# Channel

Java NIO Channels are similar to streams with a few differences:

* You can both read and write to a Channels. Streams are typically one-way (read or write).
* Channels can be read and written asynchronously.
* Channels always read to, or write from, a Buffer.

As mentioned above, you read data from a channel into a buffer, and write data from a buffer into a channel. Here is an illustration of that:

| ![Java NIO: Channels and Buffers](https://jenkov.com/images/java-nio/overview-channels-buffers.png) |
| :-------------------------------------------------------------------------------------------------: |
|           Java NIO: Channels read data into Buffers, and Buffers write data into Channels           |

### Channel Implementations

Here are the most important Channel implementations in Java NIO:

* FileChannel
* DatagramChannel
* SocketChannel
* ServerSocketChannel

The `FileChannel` reads data from and to files.

The `DatagramChannel` can read and write data over the network via UDP.

The `SocketChannel` can read and write data over the network via TCP.

The `ServerSocketChannel` allows you to listen for incoming TCP connections, like a web server does. For each incoming connection a `SocketChannel` is created.

### Basic Channel Example

Here is a basic example that uses a `FileChannel` to read some data into a `Buffer`:

```
    RandomAccessFile aFile = new RandomAccessFile("data/nio-data.txt", "rw");
    FileChannel inChannel = aFile.getChannel();

    ByteBuffer buf = ByteBuffer.allocate(48);

    int bytesRead = inChannel.read(buf);
    while (bytesRead != -1) {

      System.out.println("Read " + bytesRead);
      buf.flip();

      while(buf.hasRemaining()){
          System.out.print((char) buf.get());
      }

      buf.clear();
      bytesRead = inChannel.read(buf);
    }
    aFile.close();
```

Notice the `buf.flip()` call. First you read into a Buffer. Then you flip it. Then you read out of it. I'll get into more detail about that in the next text about `Buffer`'s.


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