1、FileChanel介紹

Java NIO FileChannel是連接文件的通道，從文件中讀取數據和將數據寫入文件。Java NIO FileChannel類是NIO用于替代使用標準Java IO API讀取文件的方法。

FileInputStream的getChannel方法獲取的文件通道是只讀的，當然通過FileOutputStream的getChannel的方法獲取的文件通道是可寫的

部分API

1）、truncate截斷文件功能

/*** 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()強制在內存中的數據刷新到硬盤中去

/*** 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可以看出是拷貝從源的position位置的count 字節大小

/*** 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;

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2、復制文件常用方法

1、通過普通輸入輸出流復制文件

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;//讀取到輸入流數據，然后寫入到輸出流中去，實現復制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();}}}

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2、通過 FileChannel復制文件

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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();}}

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3、總結

一般復制使用輸入輸出流進行操作，用源文件創建出一個輸入流，用目標文件創建出一個輸出流，把輸入流的數據讀取寫入到輸出流，用fileChannel，直接連接輸入輸出流的文件通道，將數據直接寫入到目標文件中，效率很高，尤其是復制文件比較大的時候，我們一般采用fileChannel復制文件。

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