Question

我想从（Tomcat servlet）InputStream中读取并使用AsynchronousFileChannel异步地将（大）内容复制到文件中。我可以with a regular FileChannel执行此操作并阅读the missing transferTo。但是如果我使用Java 7 AsyncFileChannel，我总是得到BufferOverflowException。

    try (AsynchronousFileChannel output = AsynchronousFileChannel.open(path, StandardOpenOption.CREATE, StandardOpenOption.WRITE);
         output.lock(); // need to lock, this is one key reason to use channel

        ReadableByteChannel input = Channels.newChannel(inputStream); // servlet InputStream
        ByteBuffer buf = ByteBuffer.allocate(4096);
        int position = 0;
        int count;
        Future<Integer> lastWrite = null;
        while ((count = input.read(buf)) >= 0 || buf.position() > 0) {
            logger.info("read {} bytes", count);
            buf.flip();
            output.write(buf, position);
            if (count > 0) position += count;
            buf.compact();
        }
        if (lastWrite != null) lastWrite.get(10, TimeUnit.SECONDS);

然后在跑步时我得到

14:12:30.597 [http-bio-9090-exec-3] INFO  c.b.p.c.BlobUploadServlet - read 4096 bytes
14:12:30.597 [http-bio-9090-exec-3] INFO  c.b.p.c.BlobUploadServlet - read 0 bytes
... many more with 0 bytes read ...
14:12:30.597 [http-bio-9090-exec-3] INFO  c.b.p.c.BlobUploadServlet - read 3253 bytes
14:12:30.605 [http-bio-9090-exec-3] ERROR c.b.p.c.BlobUploadServlet - null
java.nio.BufferOverflowException: null
at java.nio.HeapByteBuffer.put(HeapByteBuffer.java:183) ~[na:1.7.0_17]
at java.nio.channels.Channels$ReadableByteChannelImpl.read(Channels.java:393) ~[na:1.7.0_17]

如何修复BufferOverflow？还有什么是在读取0字节时暂停循环并等待的正确方法？

Answer 1

原版海报太迟了，但无论如何。

我试图重现你的问题（但是样本略有不同，我借助频道复制了大文件）：

public static void main(String[] args) throws IOException,  InterruptedException, ExecutionException {
    final InputStream inputStream = new FileInputStream("/home/me/Store/largefile");
    final ReadableByteChannel inputChannel = Channels.newChannel(inputStream);
    final AsynchronousFileChannel outputChannel = AsynchronousFileChannel.open(
                    FileSystems.getDefault().getPath(
                    "/home/me/Store/output"),
                    StandardOpenOption.CREATE, StandardOpenOption.WRITE);
    outputChannel.lock();

    final ByteBuffer buffer = ByteBuffer.allocate(4096);
    int position = 0;
    int recievedBytes = 0;
    Future<Integer> lastWrite = null;

    while ((recievedBytes = inputChannel.read(buffer)) >= 0
            || buffer.position() != 0) {
        System.out.println("Recieved bytes: " + recievedBytes);
        System.out.println("Buffer position: " + buffer.position());
        buffer.flip();
        lastWrite = outputChannel.write(buffer, position);
        // do extra work while asynchronous channel is writing bytes to disk,
        // in perfect case more extra work can be done, not just simple calculations
        position += recievedBytes;
        // extra work is done, we should wait, because we use only one buffer which can be still busy
        if (lastWrite != null)  lastWrite.get();
        buffer.compact();
    }

    outputChannel.close();
    inputChannel.close();
    inputStream.close();
}

在循环的每次迭代中，我们从输入流中读取一大块数据，然后我们将此块“推送”到输出流中。当前线程不等待写入完成，它继续，所以我们可以做额外的工作。但在新迭代之前，我们应该等待写完成。尝试发表评论

if (lastWrite != null) lastWrite.get();

你会得到

java.nio.BufferOverflowException。

您的代码给了我一个使用Future来处理上次写操作的提示。但你错过了等待最后一次手术。

此外，我省略了一些补丁内容（仅为了简单起见，在处理文件时不需要额外的调整）。

如何将InputStream复制到AsynchronousFileChannel

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