英文:
Incomplete file returned by GridFS
问题
我正在进行一个Java项目，使用GridFS规范将文件存储到MongoDB并从中检索文件。我正在使用MongoDB Java驱动文档中在<https://mongodb.github.io/mongo-java-driver/4.1/driver/tutorials/gridfs/>提供的代码片段。
在使用OpenDownloadStream检索文件时，我注意到，如果文件被分成多个块，它只返回第一个块，而不是完整的文件。
ObjectId fileId;GridFSDownloadStream downloadStream = gridFSBucket.openDownloadStream(fileId);int fileLength = (int) downloadStream.getGridFSFile().getLength();byte[] bytesToWriteTo = new byte[fileLength];downloadStream.read(bytesToWriteTo); /*读取文件内容*/downloadStream.close();System.out.println(new String(bytesToWriteTo, StandardCharsets.UTF_8));
有解决这个问题的方法吗？
英文:
I'm working on a Java project to store and retrieve files from MongoDB using GridFS specification. I'm using the code snippets provided in MongoDB Java driver documentation from <https://mongodb.github.io/mongo-java-driver/4.1/driver/tutorials/gridfs/>.
While using OpenDownloadStream to retrieve the file, I noticed that if the file is divided into more than one chunks, it returns only the first chunk, and not the full file.
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));
Any solutions to this?
答案1
得分: 2
看一下实现了GridFSDownloadStream接口的GridFSDownloadStreamImpl类，它似乎是通过分块来实现read(byte[])方法：
@Overridepublic int read(final byte[] b) {return read(b, 0, b.length);}@Overridepublic 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，你必须循环直到实际读取了所有预期的字节：
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，我无法对上述代码进行测试，请谨慎使用。
英文:
Looking at the class GridFSDownloadStreamImpl which implements GridFSDownloadStream, it looks like the method read(byte[]) reads chunk by chunk:
@Overridepublic int read(final byte[] b) {return read(b, 0, b.length);}@Overridepublic 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;}
Therefore, you have to loop until all expected bytes are actually read:
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();
Note that I was not able to test above code so please use with caution.
答案2
得分: 1
我最终使用了 readAllBytes() 方法，它会返回整个文件。
GridFSDownloadStream downloadStream = gridFSBucket.openDownloadStream(fileId);int fileLength = (int) downloadStream.getGridFSFile().getLength();byte[] bytesToWriteTo = new byte[fileLength];bytesToWriteTo = downloadStream.readAllBytes();downloadStream.close();
英文:
I ended up using readAllBytes() method and it returns the whole file.
GridFSDownloadStream downloadStream = gridFSBucket.openDownloadStream(fileId);int fileLength = (int) downloadStream.getGridFSFile().getLength();byte[] bytesToWriteTo = new byte[fileLength];bytesToWriteTo = downloadStream.readAllBytes();downloadStream.close();
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