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.
* <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();
}
}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。
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