java文件系统中的的NIO与IO

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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.
 *
 * 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. 
 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.
 *
 * 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. 
 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.
 * 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. 
 public abstract long transferTo(long position, long count,
 WritableByteChannel target)
 throws IOException;
 上述是FileChannel新增的方法。