JcloudsObjectWritingByteArrayOutputStream.java

import java.io.IOException;

import com.google.common.base.Preconditions;
import com.google.common.util.concurrent.UncheckedExecutionException;

import org.apache.commons.io.output.ByteArrayOutputStream;
import org.jclouds.blobstore.BlobStore;
import org.jclouds.blobstore.domain.Blob;

/**
 * An OutputStream that buffers the contents in memory, and writes them to an object store when the stream is closed.
 */
public class JcloudsObjectWritingByteArrayOutputStream extends ByteArrayOutputStream {
    // prevents the blob from being written to the blob store multiple times, if close() is called more than once
    private boolean isBlobWritten;

    private final BlobStore blobStore;
    private final String containerName;
    private final String blobName;

    public JcloudsObjectWritingByteArrayOutputStream(BlobStore blobStore, String containerName, String blobName) {
        this.blobStore = Preconditions.checkNotNull(blobStore);
        this.containerName = Preconditions.checkNotNull(containerName);
        this.blobName = Preconditions.checkNotNull(blobName);
    }

    // the client will have to call this when he's finished writing, so this is our chance to upload the blob,
    // now that we have the full payload in memory
    @Override
    public void close() throws IOException {
        if (!this.isBlobWritten) {
            writeBytesToBlob();
        }

        super.close();
    }

    private void writeBytesToBlob() throws IOException {
        byte[] payload = toByteArray();

        try {
            Blob blob = this.blobStore.blobBuilder(this.blobName)
                                      .payload(payload)
                                      .contentLength(payload.length)
                                      .build();
            this.blobStore.putBlob(this.containerName, blob);
        }
        catch (UncheckedExecutionException e) {  // this exception is not useful, so grab the cause
            throw new IOException(getExceptionMessage(), e.getCause());
        }
        catch (Exception e) {
            throw new IOException(getExceptionMessage(), e);
        }
        this.isBlobWritten = true;
    }

    private String getExceptionMessage() {
        return String.format("Failed to write the resource! object store: %s, container: %s, resource: %s",
                             this.blobStore.toString(), this.containerName, this.blobName);
    }
}