synchronized 无效问题？

Question

public class App2 {
    public static void main( String[] args ) throws Exception
    {
        System.out.println("虚拟机内核数："+Runtime.getRuntime().availableProcessors());

        Runnable r = new Runnable() {
            Integer count = 100;
            @Override
            public void run() {
                threadRun(count);
            }
        };

        Thread ct1 = new Thread(r,"子线程1号");
        Thread ct2 = new Thread(r,"子线程2号");
        Thread ct3 = new Thread(r,"子线程3号");
        Thread ct4 = new Thread(r,"子线程4号");
        Thread ct5 = new Thread(r,"子线程5号");
        Thread ct6 = new Thread(r,"子线程6号");
        Thread ct7 = new Thread(r,"子线程7号");
        Thread ct8 = new Thread(r,"子线程8号");
        Thread ct9 = new Thread(r,"子线程9号");

        ct1.start();
        ct2.start();
        ct3.start();
        ct4.start();
        ct5.start();
        ct6.start();
        ct7.start();
        ct8.start();
        ct9.start();
    }
    /*
    * synchronized 无效问题？
    * */
    synchronized private static void threadRun( Integer count){
        try {
            count++;
            System.out.println(Thread.currentThread().getName() + "--计算count,计算后值为：" + count);
        }catch (Exception e){}
    }
}
​​​​​​​}

Answer 1

Integer 属于不可变对象，你每次修改 count 的值，其实 count 的引用都已经被改变了。你可以自己模拟一个内部值的可变的 Integer 对象：

public class MutableInt {
    private int value;
    
    public MutableInt(int initial) {
        value = initial;
    }
    
    public int get() {
        return value;
    }
    
    public String toString() {
        return String.valueOf(value);
    }
    
    public void inc() {
        value++;
    }
}

然后你把 count 的类型声明为 MutableInt，count++ 替换为 count.inc() —— 就可以看到你想要的结果：

public static void main(String[] args) throws Exception {
    System.out.println("虚拟机内核数："
            + Runtime.getRuntime().availableProcessors());

    
    
    Runnable r = new Runnable() {
        MutableInt count = new MutableInt(100);
        
        @Override
        public void run() {
            threadRun(count);
        }
    };

    Thread ct1 = new Thread(r, "子线程1号");
    Thread ct2 = new Thread(r, "子线程2号");
    Thread ct3 = new Thread(r, "子线程3号");
    Thread ct4 = new Thread(r, "子线程4号");
    Thread ct5 = new Thread(r, "子线程5号");
    Thread ct6 = new Thread(r, "子线程6号");
    Thread ct7 = new Thread(r, "子线程7号");
    Thread ct8 = new Thread(r, "子线程8号");
    Thread ct9 = new Thread(r, "子线程9号");

    ct1.start();
    ct2.start();
    ct3.start();
    ct4.start();
    ct5.start();
    ct6.start();
    ct7.start();
    ct8.start();
    ct9.start();
}

/*
 * synchronized 无效问题？
 */
synchronized private static void threadRun(MutableInt count) {
    try {
        count.inc();
        System.out.println(Thread.currentThread().getName()
                + "--计算count,计算后值为：" + count);
    } catch (Exception e) {
    }
}