如何使用GWT检查图像的给定URL是否存在？

import com.google.gwt.http.client.RequestBuilder;
import com.google.gwt.http.client.RequestCallback;
import com.google.gwt.http.client.RequestException;
import com.google.gwt.http.client.Response;

public void checkImageUrl(String url) {
    RequestBuilder requestBuilder = new RequestBuilder(RequestBuilder.HEAD, url);
    requestBuilder.setTimeout(5000);

    try {
        requestBuilder.sendRequest(null, new RequestCallback() {
            @Override
            public void onResponseReceived(Request request, Response response) {
                if (response.getStatusCode() == Response.SC_OK) {
                    // 图像存在
                } else {
                    // 图像不存在或无法访问
                }
            }

            @Override
            public void onError(Request request, Throwable exception) {
                // 请求失败
            }
        });
    } catch (RequestException e) {
        // 请求异常
    }
}