Java process out of memory when creating thread on CentOS 6

Question

I get this error basically less then an hour after i fire this nohup java -cp server.jar:mysql-connector.jar com.server.test.EchoMulti &. The error:

Exception in thread "main" java.lang.OutOfMemoryError: unable to create new native thread    
    at java.lang.Thread.start0(Native Method)                                            
    at java.lang.Thread.start(Unknown Source)                                            
    at com.server.test.EchoMulti.main(EchoMulti.java:14)

I'm new to servers and Java. This is the first thing I did on a server and in Java and it's a multiplayer for Android game. The game is doing quite good so there are many people trying to play the multiplayer. I already fixed the mySQL max_connections but now i got whis problem. I would also like to know if there is a way to restart process automatically when it gets an error that causes a crach. And a way to automatically start it on reboot.

The EchoMulti class that's causing it:

package com.server.test;
import java.net.*;
import java.io.*;

public class EchoMulti {

    public static void main(String[] args) throws IOException {

        int portNumber = 25003;
        boolean listening = true;

        try (ServerSocket serverSock = new ServerSocket(portNumber)) { 
            while (listening) {
                new EchoServer(serverSock.accept()).start();
            }
        } catch (IOException e) {
            System.err.println("Could not listen on port " + portNumber);
            System.exit(-1);
        }
    }

}

The EchoServer class:

package com.server.test;
import java.net.*;
import java.io.*;
import java.sql.*;
import java.lang.Thread;


public class EchoServer extends Thread {
     private Socket sock = null;

     public EchoServer(Socket sock) {
         super("EchoServer");
         this.sock = sock;
     }

public void run(){
    String url = "jdbc:mysql://IP:3306/DB";
    String username = "USER";
    String password = "PASSWORD";
    Connection con = null;
    Statement sta = null;
    ResultSet resultSet = null;

    try {
        //System.out.println("Connecting database...");
        con = DriverManager.getConnection(url, username, password);
        //System.out.println("Database connected!");
    } catch (SQLException e) {
        throw new RuntimeException("Cannot connect the database!", e);
    }

    try (
        PrintWriter out = new PrintWriter(sock.getOutputStream(), true);                   
        BufferedReader in = new BufferedReader(new InputStreamReader(sock.getInputStream()));
    ) {
        String inputLine;
        while ((inputLine = in.readLine()) != null) {
            if(inputLine.equalsIgnoreCase("get")){
                    SEND SOMETHING
            } else {
                String catched[] = inputLine.split("\\;");
                if(catched[0].equalsIgnoreCase("sub")){
                    SEND SOMETHING
                } else if(catched[0].equalsIgnoreCase("res")){
                    SEND SOMETHING
                }
            }
        }
    } catch (IOException e) {
        System.out.println("Exception caught when trying to listen on port 25003 or listening for a connection");
        System.out.println(e.getMessage());
    } finally {
        //System.out.println("Closing the connection.");
        if (con != null) try { con.close(); } catch (SQLException ignore) {}
        try { sock.close(); } catch (IOException ignore) {} finally {}
    }
}
public void close() throws IOException {
    if (sock != null) sock.close();
}
}

Answer 1

You can also have a memory leak somewhere, or something preventing the correct garbage collection of used memory.

I would do the following three things.

Modify the EchoServer class You can add a static member to understand if, when the crash occurs, the number of existing thread in memory is the one you expect (they should be as many as the connected players).

public class EchoServer extends Thread {
     private Socket sock = null;
     private static Integer CONNECTED = 0;
     public EchoServer(Socket sock) {
         try{
         super("EchoServer");
         this.sock = sock;
         CONNECTED++;
         } catch (Exception ex)
         {
            Logger.getLogger(EchoServer.class.getName()).log(Level.SEVERE, "Currently CONNECTED: "+CONNECTED);
         }
     }

     .... your stuff here

     @Override
     public void finalize(){
         super.finalize();  
         CONNECTED--;
     }
}

If have more threads than connected players something is wrong.

Use a profiler Use a profiler to observe the memory usage and identify bottlenecks. Also be sure there aren't "strange" objects surviving generations or that the memory is not increasing over time without an increment of the players. NetBeans has a nice profiler that can be connected via TCP to live apps too.

Use -Xmx Use java -Xmx1g to launch the server.