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 1

Current Sum for Thread 2 equals 3

Current Sum for Thread 2 equals 6

Current Sum for Thread 2 equals 10

Current Sum for Thread 2 equals 15

Current Sum for Thread 1 equals 1

Final Sum for Thread 2 equals 15

Thread 2 has been finished.

Current Sum for Thread 1 equals 3

Current Sum for Thread 1 equals 6

Current Sum for Thread 1 equals 10

Current Sum for Thread 1 equals 15

Final Sum for Thread 1 equals 15

Thread 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