移动端上传回调，怎么获得上传的url

PutObjectRequest put = new PutObjectRequest(testBucket, testObject, uploadFilePath);

ObjectMetadata metadata = new ObjectMetadata();
metadata.setContentType("application/octet-stream");

put.setMetadata(metadata);

put.setCallbackParam(new HashMap() {

{
    put("callbackUrl", "110.75.82.106/mbaas/callback");
    put("callbackBody", "test");
}

});

OSSAsyncTask task = oss.asyncPutObject(put, new OSSCompletedCallback() {

@Override
public void onSuccess(PutObjectRequest request, PutObjectResult result) {
    Log.d("PutObject", "UploadSuccess");
    // 只有设置了servercallback，这个值才有数据
    String serverCallbackReturnJson = result.getServerCallbackReturnBody();
}

网上查找回调都是这个代码 ， 看了半天文档就知道要添加一个callback，但是怎么添加，还需要json，这实在 看不懂 ，希望大牛帮助一下，谢谢