i++ 线程不安全示例

public class TestAtomicInteger{
    private static int count = 0;

    public static void main(String[] args) throws InterruptedException {

        for(int i=0;i<1000;i++){
           new Thread(new Runnable() {
               @Override
               public void run() {
                   for(int j=0;j<100;j++){
                       count++;
                   }
               }
           }).start();
        }
        Thread.sleep(5*1000);
        System.out.println(count);
    }
}

输出截图:

result.png

AtomicInteger解决线程安全问题

public class TestAtomicInteger{
    private static AtomicInteger count= new AtomicInteger(0);

    public static void main(String[] args) throws InterruptedException {

        for(int i=0;i<10000;i++){
            new Thread(new Runnable() {
                @Override
                public void run() {
                    for(int j=0;j<1000;j++){
                        count.getAndIncrement();
                    }
                }
            }).start();
        }
        Thread.sleep(5*1000);
        System.out.println(count);
    }
}

输出截图:

result.png

思考为什么？

第一种写法报错，第二种没有问题

//第一种, idea提示错误
public class TestAtomicInteger{

    public static void main(String[] args) throws InterruptedException {
        int count = 0;

        for(int i=0;i<1000;i++){
            new Thread(new Runnable() {
                @Override
                public void run() {
                    for(int j=0;j<100;j++){
                        count++;    //错误提示：Variable 'count' is accessed from within inner class, needs to be final or effectively final
                    }
                }
            }).start();
        }
        Thread.sleep(5*1000);
        System.out.println(count);
    }
}

//第二种， 运行结果正常
public class TestAtomicInteger{

    public static void main(String[] args) throws InterruptedException {
        AtomicInteger count= new AtomicInteger(0);

        for(int i=0;i<10000;i++){
           new Thread(new Runnable() {
               @Override
               public void run() {
                   for(int j=0;j<1000;j++){
                       count.getAndIncrement();
                   }
               }
           }).start();
        }
        Thread.sleep(5*1000);
        System.out.println(count);
    }
}

个人理解第一种为值传递，第二种为引用传递；具体不是很清晰，大概感觉是这么个原因