管理线程同步中的静态关键字 - Java

Question

我有一个关于下面发布的代码的问题。为什么我需要在 SumArray 对象的声明前面加上“static”这个词才能使这段代码正常运行？为什么不保证（由它自己）一次为一个线程调用同步方法？事实上，如果我删除“静态”一词，那么 SumArray 方法将被 2 个线程调用可互换。尽管如此，两个线程的总和将是正确的。class SumArray {    private int sum;    synchronized int sumArray(int numbers[]) {        sum = 0;        for (int i = 0; i < numbers.length; i++) {            sum += numbers[i];            System.out.println("Current Sum for " + Thread.currentThread().getName() + " equals " + sum);            try {                Thread.sleep(2000);            } catch (InterruptedException e) {                System.out.println("Main Thread has been stopped");            }        }        return sum;    }}class MyThread implements Runnable {    Thread t;    int a[];    int ans;    static SumArray sa = new SumArray();    MyThread(String name, int numbers[]) {        t = new Thread(this, name);        t.start();        a = numbers;    }    public void run() {        System.out.println(t.getName() + " is starting.");        ans = sa.sumArray(a);        System.out.println("Final Sum for " + t.getName() + " equals " + ans);        System.out.println(t.getName() + " has been finished.");    }}public class Main {    public static void main(String[] args) {        int a[] = { 1, 2, 3, 4, 5 };        new MyThread("Thread 1", a);        new MyThread("Thread 2", a);    }}代码的示例结果 static SumArray sa = new SumArray();Thread 2 is starting.Thread 1 is starting.Current Sum for Thread 2 equals 1Current Sum for Thread 2 equals 3Current Sum for Thread 2 equals 6Current Sum for Thread 2 equals 10Current Sum for Thread 2 equals 15Current Sum for Thread 1 equals 1Final Sum for Thread 2 equals 15Thread 2 has been finished.Current Sum for Thread 1 equals 3Current Sum for Thread 1 equals 6Current Sum for Thread 1 equals 10Current Sum for Thread 1 equals 15Final Sum for Thread 1 equals 15Thread 1 has been finished.结果与 SumArray sa = new SumArray();

Answer 1

因为在线程内部您创建了一个新对象并且它有自己的 sumArray 方法。当它是静态时，整个 jvm 中只有一个 sumArray 函数属于类，而不是对象