java从jdk1.4后就引入了java NIO机制：
NIO的显著特点就是通道（channel）、缓冲（buffer）、选择器（selector），NIO机制中添加了传统I/O机制中没有的非阻塞调用（这对于网络通信很有用，可以有效利用CPU），但是这个只能对于网络通道（Socketchannel）才适用，filechannel还是阻塞调用。
我们现在专门分析的是java中的文件I/O机制，而不管网络的socket通信机制。
Java中传统的文件系统I/O机制是Filesystem和File，java中的Filesystem是java中的内部类，不提供对外的显示特性，File类中的包含了Filesystem的对象，从而对于File的操作，比如rename、create etc 都转成成java中的内部类Filesystem的操作。下面的是java中的Filesystem的抽象类：
abstract class FileSystem {
/**
* Return the FileSystem object representing this platform's local
* filesystem.
*/
public static native FileSystem getFileSystem();
/* -- Normalization and construction -- */
/**
* Return the local filesystem's name-separator character.
*/
public abstract char getSeparator();
/**
* Return the local filesystem's path-separator character.
*/
public abstract char getPathSeparator();
。。。。。。
}
每个操作系统中都有一个具体的文件系统，而在windows下，通过Filesystem的getFilesystem（）操作获取本地文件系统，win32Filesystem； 而对于linux系统下，获取的是unixFilesystem；
而java中传统的I/O机制中的File对象，通过包含Filesystem对象，来达到对于文件系统下文件的管理操作create、delete、rename等。而关于文件的I/O数据流，输入和输出，采用的是Fileinputstream和Fileoutputstream。下面以Fileinputstream为例，Fileinputstream中的文件操作函数包括如下：
private native void open(String name) throws FileNotFoundException;
/**
* Reads a byte of data from this input stream. This method blocks
* if no input is yet available.
*
* @return the next byte of data, or <code>-1</code> if the end of the
* file is reached.
* @exception IOException if an I/O error occurs.
*/
public int read() throws IOException {
Object traceContext = IoTrace.fileReadBegin(path);
int b = 0;
try {
b = read0();
} finally {
IoTrace.fileReadEnd(traceContext, b == -1 ? 0 : 1);
}
return b;
}
private native int read0() throws IOException;
/**
* Reads a subarray as a sequence of bytes.
* @param b the data to be written
* @param off the start offset in the data
* @param len the number of bytes that are written
* @exception IOException If an I/O error has occurred.
*/
private native int readBytes(byte b[], int off, int len) throws IOException;
public native long skip(long n) throws IOException;
等，上面是几个重要的函数，每次File读取操作的时候都有个文件读取位置，在linux文件系统下是文件描述符FileDescriptor，而windows系统下是handler，读取位置是通过FileDescriptor或者Handler来完成的，每次只能从上一次的位置读取文件操作。
但是Java中的NIO（New I/O）中引入了FileChannel，在FileChannel中有如下新特性：
- 字节读取可以以相对位置读取，也可以以绝对位置读取
- 一个文件的区域可以直接映射进入到内存中去
- 字节可以从一个文件传送到另外一个文件，通过transferto方法，直接在内核空间进行文件传输，而不用在用户态和内核态之间来回切换，有效减少了文件传输时间（在linux下有个相应的函数是sendfile，直接是在内核态进行文件传输，而无须在用户态和内核态之间来回进行数据切换）
相应的Filechannel是一个抽象类：
public abstract class FileChannel
extends AbstractInterruptibleChannel
implements SeekableByteChannel, GatheringByteChannel, ScatteringByteChannel {。。。}
新增加的方法是：
/**
* Reads a sequence of bytes from this channel into the given buffer,
* starting at the given file position.
*
* <p> This method works in the same manner as the {@link
* #read(ByteBuffer)} method, except that bytes are read starting at the
* given file position rather than at the channel's current position. This
* method does not modify this channel's position. If the given position
* is greater than the file's current size then no bytes are read. </p>
public abstract int read(ByteBuffer dst, long position) throws IOException;
/**
* Writes a sequence of bytes to this channel from the given buffer,
* starting at the given file position.
*
* <p> This method works in the same manner as the {@link
* #write(ByteBuffer)} method, except that bytes are written starting at
* the given file position rather than at the channel's current position.
* This method does not modify this channel's position. If the given
* position is greater than the file's current size then the file will be
* grown to accommodate the new bytes; the values of any bytes between the
* previous end-of-file and the newly-written bytes are unspecified. </p>
public abstract int write(ByteBuffer src, long position) throws IOException;
/**
* Acquires a lock on the given region of this channel's file.
public abstract FileLock lock(long position, long size, boolean shared)
throws IOException;
/**
* Forces any updates to this channel's file to be written to the storage
* device that contains it.
public abstract void force(boolean metaData) throws IOException;
/**
* Transfers bytes from this channel's file to the given writable byte
* channel.
* <p> This method is potentially much more efficient than a simple loop
* that reads from this channel and writes to the target channel. Many
* operating systems can transfer bytes directly from the filesystem cache
* to the target channel without actually copying them. </p>
public abstract long transferTo(long position, long count,
WritableByteChannel target)
throws IOException;
上述是FileChannel新增的方法。
传统的Java中的I/O机制中的FileInputStream的成员变量：
private final FileDescriptor fd;
即传统的java文件系统采用的是通过文件描述符的形式来记住文件的存取位置
而java中的NIO机制也是采用类似的机制：
// Used to make native read and write calls
private static NativeDispatcher nd;
// Memory allocation size for mapping buffers
private static long allocationGranularity;
// Cached field for MappedByteBuffer.isAMappedBuffer
private static Field isAMappedBufferField;
// File descriptor
private FileDescriptor fd;
上面是一个具体的Filechannel类，FilechannelImpl部分成员变量。
