Static方法在多线程环境下的运行

最近看了Jfinal 一致对model的那个static final dao有些疑惑，全局一个实例安全吗?同时也出了一个疑惑，静态方法执行会有并发影响吗？看代码

StaticThread.java

public class StaticThread implements Runnable {
    @Override
    public void run() {
        // TODO Auto-generated method stub
        StaticAction.print();
    }

    public static void main(String[] args) {
        for (int i = 0; i < 2; i++) {
            new Thread(new StaticThread()).start();
        }
    }
}

StaticAction.java

public class StaticAction {

public static void print() {
        int sum = 0;
        for (int i = 0; i < 10; i++) {
            System.out.println(Thread.currentThread().getName()+ " step " + i + " is running.");
            sum += i;
        }
        System.out.println("sum is " + sum);
    }
}

执行结果如下，可以猜猜:1.sum是多少？2.执行过程是什么样子的？

Thread-0 step 0 is running.
Thread-1 step 0 is running.
Thread-1 step 1 is running.
Thread-1 step 2 is running.
Thread-1 step 3 is running.
Thread-0 step 1 is running.
Thread-1 step 4 is running.
Thread-0 step 2 is running.
Thread-1 step 5 is running.
Thread-0 step 3 is running.
Thread-0 step 4 is running.
Thread-0 step 5 is running.
Thread-0 step 6 is running.
Thread-0 step 7 is running.
Thread-0 step 8 is running.
Thread-0 step 9 is running.
sum is 45
Thread-1 step 6 is running.
Thread-1 step 7 is running.
Thread-1 step 8 is running.
Thread-1 step 9 is running.
sum is 45

 

看过结果之后，你会发现静态方法执行过程是随着线程的切换而交叉执行的【一会执行下Thread1 一会执行下Thread0】，但是每个线程的执行都有自己的执行环境【for 循环每一步都不会缺失（因为发生交叉覆盖导致循环混乱），最终sum结果都是45】，因此事隔离的。所以，随着的线程的切换，静态方法的参数环境也会发生变化（jvm计数器会记录当前线程上一次执行的位置（执行到哪个方法哪一行）），所以只要不是涉及共享变量，就不会出现并发问题。