Java NIO之Channel

介绍

定义

Channel(管道):A channel represents an open connection to an entity such as a hardware device, a file, a network socket, 
or a program component that is capable of performing one or more distinct I/O operations, for example reading or writing.
管道可以将其理解为一条连接，可以通过这条连接传输数据。

java nio数据传输介质是管道，就像java io中数据传输介质是流一样。不过io中的流是分方向的输入流和输出流，
但是nio的管道是双向的既可以输出数据也可以读取数据。当然管道都是基于Buffer对象读取和写入数据的。
java nio借助Channel和Selector实现单线程的多路复用IO.在聊天服务器中会使用到这种技术。

管道传输数据示意图
channel 管道传输数据示意图

Channel实现

Channel接口的实现有:
FileChannel(操作文件相关)
SocketChannel(TCP通信模型客户端Channel)
ServerSocketChannel(TCP通信服务器端Channel)
DatagramChannel(UDP数据报Channel)
备注:以下的学习时基于FileChannel来实现和学习的，其它几个Channel实现在网络高级编程中会使用到。

Channel接口

Channel接口就只定义了两个方法isOpen()和close()方法。
/**
 * 管道表示和磁盘、文件、网络套接字或程序组件的一条连接，它可以用来执行IO操作比如读取数据或写入数据。
 * 
 * A channel represents an open connection to an entity such as a hardware
 * device, a file, a network socket, or a program component that is capable of
 * performing one or more distinct I/O operations, for example reading or
 * writing.
 *
 * Channels are, in general, intended to be safe for multithreaded access
 * as described in the specifications of the interfaces and classes that extend
 * and implement this interface.
 */
public interface Channel extends Closeable {
    /**
     * Tells whether or not this channel is open.  </p>
     */
    public boolean isOpen();
    /**
     * Closes this channel.
     */
    public void close() throws IOException;
}
jdk nio隐藏细节太多了,如果真要看源码最好还是看openjdk的源码

FileChannel文件管道应用

FileChannel是一个抽象类,可以通过以下方法获得其实例对象
FileInputStream.getChannel()
FileOutputStream.getChannel()
RandomAccessFile.getChannel()

FileChannel核心方法

/**
 * A channel for reading, writing, mapping, and manipulating a file.
 */
public abstract class FileChannel
extends AbstractInterruptibleChannel
implements SeekableByteChannel, GatheringByteChannel, ScatteringByteChannel
{
    // -- Channel operations --
    /**
     * Reads a sequence of bytes from this channel into the given buffer.
     *
     * Bytes are read starting at this channel's current file position, and
     * then the file position is updated with the number of bytes actually
     * read.  Otherwise this method behaves exactly as specified in the 
     * ReadableByteChannel interface. 
     */
    public abstract int read(ByteBuffer dst) throws IOException;
    /**
     * Writes a sequence of bytes to this channel from the given buffer.
     *
     * Bytes are written starting at this channel's current file position
     * unless the channel is in append mode, in which case the position is
     * first advanced to the end of the file.  The file is grown, if necessary,
     * to accommodate the written bytes, and then the file position is updated
     * with the number of bytes actually written.  Otherwise this method
     * behaves exactly as specified by the  WritableByteChannel
     * interface. 
     */
    public abstract int write(ByteBuffer src) throws IOException;
    // -- Other operations --
    /**
     * Returns this channel's file position.
     */
    public abstract long position() throws IOException;
    /**
     * Sets this channel's file position.
     */
    public abstract FileChannel position(long newPosition) throws IOException;
    /**
     * Returns the current size of this channel's file.
     */
    public abstract long size() throws IOException;
    /**
     * Truncates this channel's file to the given size.
     *
     * If the given size is less than the file's current size then the file
     * is truncated, discarding any bytes beyond the new end of the file.  If
     * the given size is greater than or equal to the file's current size then
     * the file is not modified.  In either case, if this channel's file
     * position is greater than the given size then it is set to that size.
     * 
     */
    public abstract FileChannel truncate(long size) throws IOException;
    /**
     * Transfers bytes from this channel's file to the given writable byte
     * channel.
     */
    public abstract long transferTo(long position, long count,
                                    WritableByteChannel target)
        throws IOException;
    /**
     * Transfers bytes into this channel's file from the given readable byte
     * channel.
     */
    public abstract long transferFrom(ReadableByteChannel src,
                                      long position, long count)
        throws IOException;
    /**
     * Reads a sequence of bytes from this channel into the given buffer,
     * starting at the given file position.
     */
    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.
     */
    public abstract int write(ByteBuffer src, long position) throws IOException;
    // -- Memory-mapped buffers --
    /**
     * Maps a region of this channel's file directly into memory.
     * 具体参考源码
     */
    public abstract MappedByteBuffer map(MapMode mode,
                                         long position, long size)
        throws IOException;
    // -- Locks --
    /**
     * Acquires an exclusive lock on this channel's file.
     *
     * An invocation of this method of the form fc.lock() behaves
     * in exactly the same way as the invocation
     */
    public final FileLock lock() throws IOException {
        return lock(0L, Long.MAX_VALUE, false);
    }
    /**
     * Attempts to acquire an exclusive lock on this channel's file.
     *
     * An invocation of this method of the form fc.tryLock()
     * behaves in exactly the same way as the invocation
     */
    public final FileLock tryLock() throws IOException {
        return tryLock(0L, Long.MAX_VALUE, false);
    }
}
FileChannel的position和Buffer的position属性是不同的意思,FileChannel的position指的是文件指针位置,
Buffer的position指的是子节数组中下标位置。

FileChannel读写数据分析

ByteBuffer buffer = ByteBuffer.allocate(10);
buffer.put("hello".getBytes());
//从Buffer对象中读取数据时,要先调用flip方法
buffer.flip();
fileChannel.write(buffer);
备注:因为调用write(buffer)写入数据到管道中时，只能读取Buffer对象中position到limit之间的数据， 
如果不调用flip方法的话，读取到的Buffer数据可能就是错误的或0。
//向Buffer对象中写入数据时,要先调用clear()方法。
buffer.clear();
fileChannel.read(buffer);
备注:因为调用read(buffer)从管道中读取数据到管道时，如果不先调用clear方法， 
那么Buffer对象就不一定是从position为0开始写入数据了，就会导致写入数据到Buffer对象是错误的。
备注:重点还是在于Buffer对象的那几个变量值，即position、limit、capacity。最关键的是position和limit，因为容量初始化之后通常不会变的。

FileChannel的文件拷贝

FileChannel自带的拷贝方法

//nio中Channle 自带提供的拷贝方法
public void testCopyFile2() throws Exception {
    // 源文件
    FileInputStream inputStream = new FileInputStream(new File("e:/nio/test.jpg"));
    // 目标文件
    FileOutputStream outputStream = new FileOutputStream(new File("e:/nio/copy.jpg"));
    // 获得源文件的通道
    FileChannel inChannel = inputStream.getChannel();
    // 获得目标文件的通道
    FileChannel outChannel = outputStream.getChannel();
    // nio自带的考本文件方法
    //1、transferFrom(ReadableByteChannel src,long position, long count)
    outChannel.transferFrom(inChannel, 0, inChannel.size());
    //2、transferTo(long position, long count,WritableByteChannel target)
    inChannel.transferTo(0, inChannel.size(), outChannel);
    //备注这两个方法是等效的,只是要注意文件读写模式。
    //关闭资源 ...
}

基于FileChannel和ByteBuffer实现拷贝

//基于ByteBuffer和FileChannel的文件拷贝
public void testCopyFile() throws Exception {
    // 源文件
    FileInputStream inputStream = new FileInputStream(new File("e:/nio/2.jpg"));
    // 目标文件
    FileOutputStream outputStream = new FileOutputStream(new File("e:/nio/2copy.jpg"));
    // 获得源文件的通道
    FileChannel inChannel = inputStream.getChannel();
    // 获得目标文件的通道
    FileChannel outChannel = outputStream.getChannel();
    ByteBuffer buffer = ByteBuffer.allocate(10);
    boolean flag = true;
    while (flag) {
        buffer.clear();
        // 从管道(和源文件建立的管道)读取数据到buffer对象中
        int data = inChannel.read(buffer);
        buffer.flip();
        if (data == -1) {// 一直读写直到没有数据时退出循环
            flag = false;
        }
        // 从buffer对象中读取数据到管道(通过目标文件建立的管道)
        outChannel.write(buffer);
    }
    //关闭资源
}
我对ByteBuffer对象的理解,是根据结构化的的子节数组,对现实内存的抽象。
关于Buffer的flip()和clear()方法,我的理解是当从Buffer对象中读取数据时,那么要先调用flip()方法, 
当要向Buffer对象中写入数据时,要先调用clear()方法。

总结

总结其实学习nio最重要的不是学会它们如何操作,因为步骤性的技术通常是冗长难以记忆的,只要长时间不用就可能会生疏, 
但是原理性的东西是内容量比较少的,所以掌握其最核心原理很重要。

参考

1、http://www.ibm.com/developerworks/cn/education/java/j-nio/section5.html
2、http://ifeve.com/file-channel/
3、http://www.cnblogs.com/dolphin0520/p/3916526.html