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The Java Developers Almanac 1.4Order this book from Amazon. |
e176. Using a Selector to Manage Non-Blocking SocketsAlthough 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.The first step is to register a channel with a selector. The registration process yields an object called a selection key 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. This example creates two sockets and registers them with a selector. The example then uses the selector to listen for events. See also e179 Using a Selector to Manage Non-Blocking Server Sockets. // 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 // e173 Creating a Non-Blocking Socket. SocketChannel sChannel1 = createSocketChannel("hostname.com", 80); SocketChannel sChannel2 = createSocketChannel("hostname.com", 80); // 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 e174 Reading from a SocketChannel } if (selKey.isValid() && selKey.isWritable()) { // Get channel that's ready for more bytes SocketChannel sChannel = (SocketChannel)selKey.channel(); // See e175 Writing to a SocketChannel } }
e174. Reading from a SocketChannel e175. Writing to a SocketChannel e177. Creating a Non-Blocking Server Socket e178. Accepting a Connection on a ServerSocketChannel e179. Using a Selector to Manage Non-Blocking Server Sockets e180. Detecting When a Non-Blocking Socket Is Closed by the Remote Host
© 2002 Addison-Wesley. |