【原】java之通过FileChannel实现文件复制

陈喻 2021-10-19

展开全文

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.
     * <p>
     * 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.
     * <p>
     * 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.
     * <p>
     * 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.
     * <p>
     * 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.
     * <p>
     * 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();
}
}