Static方法在多线程环境下的运行
2024-09-17 15:55:08
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。
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