Thread synchronization lock.

Start with the question of buying a ticket

package com.google.thread;
/**
 * Security issues under the example of buying a ticket.
 * Reason for appearing
 *
 * Solutions
 *
 * Synchronized code blocks.
 *
 * Restrooms on the train.
 */
class Ticker implements Runnable{
    public int num = 100;
    Object obj = new Object();
     public   void run() {
         while ( true ) {
             // synchronized code block 
                if (num > 0 ) {
                     try {
                        Thread.sleep(10);
                        System.out.println(Thread.currentThread().getName() + "...." + num--);
                    } catch (InterruptedException e) {
                        e.printStackTrace ();
                    }
                    
                }
            
        }
    }
}
/* 
 Thread safety issues. Ticket 0 will be sold.
Thread-3....-1
Thread-1....2
Thread-2....0
 */
public class ThreadDemo extends Thread {
    public static void main(String[] args) {
        Ticker t = new Ticker();
        
        Thread t1 = new Thread(t); // Create a thread 
        Thread t2= new Thread(t);
        Thread t3=new Thread(t);
        Thread t4=new Thread(t);
        
        t1.start(); // Start thread 
        t2.start();
        t3.start();
        t4.start();
    }
}

 

Thread safety issues.　　　

　　Ticket 0 will be sold.

　　Multiple threads are manipulating shared data.

　　There are multiple threads of code that operate on shared data.

　　When a thread is executing multiple pieces of code that operate on shared data, other threads participate in the operation. This can lead to thread safety issues.

Solution