无法停止线程循环

Question

Can someone explain to me why my thread "m" isn't stopping when the logout jbutton is pushed? program seems to being hanging or in some infinite loop.I'm having trouble understanding what is going on with the thread in this case.

public class Log_GUI extends Jframe{
    private GUI talking;
    private volatile boolean[]go = new boolean[1]; 


                    go[0] = true;
                    setVisible(false);
                    GUI talking = new GUI(read_me,display_me,usr_name);
                    talking.create(go);


                    Thread m = new Thread(){
                      public void run(){

                          String me;
                          while (go[0]){

                              try
                              {


                                  try {
                                      me = (String) read_me.readObject();
                                      System.out.println(me);
                                      talking.sendme(me);

                                  }catch (ClassNotFoundException ClassNotfoundException){
                                  }
                              }catch (IOException IOException){
                              }
                          }
                      }

                    }; m.start();
                }

            } catch (ClassNotFoundException ClassNotFoundException) {

            }

    }catch (IOException IOException){

        }

        System.out.println("hello");
}
}



    public class GUI extends Jframe{

     public void create(boolean[]go){
       .........
         sign_out.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {

            go[0]=false;

        }
    });
    }
    }

Answer 1

You have a concurrency issue. volatile boolean[]go = new boolean[1]; only solves visibility and atomicity for variable go but it does not provide any guarantees for the values inside the array.

In a nutshell if thread 2 modifies go[0] to be true , thread 1 might never see it... if thread 2 creates a new array and assigns it to go then thread 1 will see it...

In method actionPerformed() you must do something like

boolean[] copy = copy(go);
copy[0] = false;
go = copy; // correct way to do it is CAS unless you know you are the only writer...

Also i dont understand the purpose of keeping array, in your case you could just keep one boolean flag (unless you intend to store multiple flags)...

If so you could do cheap concurrency using bitset approach, when you use single volatile int or long and set single bit in it... that allows you to have atomicity of change and visibility using single volatile for 32 or 64 flags...

Answer 2

尝试一起删除局部变量go[] ，始终从talking.stop();选择最新状态talking.stop();