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java之通过FileChannel实现文件复制

1、FileChanel介绍

Java NIO FileChannel是连接文件的通道，从文件中读取数据和将数据写入文件。Java NIO FileChannel类是NIO用于替代使用标准Java IO API读取文件的方法。

FileInputStream的getChannel方法获取的文件通道是只读的，当然通过FileOutputStream的getChannel的方法获取的文件通道是可写的

部分API

1）、truncate截断文件功能

 /**
 * Truncates the file underlying this channel to a given size. Any bytes
 * beyond the given size are removed from the file. If there are no bytes
 * beyond the given size then the file contents are unmodified.
 * 
 * If the file position is currently greater than the given size, then it is
 * set to the new size.
 *
 * @param size
 * the maximum size of the underlying file.
 * @throws IllegalArgumentException
 * if the requested size is negative.
 * @throws ClosedChannelException
 * if this channel is closed.
 * @throws NonWritableChannelException
 * if the channel cannot be written to.
 * @throws IOException
 * if another I/O error occurs.
 * @return this channel.
 */
 public abstract FileChannel truncate(long size) throws IOException;

2）、force()强制在内存中的数据刷新到硬盘中去

 /**
 * Requests that all updates to this channel are committed to the storage
 * device.
 * 
 * When this method returns, all modifications made to the platform file
 * underlying this channel have been committed if the file resides on a
 * local storage device. If the file is not hosted locally, for example on a
 * networked file system, then applications cannot be certain that the
 * modifications have been committed.
 * 
 * There are no assurances given that changes made to the file using methods
 * defined elsewhere will be committed. For example, changes made via a
 * mapped byte buffer may not be committed.
 * 
 * The <code>metadata</code> parameter indicates whether the update should
 * include the file's metadata such as last modification time, last access
 * time, etc. Note that passing <code>true</code> may invoke an underlying
 * write to the operating system (if the platform is maintaining metadata
 * such as last access time), even if the channel is opened read-only.
 *
 * @param metadata
 * {@code true} if the file metadata should be flushed in
 * addition to the file content, {@code false} otherwise.
 * @throws ClosedChannelException
 * if this channel is already closed.
 * @throws IOException
 * if another I/O error occurs.
 */
 public abstract void force(boolean metadata) throws IOException;

3）、transferFrom可以看出是拷贝从源的position位置的count 字节大小

 /**
 * Reads up to {@code count} bytes from {@code src} and stores them in this
 * channel's file starting at {@code position}. No bytes are transferred if
 * {@code position} is larger than the size of this channel's file. Less
 * than {@code count} bytes are transferred if there are less bytes
 * remaining in the source channel or if the source channel is non-blocking
 * and has less than {@code count} bytes immediately available in its output
 * buffer.
 * 
 * Note that this channel's position is not modified.
 *
 * @param src
 * the source channel to read bytes from.
 * @param position
 * the non-negative start position.
 * @param count
 * the non-negative number of bytes to transfer.
 * @return the number of bytes that are transferred.
 * @throws IllegalArgumentException
 * if the parameters are invalid.
 * @throws NonReadableChannelException
 * if the source channel is not readable.
 * @throws NonWritableChannelException
 * if this channel is not writable.
 * @throws ClosedChannelException
 * if either channel has already been closed.
 * @throws AsynchronousCloseException
 * if either channel is closed by other threads during this
 * operation.
 * @throws ClosedByInterruptException
 * if the thread is interrupted during this operation.
 * @throws IOException
 * if any I/O error occurs.
 */
 public abstract long transferFrom(ReadableByteChannel src, long position,
 long count) throws IOException;

2、复制文件常用方法

1、通过普通输入输出流复制文件

 public void copyFile(File srcFile, File dstFile) throws FileNotFoundException {
 InputStream inputStream = null;
 OutputStream outputStream = null;
 try {
 inputStream = new BufferedInputStream(new FileInputStream(srcFile));
 outputStream = new BufferedOutputStream(new FileOutputStream(dstFile));
 byte[] bytes = new byte[1024];
 int i;
 //读取到输入流数据，然后写入到输出流中去，实现复制
 while ((i = inputStream.read(bytes)) != -1) {
 outputStream.write(bytes, 0, i);
 }
 } catch (Exception e) {
 e.printStackTrace();
 } finally {
 try {
 if (inputStream != null)
 inputStream.close();
 if (outputStream != null)
 outputStream.close();
 } catch (IOException e) {
 e.printStackTrace();
 }
 }
 }

2、通过 FileChannel复制文件

 public void copyFile(File srcFile, File dstFile) throws IOException {
 if (srcFile == null || !srcFile.exists()) {
 return;
 }
 if (dstFile == null || !dstFile.exists()) {
 return;
 }

 FileInputStream fileIns = null;
 FileOutputStream fileOuts = null;
 FileChannel source = null;
 FileChannel destination = null;

 try {
 fileIns = new FileInputStream(srcFile);
 fileOuts = new FileOutputStream(dstFile);
 source = fileIns.getChannel();
 destination = fileOuts.getChannel();
 destination.transferFrom(source, 0, source.size());
 } catch (Exception e) {
 e.printStackTrace();
 } finally {
 if (fileIns != null)
 fileIns.close();
 if (fileOuts != null)
 fileOuts.close();
 if (source != null)
 source.close();
 if (destination != null)
 destination.close();
 }
 }

3、总结

一般复制使用输入输出流进行操作，用源文件创建出一个输入流，用目标文件创建出一个输出流，把输入流的数据读取写入到输出流，用fileChannel，直接连接输入输出流的文件通道，将数据直接写入到目标文件中，效率很高，尤其是复制文件比较大的时候，我们一般采用fileChannel复制文件。

作者：chen.yu
深信服三年半工作经验，目前就职游戏厂商，希望能和大家交流和学习,
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