Single Threaded Execution----能通过这座桥的，只有一个人

Single Threaded Execution----能通过这座桥的，只有一个人

 

当有一座小桥，每次只能通过一个人，如果同时有两个人在桥上，桥会断开如今有三个姓和名的首字母相同的人多次通过此桥

 

package com.quest.mchen2.thread.single_thread_execution;


public class Gate {
     
      private int counter=0;
      private String name= "Nobody";
      private String address= "Nowhere";
     
      //为了防止多线程情况下对象先后交叉执行，从而导致将共享实例的字段改写了
      public synchronized void pass(String name, String address){
            this. counter++;
            this. name=name;
            this. address=address;
            check();
     }
     
      @Override
      public synchronized String toString() {
            // TODO Auto-generated method stub
            return "No."+ counter+ ":"+ name+ ", "+ address;
     }
     
      private void check(){
            if( name.charAt(0)!= address.charAt(0)){
                System. out.println( "*****BROKEN*****" +toString());
           }
     }
}


package com.quest.mchen2.thread.single_thread_execution;

public class UserThread extends Thread{

      private final Gate gate;
      private final String myname;
      private final String myaddress;
     
      public UserThread(Gate gate, String myname, String myaddress){
            this. gate=gate;
            this. myname=myname;
            this. myaddress=myaddress;
     }
     
      @Override
      public void run() {
            // TODO Auto-generated method stub
           System. out.println( myname+ " BEGIN");
            while( true){
                 gate. pass( myname, myaddress);
           }
     }
     
}


package com.quest.mchen2.thread.single_thread_execution;

public class Main {

      /**
      * @param args
      */
      public static void main(String[] args) {
            // TODO Auto-generated method stub
           
           System. out.println( "Testing Gate, hit CTRL+C to exit");
           Gate gate= new Gate();
            new UserThread(gate, "Alice", "Alaska").start();
            new UserThread(gate, "Bobby", "Brazil").start();
            new UserThread(gate, "Chris", "Canada").start();
     }

}

 

由于在Gate类的pass方法加上了synchronized  所以可以阻止name与address的首字母出现的情况

toString()加上synchronized是因为当别的线程X调用toString。线程X在引用name字段的值之后到引用addresszi字段之间，UserThread的线程

可能会改掉address的值，这样一来，toString方法可能对线程使用name与address第一个字母不一致的值来构成字符串