多线程（二）多线程安全与同步

一，环境

  idea

二.什么是线程安全问题，为什么会有线程安全问题

线程安全问题产生于多个线程同时访问共享资源（通常查询不会产生）

三.举例

假如我现在想讲一个数循化加一，最终增加到1000.但是需要用5个线程来加

class Count implements Runnable{
    private int count=1;

    public void run() {
        while (count<=1000){
            count=count+1;
            System.out.println(Thread.currentThread().getName()+",count:"+count);
        }

    }
}
public class ThreadTest {
    public static void main(String[] args) {
        Count  count=new Count();
        Thread t1=new Thread(count,"线程一");
        Thread t2=new Thread(count,"线程二");
        Thread t3=new Thread(count,"线程三");
        Thread t4=new Thread(count,"线程四");
        Thread t5=new Thread(count,"线程五");
        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();
    }

}

结果：

代码显示:最多会增加到1000循环就会结束那么为什么会出现1001呢！！

由于现在是多线程增加，有可能当count增加到999的时候同时又两个线程都进入了while循环里，然后就连续增加了两次

那么怎么解决呢！！！

四.使用锁来解决

4.1同步代码块

class Count implements Runnable{
    private volatile int  count=1;
    private static Object oj = new Object();

    public void run() {
        while (count<=1000){
            synchronized (oj) {//oj就是锁对象可以为任意对象也可为this
                if(count<=1000) {
                    count = count + 1;
                    System.out.println(Thread.currentThread().getName() + ",count:" + count);
                }
            }
        }

    }

}
public class ThreadTest {
    public static void main(String[] args) {
        Count  count=new Count();
        Thread t1=new Thread(count,"线程一");
        Thread t2=new Thread(count,"线程二");
        Thread t3=new Thread(count,"线程三");
        Thread t4=new Thread(count,"线程四");
        Thread t5=new Thread(count,"线程五");
        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();
    }

}

4.2使用同步方法

class Count implements Runnable{
    private volatile int  count=1;
    private static Object oj = new Object();

    public void run() {
        while (count<=1000){
            
            sole();
               
            }
        

    }
    public synchronized void sole(){//这把锁的锁对象就是this
        if(count<=1000) {
            count = count + 1;
            System.out.println(Thread.currentThread().getName() + ",count:" + count);
        }
    }
}
public class ThreadTest {
    public static void main(String[] args) {
        Count  count=new Count();
        Thread t1=new Thread(count,"线程一");
        Thread t2=new Thread(count,"线程二");
        Thread t3=new Thread(count,"线程三");
        Thread t4=new Thread(count,"线程四");
        Thread t5=new Thread(count,"线程五");
        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();
    }

}