就是多线程处理list；以下代码如何能保证list安全呢

Question

package com.gray.test;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;

public class ThreadTest {
public static void main(String[] args) throws Exception {
// 开始时间
long start = System.currentTimeMillis();
// 模拟数据List
List list = new ArrayList();
for (int i = 0; i < 3000; i++) {
list.add(i + "");
}

    ExecutorService exec = Executors.newFixedThreadPool(3);
     CountDownLatch doneSignal = new CountDownLatch(3);
    exec.execute(new PoiWriter(doneSignal,list,0,999));
    exec.execute(new PoiWriter(doneSignal,list,1000,1999));
    exec.execute(new PoiWriter(doneSignal,list,2000,2999));
    doneSignal.await();
    exec.shutdown();
    System.out.println("执行任务消耗了 ：" + (System.currentTimeMillis() - start) + "毫秒");


}


protected static class PoiWriter implements Runnable {
    private  CountDownLatch doneSignal;
    private List list;
    private int start;

    private int end;

    public PoiWriter(CountDownLatch doneSignal,List list, int start, int end) {
        this.doneSignal = doneSignal;
        this.start = start;
        this.list = list;
        this.end = end;
    }

    @Override
    public void run() {
        try {
            for (int i = start; i <= end ;i++) {
                list.set(i, Thread.currentThread().getName() +"<--------->" + list.get(i));
                System.out.println(list.get(i));
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            doneSignal.countDown();
        }

    }
}

}

Answer 1

用synchronized包裹list操作的部分代码