如何将InputStream复制到AsynchronousFileChannel

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.我可以使用常规的FileChannel来完成它并阅读有关丢失的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);

那时跑步我得到

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字节时暂停循环和等待的正确方法？

Answer 1

对于原始海报来说为时已晚，但无论如何。

我尝试重现您的问题（但示例略有不同，我借助通道复制了大文件）：

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块数据，然后将该块“推”到输出流中。当前线程不会等待写入完成，它会继续执行，因此我们可以做额外的工作。但在新的迭代之前我们应该等待写入完成。尝试注释掉

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

你会得到

java.nio.BufferOverflowException。

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

另外，我还省略了代码片段中的一些额外调整（只是为了简单起见，在处理文件时不需要额外调整）。