实例变量共享造成的非线程安全问题

朱雀 2022-12-06 12:33 134阅读 0赞

### 文章目录 ###

*   *   *  不共享实例变量
         *  共享实例变量
         *  解决方案

### 不共享实例变量 ###

测试代码

public class MyThreadNotShared extends Thread {
    
        private int count = 5;
    
        public MyThreadNotShared(String name){
            super();
            this.setName(name);
        }
    
        @Override
        public void run() {
            super.run();
            while (count > 0) {
                count--;
                System.out.println("Thread is " + this.currentThread().getName() + ", count is " + count);
            }
        }
    }
    
    public class TestInstanceNotShared {
        public static void main(String[] args) {
            Thread a = new MyThreadNotShared("A");
            Thread b = new MyThreadNotShared("B");
            Thread c = new MyThreadNotShared("C");
    
            a.start();
            b.start();
            c.start();
        }
    }
    
    # 输出：
    Thread is A, count is 4
    Thread is C, count is 4
    Thread is B, count is 4
    Thread is C, count is 3
    Thread is A, count is 3
    Thread is A, count is 2
    Thread is C, count is 2
    Thread is B, count is 3
    Thread is C, count is 1
    Thread is C, count is 0
    Thread is A, count is 1
    Thread is A, count is 0
    Thread is B, count is 2
    Thread is B, count is 1
    Thread is B, count is 0

程序创建了3个线程，每个线程都有各自的count变量，不存在多个线程访问同一个实例变量问题。

### 共享实例变量 ###

测试代码

public class MyThreadShared extends Thread {
    
        private int count = 5;
    
        @Override
        public void run() {
            super.run();
            count--;
            System.out.println("Thread is " + this.currentThread().getName() + ", count is " + count);
        }
    }
    
    public class TestInstanceShared {
        public static void main(String[] args) {
            MyThreadShared myThread = new MyThreadShared();
    
            Thread a = new Thread(myThread,"A");
            Thread b = new Thread(myThread,"B");
            Thread c = new Thread(myThread,"C");
            Thread d = new Thread(myThread,"D");
            Thread e = new Thread(myThread,"E");
    
            a.start();
            b.start();
            c.start();
            d.start();
            e.start();
        }
    }
    
    # 输出:
    Thread is A, count is 3
    Thread is E, count is 0
    Thread is D, count is 1
    Thread is B, count is 3
    Thread is C, count is 2

A线程和B线程获取到的count都是3，出现了非现场安全问题。

对count的值处理分为3步（执行这些步骤的过程中会被其他线程所打断）：

1、获取count的值；

2、计算count-1；

3、计算结果对count赋值。

A线程和B线程得出相同值的过程如下：

1、在时间单位为1处，A线程取得count变量的值为5。

2、在时间单位为2处，B线程取得count变量的值为5。

3、在时间单位为3处，A线程执行count–计算，将计算后的值4存储到临时变量中。

4、在时间单位为4处，B线程执行count–计算，将计算后的值4也存储到临时变量中。

5、在时间单位为5处，A线程将临时变量中的值4赋值给count。

6、在时间单位为6处，B线程将临时变量中的值4也赋值给count。