这是从Google Cloud Storage下载文件的代码:

@Override
public void write(OutputStream outputStream) throws IOException {
    try {
        LOG.info(path);
        InputStream stream = new ByteArrayInputStream(GoogleJsonKey.JSON_KEY.getBytes(StandardCharsets.UTF_8));
        StorageOptions options = StorageOptions.newBuilder()
                .setProjectId(PROJECT_ID)
                .setCredentials(GoogleCredentials.fromStream(stream)).build();
        Storage storage = options.getService();
        final CountingOutputStream countingOutputStream = new CountingOutputStream(outputStream);
        byte[] read = storage.readAllBytes(BlobId.of(BUCKET, path));
        countingOutputStream.write(read);
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        outputStream.close();
    }
}

这行得通,但是这里的问题是,它必须先缓冲所有字节,然后再流回此方法的客户端。这会导致很多延迟,尤其是当存储在GCS中的文件很大时。

有没有一种方法可以从GCS获取文件,然后将直接将其流传输到OutputStream ,这里的OutputStream是用于Servlet的。

最佳答案

只是为了澄清一下,您需要OutputStream还是InputStream?一种查看方式是,将Google Cloud Storage对象中存储的数据作为文件存储,并且您有一个InputStream可以读取该文件。如果可行,请继续阅读。

Storage API中没有提供InputStreamOutputStream的现有方法。但是有2 APIs in the Cloud Storage client library公开了ReadChannel对象,该对象是从ReadableByteChannel(来自Java NIO API)扩展的。

ReadChannel reader(String bucket, String blob, BlobSourceOption... options);
ReadChannel reader(BlobId blob, BlobSourceOption... options);

一个简单的例子(取自StorageSnippets.java):
/**
   * Example of reading a blob's content through a reader.
   */
  // [TARGET reader(String, String, BlobSourceOption...)]
  // [VARIABLE "my_unique_bucket"]
  // [VARIABLE "my_blob_name"]
  public void readerFromStrings(String bucketName, String blobName) throws IOException {
    // [START readerFromStrings]
    try (ReadChannel reader = storage.reader(bucketName, blobName)) {
      ByteBuffer bytes = ByteBuffer.allocate(64 * 1024);
      while (reader.read(bytes) > 0) {
        bytes.flip();
        // do something with bytes
        bytes.clear();
      }
    }
    // [END readerFromStrings]
  }

您还可以使用 newInputStream() 方法将InputStream包装在ReadableByteChannel上。

public static InputStream newInputStream(ReadableByteChannel ch)

即使您需要OutputStream,您也应该能够将数据从InputStream复制或更好地从ReadChannel对象复制到OutputStream中。

完整的例子

将此示例运行为:PROGRAM_NAME <BUCKET_NAME> <BLOB_PATH>
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.WritableByteChannel;

import com.google.cloud.ReadChannel;
import com.google.cloud.storage.Bucket;
import com.google.cloud.storage.BucketInfo;
import com.google.cloud.storage.Storage;
import com.google.cloud.storage.StorageOptions;

/**
 * An example which reads the contents of the specified object/blob from GCS
 * and prints the contents to STDOUT.
 *
 * Run it as PROGRAM_NAME <BUCKET_NAME> <BLOB_PATH>
 */
public class ReadObjectSample {
  private static final int BUFFER_SIZE = 64 * 1024;

  public static void main(String[] args) throws IOException {
    // Instantiates a Storage client
    Storage storage = StorageOptions.getDefaultInstance().getService();

    // The name for the GCS bucket
    String bucketName = args[0];
    // The path of the blob (i.e. GCS object) within the GCS bucket.
    String blobPath = args[1];

    printBlob(storage, bucketName, blobPath);
  }

  // Reads from the specified blob present in the GCS bucket and prints the contents to STDOUT.
  private static void printBlob(Storage storage, String bucketName, String blobPath) throws IOException {
    try (ReadChannel reader = storage.reader(bucketName, blobPath)) {
      WritableByteChannel outChannel = Channels.newChannel(System.out);
      ByteBuffer bytes = ByteBuffer.allocate(BUFFER_SIZE);
      while (reader.read(bytes) > 0) {
        bytes.flip();
        outChannel.write(bytes);
        bytes.clear();
      }
    }
  }
}

09-10 08:52
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