Channel
概述
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。
正如上面所说,从通道读取数据到缓冲区,从缓冲区写入数据到通道。如下图所示:
Channel和流的区别
早一代IO操作是由CPU负责IO接口:
新一代DMA授权处理IO接口:
通道(Channel)模式:
Channel的实现
这些是Java NIO中最重要的通道的实现:
- FileChannel 从文件中读写数据
- DatagramChannel 通过UDP读写网络中的数据
- SocketChannel 能通过TCP读写网络中的数据
- ServerSocketChannel 可以监听新进来的TCP连接,像Web服务器那样。对每一个新进来的连接都会创建一个SocketChannel
Channel的获取方式
1. 支持通道的类提供了getChannel()方法
RandomAccessFile randomAccessFile = new RandomAccessFile("D:\\scl\\20190820\\a.txt","rw");
FileChannel fc = randomAccessFile.getChannel();
2. 各个通道提供了静态方法open()
FileChannel inChannel
= FileChannel.open(Paths.get("D:\\scl\\20190820\\a.txt"), StandardOpenOption.READ);
3.Files工具类的newByteChannel()
SeekableByteChannel inChannel
= Files.newByteChannel(Paths.get("D:\\scl\\20190820\\a.txt"), StandardOpenOption.READ);
Channel的使用方式
1. 配置Buffer使用
@Test
public void test3() throws IOException {
SeekableByteChannel inChannel = Files.newByteChannel(Paths.get("D:\\scl\\20190820\\a.txt"), StandardOpenOption.READ);
ByteBuffer buf = ByteBuffer.allocate(4);
while (inChannel.read(buf) != -1) {
buf.flip();
while (buf.hasRemaining()) {
System.out.print((char) buf.get());
}
buf.clear();
}
inChannel.close();
}
2. transferTo和transferFrom
2.1 transferTo
@Test
public void test1() throws IOException {
FileChannel inChannel = FileChannel.open(Paths.get("D:\\scl\\20190820\\a.txt"), StandardOpenOption.READ);
FileChannel outChannel
= FileChannel.open(Paths.get("D:\\scl\\20190820\\b.txt"), StandardOpenOption.WRITE,StandardOpenOption.CREATE);
inChannel.transferTo(0,inChannel.size(),outChannel);
inChannel.close();
outChannel.close();
}
2.2 transferFrom
@Test
public void test4() throws IOException {
FileChannel inChannel = FileChannel.open(Paths.get("D:\\scl\\20190820\\a.txt"), StandardOpenOption.READ);
FileChannel outChannel
= FileChannel.open(Paths.get("D:\\scl\\20190820\\b1.txt"), StandardOpenOption.WRITE,StandardOpenOption.CREATE);
outChannel.transferFrom(inChannel,0,inChannel.size());
inChannel.close();
outChannel.close();
}
3. MappedByteBuffer
@Test
public void test2() throws IOException {
FileChannel inChannel = FileChannel.open(Paths.get("D:\\scl\\20190820\\a.txt"), StandardOpenOption.READ);
MappedByteBuffer mapper = inChannel.map(FileChannel.MapMode.READ_ONLY, 0, inChannel.size());
byte[] ds = new byte[(int)inChannel.size()];
for (int offset = 0; offset < inChannel.size(); offset++) {
ds[offset] = mapper.get();
}
System.out.println(new String(ds));
}
注意 MappedByteBuffer 使用的是直接内存