Question

I’m using socket to writing multiple clients – server application in java. I wrote simple client – server application and everything was ok but when i tried change it to multi clients app I got the exception when i started client:

Exception in thread "pool-1-thread-1" java.lang.NullPointerException
    at MiniSerwer.run(Serwer.java:110)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:679)

I have two more classes (threads – InWorker and OutWorker to input and output).

Serwer.java (without InWorker and OutWorker) :

public class Serwer {
Serwer(int port) {
ServerSocket serversocket=null;
ExecutorService exec= Executors.newCachedThreadPool();

    try {
        serversocket=new ServerSocket(port);
    } catch (IOException e) {
        e.printStackTrace();
}
    while(true) {
    Socket socket=null;
    try {
        socket = serversocket.accept();
    } catch (IOException e) {
        e.printStackTrace();
    }
    exec.execute(new MiniSerwer(socket)); // create new thread
        }
} }

MiniSerwer – helper class to create owns thread to everyone client

class MiniSerwer implements Runnable{

    Socket socket=null;
    ExecutorService exec=null;
    ObjectOutputStream oos=null;
    ObjectInputStream ois=null;

    MiniSerwer(Socket socket) {
        this.socket=socket;
        try {
            oos=new ObjectOutputStream(socket.getOutputStream());
            oos.flush();
            ois=new ObjectInputStream(socket.getInputStream());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void run() {
        while(true) {
            exec.execute(new InWorker(socket, ois)); // input stream
            exec.execute(new OutWorker(socket, oos)); //output stream
            Thread.yield();
        }
    }
}

I change my program but it still doesn’t work. Any other suggestion ?

Server:

public class Serwer implements Runnable{

ServerSocket serversocket=null;
ExecutorService exec= Executors.newCachedThreadPool();
int port;

Serwer(int port) {
    this.port=port;
}

public void run() {
    try {
        serversocket=new ServerSocket(port);
        while(true) {
            Socket socket=null;
            try {
                socket = serversocket.accept();
                exec.execute(new MiniSerwer(socket)); // create new thread
            } catch (IOException e) {
                e.printStackTrace();
                throw new RuntimeException(e);
            }
        }
    } catch (IOException e) {
        e.printStackTrace();
}
}

public static void main(String[] args) {
    int port;
    Scanner in = new Scanner(System.in);
    System.out.println("Enter the port:");
    port = in.nextInt();
    ExecutorService exec=Executors.newCachedThreadPool();
    exec.execute(new Serwer(port));
}   
 }

MiniSerwer:

class MiniSerwer implements Runnable{

    Socket socket=null;
    ExecutorService exec=Executors.newCachedThreadPool();
    ObjectOutputStream oos=null;
    ObjectInputStream ois=null;

    MiniSerwer(Socket socket) {
        this.socket=socket;
    }

    public void run() {
        try {
            oos=new ObjectOutputStream(socket.getOutputStream());
            oos.flush();
            ois=new ObjectInputStream(socket.getInputStream());
        } catch (IOException e) {
            e.printStackTrace();
        }
        while(true) {
            exec.execute(new InWorker(socket, ois)); // input stream
            exec.execute(new OutWorker(socket, oos)); //output stream
            Thread.yield();
        }
    }
}

I have a lot of exceptions when I’m trying send message from client to server.

2 sec. after connecting on the server side (i don’t send anything) :

Exception in thread "pool-2-thread-1" java.lang.OutOfMemoryError: unable to create new native thread
    at java.lang.Thread.start0(Native Method)
    at java.lang.Thread.start(Thread.java:657)
    at java.util.concurrent.ThreadPoolExecutor.addWorker(ThreadPoolExecutor.java:943)
    at java.util.concurrent.ThreadPoolExecutor.processWorkerExit(ThreadPoolExecutor.java:992)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:679)

Answer 1

Your executor service, in your MiniSerwer class, is never initialized. That’s the root cause of your NPE.

But like I mentioned in my comment, you should not be doing all the Serwer logic in it’s constructor. The object never gets fully initialized because you never exit the constructor. Make the entire class Runnable and move that logic to the overridden run method. Then add a main method to be used in actually instantiation/running the server.

Also, in your MiniSerwer, your stream initialization can fail. You need to validate the streams aren’t null before using them in the run method. Or just move the initialization logic for them to the beginning of your run method.

EDIT

You also have a bug in your MiniSerwer implementation, you spawn an infinite number of threads to process the object input and output streams:

while(true) {
    exec.execute(new InWorker(socket, ois)); // input stream
    exec.execute(new OutWorker(socket, oos)); //output stream
    Thread.yield();
}

You will either run out of threads, run out of memory, or both. To be honest, your solution is over-engineered, I doubt the In and Out workers really need to be separated the way you have them now.