multithreading - java synchronize multiple thread issue -

i write code test multiple threads how synchronize,but cannot expected result.the code can start 3 threads,but 1 thread process shared resource.what wrong code.

class threaddemo1{    public static void main (string[] args){        multithread tt = new multithread();        new thread(tt).start();        new thread(tt).start();        new thread(tt).start();    } } class multithread implements runnable {   int tickets = 100;   object _lock = new object();   public void run () {     system.out.println(thread.currentthread().getname());     synchronized(_lock) {       while (true) {           if (tickets>0) {           try {             thread.sleep(10);           } catch (exception e) {}           system.out.println(thread.currentthread().getname() + " selling "+tickets--);         }       }     }   } } 

you sleeping while holding lock. there no reason multithread if going that.

public void run () {     system.out.println(thread.currentthread().getname());     while(tickets > 0) {         synchronized(_lock) {             if (tickets > 0) {                 system.out.println(thread.currentthread().getname() + " selling " + tickets--);             }         }         try {             thread.sleep(10);         } catch (exception e) {         }     } } 

i'm guessing sleep placeholder processing. if possible, should check , decrement inside synchronized block, lengthy processing outside it.

in order locks , multi-threading useful you, must make sure synchronized code takes little time possible, since code can run 1 thread @ time.

in code, thing wasn't single-threaded first system.println.

---

fyi, in mind, if have print statements accurate possibly out of order, better have:

public void run () {     system.out.println(thread.currentthread().getname());     while(tickets > 0) {         int oldtickets = 0;         synchronized(_lock) {             if (tickets > 0) {                 oldtickets = tickets--;             }         }         if(oldtickets > 0) {             system.out.println(thread.currentthread().getname() + " selling " + oldtickets);             try {                 thread.sleep(10);             } catch (interruptedexception e) {             }         }     } }