GWT -如何在收到响应后从RequestBuilder返回字符串值？

RequestBuilder requestBuilder = new RequestBuilder(RequestBuilder.GET, "http://example.com/api");
requestBuilder.setHeader("Content-Type", "application/json");

try {
    requestBuilder.sendRequest(null, new RequestCallback() {
        @Override
        public void onResponseReceived(Request request, Response response) {
            if (response.getStatusCode() == 200) {
                String responseText = response.getText();
                // 在这里处理响应的字符串值
            } else {
                // 处理请求失败的情况
            }
        }

        @Override
        public void onError(Request request, Throwable exception) {
            // 处理请求发生错误的情况
        }
    });
} catch (RequestException e) {
    // 处理请求发送异常的情况
}