java文件nio_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 -1 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。

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。