Sample https://github.com/spring-projects/spring-integration-samples/tree/master/basic/tcp-client-server is great to build a TCP Server application. Its simple and runs on JVM. It does not need any Application Server.
Sample uses command line input to run the program. I want the server to accept data only from the Socket port and not through the command line. If I remove the command line input, main thread is finishing and the program no longer accepts input from the port. I have to keep the main thread running all the time.
I am thinking some thing like this:
boolean isGatewayStopped = false;
while (!isGatewayStopped) {
try {
Thread.sleep(5000);
isGatewayStopped = getGatewayStatus();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
I have two questions:
is there a clean way of making the Main thread continue to run?
How to know that Gateway is stopped? If the user sends TCP data as "quit" then gateway can be stopped. is there any way to know that gateway is stopped?
Thanks
Another solution is like this:
public static void main(String[] args) throws Exception {
ConfigurableApplicationContext ctx = SpringApplication.run(Application.class, args);
System.out.println("Hit 'Enter' to terminate");
System.in.read();
ctx.close();
}
You start the ApplicationContext and wait for the stop from the console input.
EDIT
For the case when you would like to stop program via an event in the application, you can register ApplicationListener and wait on the barrier before existing from the main:
CountDownLatch exitLatch = new CountDownLatch(1);
ctx.addApplicationListener(ContextClosedEvent e -> exitLatch.countDown())
exitLatch.await();
Now you should just come up with some logic in your application to call ctx.stop() there.
You can call wait() on the gateway thread from the main thread to make the main thread wait until gateway thread finishes. You'll need to call the notify() from gateway thread (when it should stop) to indicate that it's finished and waiting threads should proceed to run (in this case main will run and exit). An example can be found here.
Or else (a different solution for a very simple app), you can try something like following to read data from a main method and stop the program when the data read is equal to a command to stop the program:
class Server
{
static Executor pool = Executors.newFixedThreadPool(5);
public static void main(String[] args) throws IOException
{
ServerSocket serverSocket = new ServerSocket(9000);
while (true)
{
final Socket s = serverSocket.accept();
Runnable r = new Runnable()
{
#Override
public void run()
{
// read data from socket 's' here
// call System.exit() if command is to stop.
}
};
pool.execute(r);
}
}
Related
The following code is taken from an example in the Jersey project. See here.
public class App {
private static final URI BASE_URI = URI.create("http://localhost:8080/base/");
public static final String ROOT_PATH = "helloworld";
public static void main(String[] args) {
try {
System.out.println("\"Hello World\" Jersey Example App");
final ResourceConfig resourceConfig = new ResourceConfig(HelloWorldResource.class);
final HttpServer server = GrizzlyHttpServerFactory.createHttpServer(BASE_URI, resourceConfig, false);
Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
#Override
public void run() {
server.shutdownNow();
}
}));
server.start();
System.out.println(String.format("Application started.\nTry out %s%s\nStop the application using CTRL+C",
BASE_URI, ROOT_PATH));
//////////////////////////////
Thread.currentThread().join();
//////////////////////////////
} catch (IOException | InterruptedException ex) {
//
}
}
}
I understand what is going on apart from the use of Thread.currentThread().join();.
I'm a Java newbie and my understanding is that this will block the execution of the current thread (in this case, the main thread), and effectively deadlock it. i.e. it will cause the current (main) thread to block until the current (main) thread finishes, which will never happen.
Is this correct? If so, why is it there?
Thread.currentThread().join() blocks the current thread forever. In your example, that prevents the main from exiting, unless the program is killed, e.g. with CTRL+C on Windows.
Without that line, the main method would exit right after the server is started.
An alternative would have been to use Thread.sleep(Long.MAX_VALUE);.
It's a common misunderstanding that if the main thread exits, the program will exit.
This is only true if there is no non-daemon thread running. This may be true here, but usually it is better IMHO to make the background threads this main is "waiting" for non-dameon and let the main thread exit when it doesn't have anything to do. I have see developers put Thread.sleep() wrapped in an infinite loop. etc.
It's an example. It's just not a very good one.
They're trying to show you how to make a thread that runs forever.
Thread.currentThread().join(); is a statement that takes forever to complete. You're supposed to replace it with your own code that runs forever and, presumeably does something useful.
I have a thread pool on the function that the clients calling.. to make only (n) clients execute this function upload() and the others wait.. i tried to call sleep() in the implementation of the function but it didn't work ...
note: I'm doing this to have time to see that other clients doesn't execute the function while there are (n) clients execute it...
i need fast help please ..
the code of Server:
public class Server extends UnicastRemoteObject implements ExcutorInterface
{
public Server()throws RemoteException
{
System.out.println("Server is in listening mode");
}
public static void main(String arg[]) throws InterruptedException
{
try{
LocateRegistry.createRegistry(1234);
Server p=new Server();
Naming.bind("//127.0.0.1:1234/obj",p);
}catch(Exception e)
{
System.out.println("Exception occurred : "+e.getMessage());
}
}
#Override
public void executeJob() throws RemoteException {
System.out.println("Inside executeJob...");
doJob a=new doJob("req_id","usrname","pwd");
ExecutorService threadExecutor = Executors.newFixedThreadPool(2);
threadExecutor.execute(a);
threadExecutor.shutdown();
}
}
the code of doJob :
public class doJob implements Runnable {
String request_id="", usrnamee="", pswd="";
public static int i = 1;
public doJob(String request_id,String usrnamee,String pswd) {
this.request_id=request_id;
this.usrnamee=usrnamee;
this.pswd=pswd;
}
public void upload() throws InterruptedException, IOException {
Thread.sleep(1000*15);
}
public void run() {
upload();
}
}
and I call executeJob(); in the client
One suggestion is to make "threadExecutor" a static member variable of
server.
If you want only n clients then make the pool have n threads
ExecutorService threadExecutor = Executors.newFixedThreadPool(n);
Shutting down within execute method id does not seem right.
The pool should be shutdown only when you decide to shutdown the
server.
Till then it should be alive and process the client requests.
So you have to remove the shutdown and newFixedThreadPool statements
out of the executeJob method.
To elaborate on my comment, you should surround the Thread.sleep in a try/catch and make sure the thread sleeps as long as you wish it to do so. It would look something like this:
long wakeTime = new Date().getTime() + (1000 * 15);
while ((new Date()).getTime() < wakeTime) {
try {
Thread.sleep(1000*15);
} catch (InterruptedException e) {
// do nothing
}
}
I suspect your thread was waking early because of a signal perhaps because of your call to threadExecutor.shutdown() immediately after threadExecutor.execute(a). You might want to consider calling threadExecutor.awaitTermination() as well.
Edits after learning that the task never executes:
Because threadExecutor.shutdown() doesn't wait for the tasks to complete, it looks like your program is immediately exiting. You should try using threadExecutor.awaitTermination() after your call to threadExecutor.shutdown(), placing it in a loop similar to the one suggested for Thread.sleep().
Get rid of the thread pool and use a counting semaphore to control inline execution of the upload.
I hope Thread.sleep() will help you to resolve.
Also you can use wait().
I'm writing a client/server application in Java using sockets. In the server, I have a thread that accepts client connections, this thread runs indefinitely. At some point in my application, I want to stop accepting client connection, so I guess destroying that thread is the only way. Can anybody tell me how to destroy a thread?
Here's my code:
class ClientConnectionThread implements Runnable {
#Override
public void run() {
try {
// Set up a server to listen at port 2901
server = new ServerSocket(2901);
// Keep on running and accept client connections
while(true) {
// Wait for a client to connect
Socket client = server.accept();
addClient(client.getInetAddress().getHostName(), client);
// Start a new client reader thread for that socket
new Thread(new ClientReaderThread(client)).start();
}
} catch (IOException e) {
showError("Could not set up server on port 2901. Application will terminate now.");
System.exit(0);
}
}
}
As you can see, I have an infinite loop while(true) in there, so this thread will never stop unless somehow I stop it.
The right way to do this would be to close the server socket. This will cause the accept() to throw an IOException which you can handle and quit the thread.
I'd add a public void stop() method and make the socket a field in the class.
private ServerSocket serverSocket;
public ClientConnectionThread() {
this.serverSocket = new ServerSocket(2901);
}
...
public void stop() {
serverSocket.close();
}
public void run() {
while(true) {
// this will throw when the socket is closed by the stop() method
Socket client = server.accept();
...
}
}
Generally you don't. You ask it to interrupt whatever it is doing using Thread.interrupt().
A good explanation of why is in the Javadoc.
From the link:
Most uses of stop should be replaced by code that simply modifies some
variable to indicate that the target thread should stop running. The
target thread should check this variable regularly, and return from
its run method in an orderly fashion if the variable indicates that it
is to stop running. (This is the approach that the Java Tutorial has
always recommended.) To ensure prompt communication of the
stop-request, the variable must be volatile (or access to the variable
must be synchronized).
It should be noted that in all situations where a waiting thread doesn't respond to Thread.interrupt, it wouldn't respond to Thread.stop either.
For your specific situation you will have to call serverSocket.close, since it does not respond to Thread.interrupt.
I took this code:
28 public static void main(String[] args) throws IOException {
29 HttpServer httpServer = startServer();
30 System.out.println(String.format("Jersey app started with WADL available at "
31 + "%sapplication.wadl\nTry out %shelloworld\nHit enter to stop it...",
32 BASE_URI, BASE_URI));
33 System.in.read();
34 httpServer.stop();
35 }
Does line 33 "System.in.read()" means that it will block until there is input? Will this also work when starting the Java application using UNIX rc script - not manually started from a command line?
I'd like to write a Java application to listen for HTTP connections. The application will be started automatically when the system boots (using UNIX rc scripts). It means that the application will run continuously - theoretically forever, until purposefully stopped. What is the best way to implement this in the Java main() method?
It looks like a weird black magic but following does the trick in very elegant way
Thread.currentThread().join();
As a result the current thread, main for instance, waits on join() for thread main, that is itself, to end. Deadlocked.
The blocked thread must not be a daemon thread of course.
Leaving the main method in Java does not automatically end the program.
The JVM exists if no more non-daemon threads are running. By default the only non-daemon thread is the main thread and it ends when you leave the main method, therefore stopping the JVM.
So either don't end the main thread (by not letting the main method return) or create a new non-daemon thread that never returns (at least not until you want the JVM to end).
Since that rule is actually quite sensible there is usually a perfect candidate for such a thread. For a HTTP server, for example that could be the thread that actually accepts connections and hands them off to other threads for further processing. As long as that code is running, the JVM will continue running, even if the main method has long since finished running.
#Joachim's answer is correct.
But if (for some reason) you still want to block the main method indefinitely (without polling), then you can do this:
public static void main(String[] args) {
// Set up ...
try {
Object lock = new Object();
synchronized (lock) {
while (true) {
lock.wait();
}
}
} catch (InterruptedException ex) {
}
// Do something after we were interrupted ...
}
Since the lock object is only visible to this method, nothing can notify it, so the wait() call won't return. However, some other thread could still unblock the main thread ... by interrupting it.
while (true) { ... } should go on for a pretty long time. Of course, you'll have to figure out some way of stopping it eventually.
A common trick is to have some volatile boolean running = true, then have the main loop be while (running) { ... } and define some criteria by which a thread sets running = false.
Back to Threads, thats exactly what i wanted. Btw this awesome tutorial helped me a lot.
Main.java
public class Main {
public static void main(String args[]) {
ChatServer server = null;
/*if (args.length != 1)
System.out.println("Usage: java ChatServer port");
else*/
server = new ChatServer(Integer.parseInt("8084"));
}
}
and ChatServer.java Class extends a Runnable
public class ChatServer implements Runnable
{ private ChatServerThread clients[] = new ChatServerThread[50];
private ServerSocket server = null;
private Thread thread = null;
private int clientCount = 0;
public ChatServer(int port)
{ try
{ System.out.println("Binding to port " + port + ", please wait ...");
server = new ServerSocket(port);
System.out.println("Server started: " + server);
start(); }
catch(IOException ioe)
{
System.out.println("Can not bind to port " + port + ": " + ioe.getMessage()); }
}
public void start() {
if (thread == null) {
thread = new Thread(this);
thread.start();
}
}
.... pleas continue with the tutorial
So in the main Method a Runnable is being instantiated and a new Thread as shown in
public void start() {
is being instantiated with the runnable.
That cases the JVM to continue executing the process until you quit the project or the debugger.
Btw thats the same as Joachim Sauer posted in his answere.
Java program terminates when there are no non-daemon threads running. All you need is to have one such running thread. You could do it using infinite loops but that would consume CPU cycles. The following seems like a reasonable way to do it.
ExecutorService executor = Executors.newSingleThreadExecutor();
executor.submit(() -> {}); //submit a task that does what you want (in this case, nothing)
Also we can achieve the same with the ReentrantLock and call wait() on it:
public class Test{
private static Lock mainThreadLock = new ReentrantLock();
public static void main(String[] args) throws InterruptedException {
System.out.println("Stop me if you can");
synchronized (mainThreadLock) {
mainThreadLock.wait();
}
}
There is a console Java application which is supposed to run until it is stopped by Ctrl+C or closing the console window. How that application can be programmed to execute a clean up code before exit?
You could use a Shutdown Hook.
Basically you need to create a Thread which will perform your shutdown actions, and then add it as a shutdown hook. For example:
class ShutdownHook extends Thread
{
public void run()
{
// perform shutdown actions
}
}
// Then, somewhere in your code
Runtime.getRuntime().addShutdownHook(new ShutdownHook())
A Shutdown hook is the way to go, but be aware that there is no guarantee that the code is actually executed. JVM crashes, power failures, or a simple "kill -9" on your JVM can prevent the code from cleaning up. Therefore you must ensure that your program stays in a consistent state even if it has been aborted abruptly.
Personally, I simply use a database for all state-storage. Its transactions model makes sure that the persistent storage is in a sane state no matter what happens. They spend years making that code fool-proof, so why should I waste my time on problems already solved.
Program to delete temp file bat.bat when program is exited:
public class Backup {
public static void createBackup(String s)
{
try{
String fileName ="C:\\bat"+ ".bat";
FileWriter writer=new FileWriter(fileName);
String batquery="cd C:\\Program Files\\MySQL\\MySQL Server 5.0\\bin"
+ "\nmysqldump -uroot -proot bankdb > \""+s+".sql\""
+"\nexit";
writer.append(batquery);
writer.close();
}
catch(Exception e){e.getMessage();}
try{
Process p =Runtime.getRuntime().exec("cmd /c start C:\\bat.bat");
}
catch(Exception e){e.getMessage();}
ShutDownHook sdh=new ShutDownHook();
Runtime.getRuntime().addShutdownHook(sdh);
}
}
class ShutDownHook extends Thread
{
public void run()
{
try
{
File f=new File("c:/bat.bat");
f.delete();
}
catch(Exception e){e.getMessage();}
}
}
The code written inside a Threads run() method will execute when the runtime object terminates...
class ShutdownHookclass extends Thread {
public void run() {
// perform shutdown actions
}
}
//could be written anywhere in your code
Runtime.getRuntime().addShutdownHook(new ShutdownHookclass())