Develop Reference

Continue executing a thread while another thread is running in java

tl, dr;
I have a GUI thread that creates an object of another class (the seconds class has implemented Runnable, but here we don't execute the run() method, instead, we call a normal method) and calls a method. In that method, the first thread (current thread) is called again (to show sth on the LCD), then sends some data to the Internet, and waits 3 seconds for the server response. The problem is that the information is printed after 3 seconds. I know about the stack and program counter, but I wonder if there is another option that I can do my job.
I have the main method, which runs 3 threads (for short, I just write the requisite code. Tell me to add more, if needed):
public static void main(String[] args) throws UnknownHostException, InterruptedException {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
GUI.getInstance().setVisible(true); //GUI is singleton, using swing and JFrame
} catch (Exception e) {
e.printStackTrace();
}
}
});
MQTTConnection.getInstance().tryToConnect(); //It's the connection class, which has a thread (the thread is handled by a library that keeps the connection alive. I have no threads there) and is a singleton too.
Thread t1 = new Thread(new SendDataThread()); //A thread which sends some data every 20 seconds.
t1.start();
}
And in SendDataThread, I have a function that creates some random data and sends them (using the MQTTConnection class).
This is the SendDataThread:
public class SendDataThread implements Runnable {
public void sendLog() {
boolean serverOnline = false;
StringBuilder data = new StringBuilder();
data.append(createData());
GUI.getInstance().printNeutral(data.toString()); //Prints BLACK on a `JTextPane`
if(MQTTConnection.getInstance().publishLog(MQTTConnection.getInstance().MQTT_PUBLISH_ESP_SEND_LOG, data.toString())) //This line has a 3 second timeout. If the server doesn't respond, it will return false. I've added the 3 seconds timeout too. Please continue reading.
serverOnline = true;
if(serverOnline)
GUI.getInstance().printOK("Server Submitted"); //Prints in GREEN
else
GUI.getInstance().printProblem("Check your connection!"); //Prints in RED
GUI.getInstance().printNeutral("-------------------------------------------------");
}
#Override
public void run() {
while(true) {
sendLog();
try {
Thread.sleep(20000); //sleeps 20 about seconds
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
//.....
}
And this is the 3 seconds timeout method, in MQTTConnection:
boolean publishLog(String topic, String data){
mqtt_responds = false;
publish(topic, data);
System.out.println("MQTT is connected");
long lastTime = System.currentTimeMillis();
while(System.currentTimeMillis() - lastTime < callback_timeout) {
if(mqtt_responds){
mqtt_responds = false;
System.out.println("Server submitted");
return true;
}
try {
Thread.sleep(200);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("Timeout");
return false;
}
Till now, everything work right. The problem starts where I have a button in the GUI class, which the user can manually send random logs:
JButton sendLogBtn = new JButton("Send Log");
sendLogBtn.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
SendDataThread sdt = new SendDataThread();
sdt.sendLog();
}
});
sendLogBtn.setBounds(10, 331, 89, 23);
panel.add(sendLogBtn);
This button creates an object of SendDataThread and calls the sendLog() method. The issue happens here: after sendLog() is called, sendLog(), calls this GUI thread again:
--> GUI.getInstance().printNeutral(data.toString()); //Prints BLACK on a `JTextPane`
But the log is printed after 3 seconds (After the sendLog() method has finished working, the timeout!)
How can I fix this?

In the button's actionPerformed you are calling sendLog. sendLog does exactly what you said, ie reports some logs and waits about 3 seconds (assuming callback_timeout is about equal to 3000).
To fix this, you need to make sure that the 3sec blocking is not on the EDT and also to make sure that the logs are instead posted on the EDT.
As a quick workaround you can do:
sendLogBtn.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
new Thread(() -> new SendDataThread().sendLog()).start();
}
});
and then, as always, post your logs in the EDT like for example:
SwingUtilities.invokeLater(() -> GUI.getInstance().printNeutral(...));
AND
SwingUtilities.invokeLater(() -> GUI.getInstance().printProblem(...));
AND
SwingUtilities.invokeLater(() -> GUI.getInstance().printOk(...));
As for the question in your comment, I don't really understand what you are asking, but I should say that (as far as I know) the EDT is a Thread where all the Swing code is (and should be) posted on for execution. This way the Swing code does not have to be synchronized, because all GUI related stuff is executed sequentially (on the EDT). AWT for example was not intended to be single threaded as far as I know. Swing is however single threaded.

Dealing with multithreading in UI

I am actually using sockets listeners in a Java program.
The idea is to let my program run until it catches an external event. As soon as it catches it, I have to update my layout.
In order to let my application run while it was listening on a specific port, I did something like this :
// new thread allows me to let my program continue
new Thread() {
public void run() {
try {
while (true) {
try (ServerSocket listener = new ServerSocket(59090)) {
System.out.println("The server is running...");
// listening
while (true) {
try (Socket socket = listener.accept()) {
// catches event
System.out.println("event caught");
InputStream raw = socket.getInputStream();
headerData = getHeaders(raw);
// some code
// ...
// ...
// Update UI
}
}
}
}
} catch (IOException e) {
e.printStackTrace();
}
}
}.start();
The idea is that I constantly listen on the port 59090 and I execute my specific code once I use that port and that works well.
The problem now is when I catch an event.
It seems that I can't act on the UI if I am not in the "main" Thread. Am I right ?
Is it possible to send an information to my "main thread" in order to tell it to update UI ? Otherwise is it possible to "switch" my main thread to this one ?
EDIT:
I've seen that in Android you could use AsyncTask and the method onPostExecute() allows to send informations to the UI thread.
onPostExecute(Result), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.
Is there an equivalent in Java?
Thank you

Thread does not stopped using flag as volatile

In following code i am trying to start and stop the server on button using java applet.Start works good using thread but i want to stop the server on button. I have used volatile variable.Still i am not getting the server stop..
here is code:
public class TCPServer extends Thread {
public static final int SERVERPORT = 8002;
private boolean running = false;
public volatile boolean stop = false;
public Socket client = null;
public static void main(String[] args) {
ServerBoard frame = new ServerBoard();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setVisible(true);
}
public void run() {
super.run();
running = true;
try {
System.out.println("Server Has Started........ \n Waiting for client........");
ServerSocket serverSocket = new ServerSocket(SERVERPORT);
try {
while (!stop && running) {
client = serverSocket.accept();
System.out.println("Connection Accepted......");
BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
String usercmnd = in.readLine();
if (usercmnd != null) {
Runtime runtime = Runtime.getRuntime();
Process p = runtime.exec(usercmnd);
}
}
if (stop) {
serverSocket.close();
client.close();
System.out.println("Server Has Stopped");
System.exit(0);
}
} catch (Exception e) {
System.out.println("Error");
e.printStackTrace();
}
} catch (Exception e) {
System.out.println("Error");
e.printStackTrace();
}
}
public void requestStop() {
stop = true;
}
}
But whenever i click the stop button which will stop the server.Its not showing any output on console as i expected by the code.Its also not showing any error
here is code of stop button
stopServer = new JButton("Stop");
stopServer.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
stopServer.setEnabled(false);
startServer.setEnabled(true);
mServer = new TCPServer();
mServer.requestStop();
}
});

It doesn't stop because you create a new instance before stopping it and you don't even start it first while you are supposed to call requestStop() on your current instance of TCPServer.
// Here you create a new instance instead of using the existing one
mServer = new TCPServer();
mServer.requestStop();

in your ActionListener implementation for the stop button, you are accessing a different instance of TCPServer (as you are creating a new one). So you set the value "Stop" to a second object. It has no impact on the first instance that was created with the start button.
Try to instanciate the TCServer outside of your implementation of the action listener for both buttons and use that single instance for both.

Due to https://docs.oracle.com/javase/7/docs/api/java/net/ServerSocket.html#accept()
ServerSocket::accept is a blocking method, so stop variable can only be checked between successful connections.
You can either set a ServerSocket timeout using ServerSocket::setSoTimeout (and catch SocketTimeoutException), or interrupt Server thread and catch InterruptedException.
Both exceptions will be throwed from ServerSocket::accept.
Note, that thread interruption is highly preferred over timeouts and repeatly exception catching.

Try this:
public void requestStop() {
stop = true;
interrupt();
}
However, in this case, we can not guarantee, that already processing logic will successfully shutdown.
Also, you try invoke requestStop for new instance of TCPServer, instead already existing.

your code client = serverSocket.accept(); is blocking. So once you clicked "stopServer" button, you requested the stop, but it will be acted upon only once a client sends a next request to the server. What you need to do is to run your logic in your method run() in a separate thread and catch there InterruptedException and ClosedByInterruptException and clean up and return there. On your stopButton click you will invoke interrupt() method on your Thread. Read about it in here

Components I used to draw a form won't appear after compile

I'm trying to create a chat program in java but I had a problem when I run the server form, that the components I used to draw won't appear.
this is the code I used in the run of the form :
public void run() {
Server s = new Server();
s.setVisible(true);
// Etablir la connexion
try
{
ServerSocket ecoute;
ecoute = new ServerSocket(1111);
Socket service = null;
System.out.println("Serveur en attente d'un client !");
while(true)
{
service = ecoute.accept();
System.out.println("Client connécté !");
DataInputStream is = new DataInputStream(service.getInputStream());
s.jTextArea1.setText("Client dit : " + is.readUTF().toUpperCase());
service.close();
}
}
catch(IOException e)
{
e.printStackTrace();
}
}

You said nothing happens when this code is ran. The presence of a public void run() method tells me that this is a thread, or at least a Runnable.
Because of the while(true), if this thread is not started in the proper manner, it will not run independently; that is it will hold up the entire program.
Instead of calling thread.run();, call thread.start();. This will call the run method for you, after starting a new thread that will run in parallel to the main thread.
If this code is not in a thread, and you just used public void run() by chance, then it will still provide the same problem for you.
For more information, refer to the Documentation on Threads

How to stop ServerSocket Thread correctly? Close Socket failed

I know this has been discussed some times before, but I can't find an appropriate solution for my problem. I want to run a ServerSocket thread in the background, listening to the specified port. It's working actually, but only once. Seems that the port the server is listening to is never closed correctly and still active when I try to restart (O don't restart the thread itself). Can some tell why it is not working correctly? Thanks in advance for any help...!
edit:
I have same problem on the client side. I have a sender thread and also that one cannot not be stopped. What is the best way to do that?
The ClientConnector is just a class which connects to the server port and sends the data.
It's not a thread or anything like that.
That's my sender class:
private class InternalCamSender extends Thread {
private int sendInterval = 500; // default 500 ms
private ClientConnector clientConn = null;
public InternalCamSender() {
this.sendInterval = getSendingInterval();
this.clientConn = new ClientConnector();
}
#Override
public void run() {
while(!Thread.currentThread().isInterrupted()) {
clientConn.sendCamPdu(CodingScheme.BER, createNewPDU());
try {
Thread.sleep(sendInterval);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
And I try to handle it's behaviour like that:
if(jButton_startSending.getText().equals(STARTSENDING)) {
new Thread() {
public void run() {
iSender = new InternalCamSender();
iSender.start();
jButton_startSending.setText(STOPSENDING);
}
}.start();
} else {
new Thread() {
public void run() {
if(iSender.isAlive()) {
iSender.interrupt();
try {
iSender.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
iSender = null;
jButton_startSending.setText(STARTSENDING);
}
}.start();
}
Somehow I cannot stop the InternalCamSender like that. I tried with a volatile boolean before, was the same. I read the http://download.oracle.com/javase/1.5.0/docs/guide/misc/threadPrimitiveDeprecation.html page and tried also the example What should I use instead of Thread.stop? but even that was not stopping the thread? I am lost.
Any ideas?
edit:
found the answer for my clinet sending problem here http://www.petanews.de/code-snippets/java/java-threads-sauber-beenden-ohne-stop/
even i don't know why that is working. I am sure I tried that way before.
Problem solved!

You should close your resources (the streams and socket) in a finally block, rather than a catch block - this way the resources are always closed, whether an exception is caught or not.
It's also a bad practice to call System.exit() from within a catch block or within a thread - you are forcibly shutting down the whole JVM on any instance of an error. This is likely the cause of your problem with the server socket as well - whenever any exception is encountered with reading/closing the streams, you are exiting the JVM before you have a chance to close the server socket.