从Google Cloud Storage流式传输文件

Question

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，在流回此方法的客户端之前，它必须先缓冲所有字节。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获取文件并将 其直接流式传输到OutputStream ，这里的OutputStream是用于Servlet的。

Answer 1

只是为了澄清一下，您需要一个OutputStream还是一个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InputStream或的现有方法OutputStream。但是Cloud
Storage客户端库中
有2个API，它们公开了一个ReadChannel对象ReadableByteChannel（从Java
NIO API 扩展）。

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

一个简单的例子（摘自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]
  }

您还可以使用该newInputStream()方法来包装一个InputStream比ReadableByteChannel。

public static InputStream newInputStream(ReadableByteChannel ch)

即使您需要使用OutputStream，也应该能够将InputStream或更好的ReadChannel对象中的数据复制到中OutputStream。

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();
      }
    }
  }
}