要实现不需要将用户重定向到外部网站的OAuth2，可以使用OAuth2的授权码模式（authorization code flow）和后端流程（backend process）来实现。

以下是一个使用Java和Spring Boot框架的示例代码：

1. 配置OAuth2客户端信息

@Configuration
@EnableWebSecurity
public class OAuth2Config extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http
            .authorizeRequests()
            .antMatchers("/callback").permitAll()
            .anyRequest().authenticated()
            .and()
            .oauth2Login()
            .authorizationEndpoint()
            .baseUri("/oauth2/authorize")
            .and()
            .redirectionEndpoint()
            .baseUri("/callback/*")
            .and()
            .tokenEndpoint()
            .accessTokenResponseClient(this.accessTokenResponseClient());
    }

    private OAuth2AccessTokenResponseClient accessTokenResponseClient() {
        DefaultAuthorizationCodeTokenResponseClient client = new DefaultAuthorizationCodeTokenResponseClient();
        client.setRequestEntityConverter(new CustomRequestEntityConverter());
        return client;
    }
}

2. 自定义请求转换器

public class CustomRequestEntityConverter implements Converter> {

    private final OAuth2AuthorizationCodeGrantRequestEntityConverter defaultConverter = new OAuth2AuthorizationCodeGrantRequestEntityConverter();

    @Override
    public RequestEntity convert(OAuth2AuthorizationCodeGrantRequest request) {
        RequestEntity entity = this.defaultConverter.convert(request);
        HttpHeaders headers = new HttpHeaders();
        headers.addAll(entity.getHeaders());
        headers.add("Authorization", "Basic " + Base64Utils.encodeToString("client-id:client-secret".getBytes())); // 替换为实际的客户端ID和密码
        return new RequestEntity<>(entity.getBody(), headers, entity.getMethod(), entity.getUrl());
    }
}

3. 处理回调URL的控制器

@RestController
public class CallbackController {

    @PostMapping("/callback")
    public ResponseEntity handleCallback(@RequestBody Map params) {
        // 在这里处理回调URL的参数，获取访问令牌并进行后续操作
        String code = params.get("code");
        // 获取访问令牌...
        return ResponseEntity.ok("Success");
    }
}

使用以上代码，用户不会被重定向到外部网站进行授权，而是在后端完成OAuth2的授权码流程，并在回调URL中获取访问令牌进行后续操作。请注意，代码中的"client-id"和"client-secret"需要替换为实际的客户端ID和密码。