Threaded Fortune Server Example

The Threaded Fortune Server example shows how to create a server for a simple network service that uses threads to handle requests from different clients. It is intended to be run alongside the Fortune Client example.

../_images/threadedfortuneserver-example.png

The implementation of this example is similar to that of the Fortune Server example, but here we will implement a subclass of QTcpServer that starts each connection in a different thread.

For this we need two classes: FortuneServer, a QTcpServer subclass, and FortuneThread, which inherits QThread .

class FortuneServer(QTcpServer):

    Q_OBJECT
# public
    FortuneServer(QObject parent = None)
protected:
    def incomingConnection(socketDescriptor):
# private
    fortunes = QStringList()

FortuneServer inherits QTcpServer and reimplements incomingConnection() . We also use it for storing the list of random fortunes.

def __init__(self, parent):
    QTcpServer.__init__(self, parent)

    fortunes << tr("You've been leading a dog's life. Stay off the furniture.")
             << tr("You've got to think about tomorrow.")
             << tr("You will be surprised by a loud noise.")
             << tr("You will feel hungry again in another hour.")
             << tr("You might have mail.")
             << tr("You cannot kill time without injuring eternity.")
             << tr("Computers are not intelligent. They only think they are.")

We use FortuneServer’s constructor to simply generate the list of fortunes.

def incomingConnection(self, socketDescriptor):

    fortune = fortunes.at(QRandomGenerator.global().bounded(fortunes.size()))
    thread = FortuneThread(socketDescriptor, fortune, self)
    connect(thread, FortuneThread::finished, thread, FortuneThread::deleteLater)
    thread.start()

Our implementation of incomingConnection() creates a FortuneThread object, passing the incoming socket descriptor and a random fortune to FortuneThread’s constructor. By connecting FortuneThread’s finished() signal to deleteLater() , we ensure that the thread gets deleted once it has finished. We can then call start() , which starts the thread.

class FortuneThread(QThread):

    Q_OBJECT
# public
    FortuneThread(int socketDescriptor, QString fortune, QObject parent)
    def run():
signals:
    def error(socketError):
# private
    socketDescriptor = int()
    text = QString()

Moving on to the FortuneThread class, this is a QThread subclass whose job is to write the fortune to the connected socket. The class reimplements run() , and it has a signal for reporting errors.

def __init__(self, socketDescriptor, fortune, parent):
    QThread.__init__(self, parent)
    self.socketDescriptor = socketDescriptor
    self.text = fortune

FortuneThread’s constructor simply stores the socket descriptor and fortune text, so that they are available for run() later on.

def run(self):

    tcpSocket = QTcpSocket()

The first thing our run() function does is to create a QTcpSocket object on the stack. What’s worth noticing is that we are creating this object inside the thread, which automatically associates the socket to the thread’s event loop. This ensures that Qt will not try to deliver events to our socket from the main thread while we are accessing it from FortuneThread::run().

if (not tcpSocket.setSocketDescriptor(socketDescriptor)) {
    error.emit(tcpSocket.error())
    return

The socket is initialized by calling setSocketDescriptor() , passing our socket descriptor as an argument. We expect this to succeed, but just to be sure, (although unlikely, the system may run out of resources,) we catch the return value and report any error.

block = QByteArray()
out = QDataStream(block, QIODevice.WriteOnly)
out.setVersion(QDataStream.Qt_4_0)
out << text

As with the Fortune Server example, we encode the fortune into a QByteArray using QDataStream .

tcpSocket.write(block)
tcpSocket.disconnectFromHost()
tcpSocket.waitForDisconnected()

But unlike the previous example, we finish off by calling waitForDisconnected() , which blocks the calling thread until the socket has disconnected. Because we are running in a separate thread, the GUI will remain responsive.

Example project @ code.qt.io