java multithreading. sharing data in a thread-safe manner -

i'm programming tcp socket application in java , have doubt regards how in thread-save manner.
in eample, static method startserver(int port) creates passive tcp socket. in main, wait incoming connection, managed dedicated thread.

firs of all, understand interaction between threads.

let a, b 2 instance of same class:

1. instance attributes of not visibile b;
2. static attributes visible both , b.

let t1 , t2 2 threads of same class:

1. does attributes visibility works in same manner instance? in other words, static attributes can shared thread?
2. non static attributes thread-safe?

in code thread-safe?

import java.io.*; import java.net.*; import java.util.*;  public class multithreadserver extends thread      {      private socket client;      private string nonstaticattribute;      private static volatile vector<string> clientsvector;       public multithreadserver(socket c, string enc)         {client = c;          nonstaticattribute = new string("val=" + math.random());}      /** create passive socket*/     public static serversocket startserver() throws bindexception, ioexception         {clientsvector= new vector<string>();}      public void run()         {          try             {             multithreadserver.clientsvector.addelement(clientname); // variable shared in thread-safe way between threads? vector in internally synchronized              while(true)                 {                 /** line print different values each thread? (the value assigned in costructor?)                 or thread display same value (the value of costructor of last thread ? ) */                 system.out(nonstaticattribute);                 thread.sleep(1000);}             }         catch(ioexception e)             {}             }         }      /** main */     public static void main(string[] argv) throws ioexception         { // start del server         myclass.startserver();         myclass t1 = new myclass();         t1.start();         myclass t2 = new myclass();         t2.start();         ....         myclass tn = new myclass();         tn.start();         }         } 

if have correctly understood, locals instances attributes thread-safe, because have own state (in case multiple threads works independently multiple istances).
in opinion, use of static vector thread-save, because vector internally synchronized.

what's if use non synchronized resource file?

public class myclass extends thread {     // same code other example     private final static file file = new file("rubrica.txt");      void run() {     .....     synchronized (file)         {filewriter fout= new filewriter(file,true);          fout.write("add content @ end");          fout.flush();          fout.close();         }     } 

i appreciate help.
wish nice day :)