为什么 Vector<String> 在以多线程样式访问时不以同步方式运行

Question

根据我的理解，如果一个线程已经在访问它，Vector 应该阻止其他线程的整个集合。我在尝试着

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Vector;
import java.util.concurrent.CopyOnWriteArrayList;

public class MyTest1{
    
//  static ArrayList<String> l1 = new ArrayList<>(Arrays.asList("A", "B", "C"));
    static Vector<String> l1 = new Vector<>(Arrays.asList("A", "B", "C"));
//  static List<String> l1 = Collections.synchronizedList(Arrays.asList("A", "B", "C"));

    public static void main(String[] args) {
        Thread t1 = new Thread(new MyThread1());
        Thread t2 = new Thread(new MyThread2());
        t1.start();
        t2.start();
        System.out.println(Thread.currentThread() + "adding X " + l1.add("X"));
        System.out.println(Thread.currentThread() + "adding Y " + l1.add("Y"));
        System.out.println(Thread.currentThread() + "adding Z " + l1.add("Z"));
        
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(l1);
    }
    
    static class MyThread1 implements Runnable{
        @Override
        public void run() {
            System.out.println(Thread.currentThread() + "adding D " + l1.add("D"));
            System.out.println(Thread.currentThread() + "adding E " + l1.add("E"));
            System.out.println(Thread.currentThread() + "adding F " + l1.add("F"));
        }
    }
    static class MyThread2 implements Runnable{
        @Override
        public void run() {
            System.out.println(Thread.currentThread() + "removing A " + l1.remove("A"));
            System.out.println(Thread.currentThread() + "removing B " + l1.remove("B"));
            System.out.println(Thread.currentThread() + "removing C " + l1.remove("C"));
        }
    }
    
}

输出：

Thread[main,5,main]adding X true
Thread[Thread-1,5,main]removing A true
Thread[Thread-1,5,main]removing B true
Thread[Thread-0,5,main]adding D true
Thread[Thread-1,5,main]removing C true
Thread[main,5,main]adding Y true
Thread[Thread-0,5,main]adding E true
Thread[main,5,main]adding Z true
Thread[Thread-0,5,main]adding F true
[X, D, Y, E, Z, F]

输出不应该是这样的。对于特定线程，它应该是 3 个连续的添加或删除。我在这里缺少什么？请帮忙。

Answer 1

1. Vector保证单个操作的原子性
2. Vector 不保证操作序列的原子性

在有问题的代码中，

1. add和remove操作单独（独立）同步 - 在任何给定时间，只能在对象上执行此操作之一
2. 随着时间的推移，它不会在这些操作的不同调用之间同步

如果您需要跨一组操作进行同步，

1. 在集合上显式同步
2. 在当前情况下，ArrayList与同步一起使用时应该没问题

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

class MyTest1 {

    static List<String> l1 = new ArrayList<>(Arrays.asList("A", "B", "C"));

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            synchronized (l1) {
                System.out.println(Thread.currentThread() + "removing A " + l1.remove("A"));
                System.out.println(Thread.currentThread() + "removing B " + l1.remove("B"));
                System.out.println(Thread.currentThread() + "removing C " + l1.remove("C"));
            }
        });
        Thread t2 = new Thread(() -> {
            synchronized (l1) {
                System.out.println(Thread.currentThread() + "removing A " + l1.remove("A"));
                System.out.println(Thread.currentThread() + "removing B " + l1.remove("B"));
                System.out.println(Thread.currentThread() + "removing C " + l1.remove("C")); 
            }
        });
        t1.start();
        t2.start();
        synchronized (l1) {
            System.out.println(Thread.currentThread() + "adding X " + l1.add("X"));
            System.out.println(Thread.currentThread() + "adding Y " + l1.add("Y"));
            System.out.println(Thread.currentThread() + "adding Z " + l1.add("Z"));
        }

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(l1);
    }
}