ScatteringByteChannel和GatheringByteChannel

分散和集聚 通道

分散/聚集 I/O 是使用多個而不是單個緩衝區來保存數據的讀寫方法。

一個分散的讀取就像一個常規通道讀取，只不過它是將數據讀到一個緩衝區數組中而不是讀到單個緩衝區中。同樣地，一個聚集寫入是向緩衝區數組而不是向單個緩衝區寫入數據。

分散/聚集 I/O 對於將數據流劃分爲單獨的部分很有用，這有助於實現複雜的數據格式。

源代碼如下：

A channel that can read bytes into a sequence of buffers. * * <p> A <i>scattering</i> read operation reads, in a single invocation, a * sequence of bytes into one or more of a given sequence of buffers. * Scattering reads are often useful when implementing network protocols or * file formats that, for example, group data into segments consisting of one * or more fixed-length headers followed by a variable-length body. Similar * <i>gathering</i> write operations are defined in the {@link * GatheringByteChannel} interface. </p> * * * @author Mark Reinhold * @author JSR-51 Expert Group * @version 1.15, 06/04/19 * @since 1.4 */ public interface ScatteringByteChannel extends ReadableByteChannel { /** * Reads a sequence of bytes from this channel into a subsequence of the * given buffers. * * <p> An invocation of this method attempts to read up to <i>r</i> bytes * from this channel, where <i>r</i> is the total number of bytes remaining * the specified subsequence of the given buffer array, that is, * * <blockquote><pre> * dsts[offset].remaining() * + dsts[offset+1].remaining() * + ... + dsts[offset+length-1].remaining()</pre></blockquote> * * at the moment that this method is invoked. * * <p> Suppose that a byte sequence of length <i>n</i> is read, where * <tt>0</tt> <tt><=</tt> <i>n</i> <tt><=</tt> <i>r</i>. * Up to the first <tt>dsts[offset].remaining()</tt> bytes of this sequence * are transferred into buffer <tt>dsts[offset]</tt>, up to the next * <tt>dsts[offset+1].remaining()</tt> bytes are transferred into buffer * <tt>dsts[offset+1]</tt>, and so forth, until the entire byte sequence * is transferred into the given buffers. As many bytes as possible are * transferred into each buffer, hence the final position of each updated * buffer, except the last updated buffer, is guaranteed to be equal to * that buffer's limit. * * <p> This method may be invoked at any time. If another thread has * already initiated a read operation upon this channel, however, then an * invocation of this method will block until the first operation is * complete. </p> * * @param dsts * The buffers into which bytes are to be transferred * * @param offset * The offset within the buffer array of the first buffer into * which bytes are to be transferred; must be non-negative and no * larger than <tt>dsts.length</tt> * * @param length * The maximum number of buffers to be accessed; must be * non-negative and no larger than * <tt>dsts.length</tt> - <tt>offset</tt> * * @return The number of bytes read, possibly zero, * or <tt>-1</tt> if the channel has reached end-of-stream * * @throws IndexOutOfBoundsException * If the preconditions on the <tt>offset</tt> and <tt>length</tt> * parameters do not hold * * @throws NonReadableChannelException * If this channel was not opened for reading * * @throws ClosedChannelException * If this channel is closed * * @throws AsynchronousCloseException * If another thread closes this channel * while the read operation is in progress * * @throws ClosedByInterruptException * If another thread interrupts the current thread * while the read operation is in progress, thereby * closing the channel and setting the current thread's * interrupt status * * @throws IOException * If some other I/O error occurs */ public long read(ByteBuffer[] dsts, int offset, int length) throws IOException; /** * Reads a sequence of bytes from this channel into the given buffers. * * <p> An invocation of this method of the form <tt>c.read(dsts)</tt> * behaves in exactly the same manner as the invocation * * <blockquote><pre> * c.read(dsts, 0, dsts.length);</pre></blockquote> * * @param dsts * The buffers into which bytes are to be transferred * * @return The number of bytes read, possibly zero, * or <tt>-1</tt> if the channel has reached end-of-stream * * @throws NonReadableChannelException * If this channel was not opened for reading * * @throws ClosedChannelException * If this channel is closed * * @throws AsynchronousCloseException * If another thread closes this channel * while the read operation is in progress * * @throws ClosedByInterruptException * If another thread interrupts the current thread * while the read operation is in progress, thereby * closing the channel and setting the current thread's * interrupt status * * @throws IOException * If some other I/O error occurs */ public long read(ByteBuffer[] dsts) throws IOException; } 

這些 long read() 方法很像標準的 read 方法，只不過它們不是取單個緩衝區而是取一個緩衝區數組。

在 分散讀取 中，通道依次填充每個緩衝區。填滿一個緩衝區後，它就開始填充下一個。在某種意義上，緩衝區數組就像一個大緩衝區。

分散/聚集 I/O 對於將數據劃分爲幾個部分很有用。例如，您可能在編寫一個使用消息對象的網絡應用程序，每一個消息被劃分爲固定長度的頭部和固定長度的正文。您可以創建一個剛好可以容納頭部的緩衝區和另一個剛好可以容難正文的緩衝區。當您將它們放入一個數組中並使用分散讀取來向它們讀入消息時，頭部和正文將整齊地劃分到這兩個緩衝區中。

我們從緩衝區所得到的方便性對於緩衝區數組同樣有效。因爲每一個緩衝區都跟蹤自己還可以接受多少數據，所以分散讀取會自動找到有空間接受數據的第一個緩衝區。在這個緩衝區填滿後，它就會移動到下一個緩衝區。

 

 

ScatteringByteChannel是一個藉口，對應的File流、TCP、UDP及管道有四個實現，FileChannel、DatagramChannel、SocketChannel及Pipe$SourceChannel，分別在不同場景使用。