GridFS返回的文件不完整

问题描述

我正在研究一个Java项目，以使用GridFS规范从MongoDB存储和检索文件。我正在使用https://mongodb.github.io/mongo-java-driver/4.1/driver/tutorials/gridfs/中的MongoDB Java驱动程序文档中提供的代码段。

在使用 OpenDownloadStream 检索文件时，我注意到，如果将文件分为多个块，则它仅返回第一个块，而不返回完整文件。

ObjectId fileId;

GridFSDownloadStream downloadStream = gridFSBucket.openDownloadStream(fileId);
int fileLength = (int) downloadStream.getGridFSFile().getLength();
byte[] bytesToWriteto = new byte[fileLength];
downloadStream.read(bytesToWriteto);    /*read file contents */
downloadStream.close();

System.out.println(new String(bytesToWriteto,StandardCharsets.UTF_8));

有什么解决办法吗？

解决方法

看一下实现GridFSDownloadStreamImpl的类GridFSDownloadStream，看来方法read(byte[])会逐块读取：

@Override
public int read(final byte[] b) {
    return read(b,b.length);
}

@Override
public int read(final byte[] b,final int off,final int len) {
    checkClosed();

    if (currentPosition == length) {
        return -1;
    } else if (buffer == null) {
        buffer = getBuffer(chunkIndex);
    } else if (bufferOffset == buffer.length) {
        chunkIndex += 1;
        buffer = getBuffer(chunkIndex);
        bufferOffset = 0;
    }

    int r = Math.min(len,buffer.length - bufferOffset);
    System.arraycopy(buffer,bufferOffset,b,off,r);
    bufferOffset += r;
    currentPosition += r;
    return r;
}

因此，您必须循环执行，直到实际读取了所有预期的字节为止：

byte[] bytesToWriteTo = new byte[fileLength];
int bytesRead = 0;
while(bytesRead < fileLength) {
    int newBytesRead = downloadStream.read(bytesToWriteTo);
    if(newBytesRead == -1) {
        throw new Exception();
    }
    bytesRead += newBytesRead;
}
downloadStream.close();

请注意，我无法测试上述代码，因此请谨慎使用。

,

我最终使用了 readAllBytes（）方法，它返回了整个文件。

GridFSDownloadStream downloadStream = gridFSBucket.openDownloadStream(fileId);
int fileLength = (int) downloadStream.getGridFSFile().getLength();
byte[] bytesToWriteTo = new byte[fileLength];
bytesToWriteTo = downloadStream.readAllBytes();
downloadStream.close();