ArrayList非线程安全记录

一、问题描述

线上一个查询服务，偶尔会报一次查询出来的结果集合包含null。

二、问题排查

在多线程查询过程中，使用了ArrayList，多线程查询出来后执行ArrayList.add()。然而ArrayList并不是线程安全的集合，会导致null值出现。可以考虑使用SynchronizedList。

三、问题复现

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class TestSync {

    private static final Integer n = 100;

    public static void main(String[] args) {
        long startTime = System.currentTimeMillis();
        arrayList();
        System.out.println("arrayList时间：" + (System.currentTimeMillis() - startTime));

        startTime = System.currentTimeMillis();
        synchronizedList();
        System.out.println("synchronizedList时间：" + (System.currentTimeMillis() - startTime));

        startTime = System.currentTimeMillis();
        copyArrayList();
        System.out.println("copyArrayList时间：" + (System.currentTimeMillis() - startTime));

        startTime = System.currentTimeMillis();
        futureList();
        System.out.println("futureList时间：" + (System.currentTimeMillis() - startTime));
    }

    private static void arrayList() {
        List<Integer> testList = new ArrayList<>();
        CountDownLatch countDownLatch = new CountDownLatch(n);
        for (int i=0; i<n; i++) {
            new Thread(() -> {
                testList.add(1);
                countDownLatch.countDown();
            }).start();
        }
        try {
            countDownLatch.await(10, TimeUnit.SECONDS);
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println(testList);
    }

    private static void synchronizedList() {
        List<Integer> testList = Collections.synchronizedList(new ArrayList<>());
        CountDownLatch countDownLatch = new CountDownLatch(n);
        for (int i=0; i<n; i++) {
            new Thread(() -> {
                testList.add(1);
                countDownLatch.countDown();
            }).start();
        }
        try {
            countDownLatch.await(10, TimeUnit.SECONDS);
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println(testList);
    }

    private static void copyArrayList() {
        CopyOnWriteArrayList<Integer> testList = new CopyOnWriteArrayList<>();
        CountDownLatch countDownLatch = new CountDownLatch(n);
        for (int i=0; i<n; i++) {
            new Thread(() -> {
                testList.add(1);
                countDownLatch.countDown();
            }).start();
        }
        try {
            countDownLatch.await(10, TimeUnit.SECONDS);
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println(testList);
    }

    private static void futureList() {
        List<CompletableFuture<Integer>> futureList = new ArrayList<>();
        for (int i=0; i<n; i++) {
            CompletableFuture<Integer> task = CompletableFuture.supplyAsync(() -> 1);
            futureList.add(task);
        }
        CompletableFuture.allOf(futureList.toArray(new CompletableFuture[0])).join();
        List<Integer> testList = new ArrayList<>();
        futureList.forEach(data -> data.whenComplete((integer, throwable) -> {
            if (Objects.nonNull(throwable)) {
                return;
            }
            testList.add(integer);
        }));
        System.out.println(testList);
    }
}

执行结果：