Java NIO之Channel（通道）入门

Channel（通道）是专用于IO请求的独立处理器。用于在数据源与目标节点之间建立连接。负责缓冲区中数据的传输。

Java NIO的通道类似流，但又有些不同：

• 既可以从通道中读取数据，又可以写数据到通道。但流的读写通常是单向的。
• 通道可以异步地读写。
• 通道中的数据总是要先读到一个Buffer，或者总是要从一个Buffer中写入。

Channel可以看做是铁路，Buffer可以看作是列车，而Data数据可以看做是旅客。

1、Channel的实现

以下是Java NIO中最重要的通道的实现：

• FileChannel：用于从文件中读写数据
• DatagramChannel：能通过UDP读写网络中的数据
• SocketChannel：能通过TCP读写网络中的数据（客户端）
• ServerSocketChannel：可以监听新进来的TCP连接（服务端，像Web服务器那样）对每一个新进来的连接都会创建一个SocketChannel

2、Channel UML图

3、Channel使用示例（FileChannel）

3.1、JDK1.7之前

3.1.1、RandomAccessFile

FileChannel readChannel = null;
FileChannel writeChannel = null;
try {
    RandomAccessFile sourceFile = new RandomAccessFile("demo.txt", "rw");//指定源文件及操作方式
    RandomAccessFile dstFile = new RandomAccessFile("demo2.txt", "rw");//指定目标文件及操作方式
    //获取读写通道
    readChannel = sourceFile.getChannel();
    writeChannel = dstFile.getChannel();
    ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
    while (-1 != readChannel.read(byteBuffer)) {
        byteBuffer.flip();
        writeChannel.write(byteBuffer);
        byteBuffer.clear();
    }
} catch (FileNotFoundException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
} finally {
    try {
        readChannel.close();
        writeChannel.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

3.1.2、FileInputStream/FileOutputStream

FileInputStream fips = null;
FileOutputStream fops = null;
FileChannel fipsChannel = null;
FileChannel fopsChannel = null;
try {
    fips = new FileInputStream(new File("demo.txt"));
    fops = new FileOutputStream(new File("demo2.txt"));

    fipsChannel = fips.getChannel();
    fopsChannel = fops.getChannel();

    ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
    while (-1 != fipsChannel.read(byteBuffer)) {
        byteBuffer.flip();
        fopsChannel.write(byteBuffer);
        byteBuffer.clear();
    }
} catch (FileNotFoundException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
} finally {
    try {
        fips.close();
        fops.flush();
        fops.close();
        fipsChannel.close();
        fopsChannel.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

3.2、JDK1.7开始

通过FileChannel的public static FileChannel open(Path path, OpenOption... options) 静态方法

try {
    //获取读写通道 StandardOpenOption.CREATE表示——不存在则创建，存在则覆盖
    FileChannel inChannel = FileChannel.open(Paths.get("demo.txt"), StandardOpenOption.READ);
    FileChannel outChannel = FileChannel.open(Paths.get("demo2.txt"), StandardOpenOption.WRITE,StandardOpenOption.READ,StandardOpenOption.CREATE);

    //使用内存映射文件方式读写数据（只有ByteBuffer支持）
    MappedByteBuffer inBuffer = inChannel.map(FileChannel.MapMode.READ_ONLY, 0, inChannel.size());
    MappedByteBuffer outBuffer = outChannel.map(FileChannel.MapMode.READ_WRITE, 0, inChannel.size());

    //读写数据
    byte[] dst = new byte[inBuffer.limit()];
    inBuffer.get(dst);
    outBuffer.put(dst);
} catch (IOException e) {
    e.printStackTrace();
}

4、Channel通信

采用的是直接缓冲区的方式实现

public abstract long transferFrom(ReadableByteChannel src, long position, long count)
public abstract long transferTo(long position, long count,  WritableByteChannel target)

try {
    FileChannel inChannel = FileChannel.open(Paths.get("demo.txt"), StandardOpenOption.READ);
    FileChannel outChannel = FileChannel.open(Paths.get("demo4.txt"), StandardOpenOption.WRITE,StandardOpenOption.READ,StandardOpenOption.CREATE);

    //inChannel.transferTo(0, inChannel.size(), outChannel);
    outChannel.transferFrom(inChannel,0,inChannel.size());

    inChannel.close();
    outChannel.close();
} catch (IOException e) {
    e.printStackTrace();
}