Java7中引入了SeekableByteChannel接口，允許我們定位到文件的任意位置進行讀寫。注意這里的寫，不是新增式的插入，而是覆蓋，當然在文件末尾的寫，是新增。
java.nio.channels.SeekableByteChannel A byte channel that maintains a current position and allows the position to be changed. A seekable byte channel is connected to an entity, typically a file, that contains a variable-length sequence of bytes
that can be read and written. The current position can be queried and modified. The channel also provides access to
the current size of the entity to which the channel is connected. The size increases when bytes are written beyond its
current size; the size decreases when it is truncated. The position and truncate methods which do not otherwise have a value to return are specified to return the channel
upon which they are invoked. This allows method invocations to be chained. Implementations of this interface should
specialize the return type so that method invocations on the implementation class can be chained.
而文件通道：FileChannel實現了該接口：
java.nio.channels.FileChannel A channel for reading, writing, mapping, and manipulating a file. A file channel is a SeekableByteChannel that is connected to a file. It has a current position within its file which
can be both queried and modified. The file itself contains a variable-length sequence of bytes that can be read and written and whose current size can be queried. The size of the file increases when bytes are written beyond its current size; the size of the file decreases when it is truncated. The file may also have some associated metadata such as access permissions, content type, and last-modification time; this class does not define methods for metadata access.
實例：
Path filePath = Paths.get("D://error.log");
ByteBuffer buffer = ByteBuffer.allocate(1024);
buffer.put("111111".getBytes());
buffer.flip();
try {
FileChannel channel = FileChannel.open(filePath, StandardOpenOption.WRITE);
channel.position(80); // 覆蓋此處的內容為111111
channel.write(buffer);
channel.close();
buffer = ByteBuffer.allocate(1024);
channel = FileChannel.open(filePath, StandardOpenOption.READ);
channel.position(100); // 讀取此處的內容
channel.read(buffer, 10);
buffer.flip();
System.out.println(buffer);
Charset charset = Charset.forName("utf-8");
CharsetDecoder decoder = charset.newDecoder();
CharBuffer charBuffer = decoder.decode(buffer);
System.out.println(charBuffer);
} catch (IOException e) {
e.printStackTrace();
}
注意上面的例子中涉及到 ByteBuffer 到 CharBuffer 的轉換，也就是字節到字符的轉換，需要使用對應的編碼的解碼器進行解碼。