentication) -> {
            String username = authentication.getName();
            String password = authentication.getCredentials().toString();

            // 使用AuthenticationClient进行身份验证
            ResponseEntity response = authenticationClient.authenticate(username, password);

            // 根据响应结果设置身份验证成功或失败
            if (response.getStatusCode() == HttpStatus.OK) {
                return new UsernamePasswordAuthenticationToken(username, password, null);
            } else {
                throw new BadCredentialsException("Authentication failed");
            }
        };
    }
}

在上述示例中，AuthenticationClient类负责与身份验证服务进行通信，并发送POST请求进行身份验证。SecurityConfig类中使用AuthenticationClient来进行身份验证，如果验证成功，则返回一个UsernamePasswordAuthenticationToken，否则抛出一个BadCredentialsException。

请注意替换示例中的"your-authentication-service-url"为实际的身份验证服务URL，并根据需要修改身份验证请求和响应的数据结构。