Recipe 24.6. Handling Socket Errors

Problem

You want to handle errors that might occur when using sockets.

Solution

Use try/catch to handle I/O and end of file (EOF) errors.

Discussion

Both the Socket and XMLSocket classes behave similarly in regard to errors and error events. When calling the connect( ) method, Socket and XMLSocket objects can throw an error of type SecurityError when either of the following conditions is true:

• The .swf is classified as local untrusted.

• The port number is higher than 655535.

When calling send( ) (XMLSocket) or flush( ) (Socket), the method can throw an error of type IOError if the socket isn't connected. Although you can (and likely should) place the send( ) or flush( ) method calls within try/catch blocks, you should not rely on try/catch blocks as part of your application logic. Rather, use an if statement to test whether or not the socket object's connected property is true before calling send( ) of flush( ) if you want such a test to be part of the application logic. For example, the following uses an if statement as part of the application logic to call a connectToSocketServer( ) method if the Socket object isn't currently connected. It also uses a TRy/catch block to write to a log if the flush( ) method throws an error:

if ( socket.connected ) {     try {         socket.flush(  );     }     catch( error:IOError ) {         logInstance.write( "socket.flush error\n" + error );     } } else {     connectToSocketServer(  ); }

All of the Socket read methods can throw errors of type EOFError and IOError. EOF errors occur when you try to read data, but nothing is available. I/O errors occur when you try to read from a socket that is closed.

In addition to the errors thrown by methods of the Socket and XMLSocket classes, objects of those classes also dispatch error events. There are two basic types of error events that occur with socketsIOError and securityError. The IOError event is of type IOErrorEvent, and it occurs when data fails to send or load. The securityError event is of type SecurityErrorEvent, and it occurs when a socket attempts to connect to a server but fails because the server is either outside of the sandbox or because the port number is lower than 1024.

Both security error event scenarios are correctable by way of a cross-domain policy file.