如果Apache HttpClient在使用凭证提供程序时未发送身份验证头，可以尝试使用以下代码示例解决该问题：

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.auth.AuthScope;
import org.apache.http.auth.Credentials;
import org.apache.http.auth.UsernamePasswordCredentials;
import org.apache.http.client.CredentialsProvider;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.BasicCredentialsProvider;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

public class HttpClientExample {

    public static void main(String[] args) throws Exception {
        CredentialsProvider provider = new BasicCredentialsProvider();
        Credentials credentials = new UsernamePasswordCredentials("username", "password");
        provider.setCredentials(AuthScope.ANY, credentials);

        CloseableHttpClient httpClient = HttpClients.custom()
                .setDefaultCredentialsProvider(provider)
                .build();

        String url = "http://example.com/api/endpoint";
        HttpGet httpGet = new HttpGet(url);

        HttpResponse response = httpClient.execute(httpGet);
        HttpEntity entity = response.getEntity();

        if (entity != null) {
            String result = EntityUtils.toString(entity);
            System.out.println(result);
        }
    }
}

在上述代码中，我们创建了一个CredentialsProvider实例，并将用户名和密码设置为凭证。然后，我们使用HttpClients.custom()方法创建一个自定义的CloseableHttpClient实例，并将CredentialsProvider设置为默认凭证提供程序。

接下来，我们创建一个HttpGet实例，并执行HTTP请求。最后，我们从响应实体中获取响应内容。

确保将username和password替换为实际的用户名和密码，以及将http://example.com/api/endpoint替换为实际的API端点URL。

这样，Apache HttpClient将在请求中包含身份验证头，以便进行身份验证。