java文件系统中的的NIO与IO

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：

1. 字节读取可以以相对位置读取，也可以以绝对位置读取
2. 一个文件的区域可以直接映射进入到内存中去
3. 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。