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<TITLE>Using a Selector to Manage Non-Blocking Sockets (Java Developers Almanac Example)
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<h3>e176. Using a Selector to Manage Non-Blocking Sockets</h3>
Although you could poll each non-blocking socket for events, a more
convenient and efficient method is to use a selector to manage the
channels. The selector efficiently monitors the channels for changes
and reports the events through a single method call.
<P> The first step is to register a channel with a selector. The
registration process yields an object called a <font color="#0066ff"><i>selection key</i></font> which
identifies the selector/socket channel pair (a channel could be
registered with another selector for different events). When an event
occurs on a channel, the selector returns the selection key for that
channel. The selection key also contains the type of event that
occurred.
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This example creates two sockets and registers them with
a selector. The example then uses the selector to listen for events.
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See also <a href="../java.nio/NbServer.html" class="eglink"><b>e179</b> Using a Selector to Manage Non-Blocking Server Sockets</a>.
<pre> // Create a selector and register two socket channels
Selector selector = null;
try {
// Create the selector
selector = Selector.open();
// Create two non-blocking sockets. This method is implemented in
// <a href="../java.nio/NbClientSocket.html" class="eglink"><b>e173</b> Creating a Non-Blocking Socket</a>.
SocketChannel sChannel1 = createSocketChannel(<font color="#0066ff"><i>"hostname.com"</i></font>, <font color="#0066ff"><i>80</i></font>);
SocketChannel sChannel2 = createSocketChannel(<font color="#0066ff"><i>"hostname.com"</i></font>, <font color="#0066ff"><i>80</i></font>);
// Register the channel with selector, listening for all events
sChannel1.register(selector, sChannel1.validOps());
sChannel2.register(selector, sChannel1.validOps());
} catch (IOException e) {
}
// Wait for events
while (true) {
try {
// Wait for an event
selector.select();
} catch (IOException e) {
// Handle error with selector
break;
}
// Get list of selection keys with pending events
Iterator it = selector.selectedKeys().iterator();
// Process each key at a time
while (it.hasNext()) {
// Get the selection key
SelectionKey selKey = (SelectionKey)it.next();
// Remove it from the list to indicate that it is being processed
it.remove();
try {
processSelectionKey(selKey);
} catch (IOException e) {
// Handle error with channel and unregister
selKey.cancel();
}
}
}
public void processSelectionKey(SelectionKey selKey) throws IOException {
// Since the ready operations are cumulative,
// need to check readiness for each operation
if (selKey.isValid() && selKey.isConnectable()) {
// Get channel with connection request
SocketChannel sChannel = (SocketChannel)selKey.channel();
boolean success = sChannel.finishConnect();
if (!success) {
// An error occurred; handle it
// Unregister the channel with this selector
selKey.cancel();
}
}
if (selKey.isValid() && selKey.isReadable()) {
// Get channel with bytes to read
SocketChannel sChannel = (SocketChannel)selKey.channel();
// See <a href="../java.nio/ReadSocket.html" class="eglink"><b>e174</b> Reading from a SocketChannel</a>
}
if (selKey.isValid() && selKey.isWritable()) {
// Get channel that's ready for more bytes
SocketChannel sChannel = (SocketChannel)selKey.channel();
// See <a href="../java.nio/WriteSocket.html" class="eglink"><b>e175</b> Writing to a SocketChannel</a>
}
}
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<b>Related Examples</b></font></td>
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e173. <a class="eglink" href="NbClientSocket.html?l=rel">
Creating a Non-Blocking Socket
</a>
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e174. <a class="eglink" href="ReadSocket.html?l=rel">
Reading from a SocketChannel
</a>
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e175. <a class="eglink" href="WriteSocket.html?l=rel">
Writing to a SocketChannel
</a>
<br>
e177. <a class="eglink" href="NbServerSocket.html?l=rel">
Creating a Non-Blocking Server Socket
</a>
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e178. <a class="eglink" href="AcceptSocket.html?l=rel">
Accepting a Connection on a ServerSocketChannel
</a>
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e179. <a class="eglink" href="NbServer.html?l=rel">
Using a Selector to Manage Non-Blocking Server Sockets
</a>
<br>
e180. <a class="eglink" href="DetectClosed.html?l=rel">
Detecting When a Non-Blocking Socket Is Closed by the Remote Host
</a>
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See also:
<a class="eglink" href="/egs/java.nio/pkg.html?l=rel#Byte%20Buffers">
Byte Buffers
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<a class="eglink" href="/egs/java.nio/pkg.html?l=rel#File%20Locking">
File Locking
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<a class="eglink" href="/egs/java.nio/pkg.html?l=rel#Files">
Files
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<a class="eglink" href="/egs/java.nio/pkg.html?l=rel#Streams">
Streams
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