问题描述
我有一个使用GCS存储文件上传文件的案例。我想通过使用世代和先决条件 (x-goog-if-generation-match
) 来避免竞争条件。
但是,我使用的是 Java API,并且文档仅显示 json/xml api。
解决方法
我找到的解决方案是这样做的:
Blob object =
storage.get(
bucketName,filePath,Storage.BlobGetOption.fields(BlobField.GENERATION));
long generation = object.getGeneration();
// now do some other stuff that might cause race conditions...
BlobId blobId = BlobId.of(bucketName,objectName,generation);
BlobInfo blobInfo = BlobInfo.newBuilder(blobId).setContentType("text/plain").build();
List<Storage.BlobWriteOption> blobWriteOptions = new ArrayList<>();
blobWriteOptions.add(BlobWriteOption.generationMatch());
// in case generation don't match this will throw "StorageException: 412 Precondition Failed"
try (WriteChannel writer =
storage.writer(blobInfo,blobWriteOptions.toArray(new BlobWriteOption[0]))) {
writer.write(); // etc'
}
}