Java NIO SocketChannel

Opening a SocketChannel
Closing a SocketChannel
Reading from a SocketChannel
Writing to a SocketChannel
Non-blocking Mode

connect()
write()
read()
Non-blocking Mode with Selectors

Jakob Jenkov Last update: 2014-06-25

A Java NIO SocketChannel is a channel that is connected to a TCP network socket. It is Java NIO's equivalent of Java
Networking's Sockets. There are two ways a

SocketChannel

can be created:

You open a

SocketChannel

and connect to a server somewhere on the internet.
A

SocketChannel

can be created when an incoming connection arrives at a

ServerSocketChannel

.

Opening a SocketChannel

Here is how you open a

SocketChannel

:

SocketChannel socketChannel = SocketChannel.open();
socketChannel.connect(new InetSocketAddress("http://jenkov.com", 80));

Closing a SocketChannel

You close a

SocketChannel

after use by calling the

SocketChannel.close()

method. Here is how that is done:

socketChannel.close();

Reading from a SocketChannel

To read data from a

SocketChannel

you call one of the

read()

methods. Here is an example:

ByteBuffer buf = ByteBuffer.allocate(48);

int bytesRead = socketChannel.read(buf);

First a

Buffer

is allocated. The data read from the

SocketChannel

is read into the

Buffer

.

Second the

SocketChannel.read()

method is called. This method reads data from the

SocketChannel

into the

Buffer

. The

int

returned by the

read()

method tells how many bytes were witten into the

Buffer

.
If -1 is returned, the end-of-stream is reached (the connection is closed).

Writing to a SocketChannel

Writing data to a

SocketChannel

is done using the

SocketChannel.write()

method, which takes a

Buffer

as parameter. Here is an example:

String newData = "New String to write to file..." + System.currentTimeMillis();

ByteBuffer buf = ByteBuffer.allocate(48);
buf.clear();
buf.put(newData.getBytes());

buf.flip();

while(buf.hasRemaining()) {
channel.write(buf);
}

Notice how the

SocketChannel.write()

method is called inside a while-loop. There is no guarantee of how many bytes the

write()

method writes to the

SocketChannel

. Therefore we repeat the

write()

call until
the

Buffer

has no further bytes to write.

Non-blocking Mode

You can set a

SocketChannel

into non-blocking mode. When you do so, you can call

connect()

,

read()

and

write()

in asynchronous mode.

connect()

If the

SocketChannel

is in non-blocking mode, and you call

connect()

, the method may return before a connection is established. To determine whether the connection is established, you can call the

finishConnect()

method,
like this:

socketChannel.configureBlocking(false);
socketChannel.connect(new InetSocketAddress("http://jenkov.com", 80));

while(! socketChannel.finishConnect() ){
//wait, or do something else...
}

write()

In non-blocking mode the

write()

method may return without having written anything. Therefore you need to call the

write()

method in a loop. But, since this is already being done in the previous write examples, no need to do anything
differently here.

read()

In non-blocking mode the

read()

method may return without having read any data at all. Therefore you need to pay attention to the returned

int

, which tells how many bytes were read.

Non-blocking Mode with Selectors

The non-blocking mode of

SocketChannel

's works much better with

Selector

's. By registering one or more

SocketChannel

's with a

Selector

, you can ask the

Selector

for channels that are ready for
reading, writing etc. How to use

Selector

's with

SocketChannel

's is explained in more detail in a later text in this tutorial.