【面试专栏】ArrayList 非线程安全案例并提供三种解决方案

阅读原文时间：2023年07月09日阅读：1

1. 复现问题

import java.util.ArrayList;
import java.util.List;
import java.util.UUID;

/**
 * 复现问题
 *
 * @author CL
 *
 */
public class RecurrenceProblem {

    public static void main(String[] args) {
        List<String> list = new ArrayList<String>();

        // 启动30个线程
        for (int i = 0; i < 30; i++) {
            new Thread(() -> {
                list.add(UUID.randomUUID().toString().substring(0, 8));
                System.out.println(list);
            }, String.valueOf(i)).start();
        }
    }

}

运行结果：

[42251c59]
[42251c59, 5839198b]
[42251c59, 5839198b, 1283d17b]
[42251c59, 5839198b, 1283d17b, 01fce852]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828, 3fd4f004]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828, 3fd4f004, 333308f6]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828, 3fd4f004, 333308f6, 7143a717, 9ba40ac6]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828, 3fd4f004, 333308f6, 7143a717, 9ba40ac6, 06ae20ed]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828, 3fd4f004, 333308f6, 7143a717, 9ba40ac6, 06ae20ed, a8aa4fea]
Exception in thread "21" [42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828, 3fd4f004, 333308f6, 7143a717, 9ba40ac6, 06ae20ed, a8aa4fea, 0d3b8cf8]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828, 3fd4f004, 333308f6, 7143a717, 9ba40ac6, 06ae20ed, a8aa4fea, 0d3b8cf8, 6fb56487]
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828, 3fd4f004, 333308f6, 7143a717, 9ba40ac6, 06ae20ed, a8aa4fea, 0d3b8cf8, 6fb56487, b9fb25b8]
java.util.ConcurrentModificationException
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828, 3fd4f004, 333308f6, 7143a717, 9ba40ac6, 06ae20ed, a8aa4fea, 0d3b8cf8, 6fb56487, b9fb25b8, 3b98b513]
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    at java.util.ArrayList$Itr.next(ArrayList.java:851)
    at java.util.AbstractCollection.toString(AbstractCollection.java:461)
    at java.lang.String.valueOf(String.java:2994)
    at java.io.PrintStream.println(PrintStream.java:821)
    at com.c3stones.demo.RecurrenceProblem.lambda$0(RecurrenceProblem.java:22)
    at java.lang.Thread.run(Thread.java:745)
[42251c59, 5839198b, 1283d17b, 01fce852, 857eb6d1, eb874846, 7cfa654c, 794c24d5, 68a415a7, f51c911a, 9f99c03a, 84213b11, b1d1c583, 04ecf6c3, 4d16693d, 65a4715e, e968aa23, b60db864, fb3a9828, 3fd4f004, 333308f6, 7143a717, 9ba40ac6, 06ae20ed, a8aa4fea, 0d3b8cf8, 6fb56487, b9fb25b8, 3b98b513, 83bd3f54]

出现：java.util.ConcurrentModificationException异常

2. 原因剖析

java.util.ConcurrentModificationException即并发修改异常。

查看ArrayList.add(…)源码：

/**
 * Appends the specified element to the end of this list.
 *
 * @param e element to be appended to this list
 * @return <tt>true</tt> (as specified by {@link Collection#add})
 */
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

发现并没有对该方法加锁，因此在并发修改时，必然是线程不安全的，则抛出java.util.ConcurrentModificationException异常。

3. 解决方案

Vector

使用Vector代替ArrayList。

查看Vector.add(…)源码：

/**
- Appends the specified element to the end of this Vector.
  *
- @param e element to be appended to this Vector
- @return {@code true} (as specified by {@link Collection#add})
- @since 1.2
  */
  public synchronized boolean add(E e) {
  modCount++;
  ensureCapacityHelper(elementCount + 1);
  elementData[elementCount++] = e;
  return true;
  }
  
  可以看到add方法中使用synchronized实现同步。代码修改为：
import java.util.List;
import java.util.UUID;
import java.util.Vector;

/**
- 解决方案01-Vector
  *
- @author CL
  *
  */
  public class Solution01 {
  
  public static void main(String[] args) {
  List list = new Vector();
```
// 启动30个线程
for (int i = 0; i < 30; i++) {
    new Thread(() -> {
        list.add(UUID.randomUUID().toString().substring(0, 8));
        System.out.println(list);
    }, String.valueOf(i)).start();
}
```
  }
}

运行结果：

[759f9961]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda]
[759f9961, ff871e85, 4a3939a2, 59e796cf]
[759f9961, ff871e85, 4a3939a2]
[759f9961, ff871e85]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea, 8cbe36f6, 62494d83]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea, 8cbe36f6, 62494d83, 846001e0, ec1efeaf]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea, 8cbe36f6, 62494d83, 846001e0, ec1efeaf, 12155931]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea, 8cbe36f6, 62494d83, 846001e0, ec1efeaf, 12155931, 12e61627]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea, 8cbe36f6, 62494d83, 846001e0]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea, 8cbe36f6]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea, 8cbe36f6, 62494d83, 846001e0, ec1efeaf, 12155931, 12e61627, fa205c82]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea, 8cbe36f6, 62494d83, 846001e0, ec1efeaf, 12155931, 12e61627, fa205c82, 04726e78]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea, 8cbe36f6, 62494d83, 846001e0, ec1efeaf, 12155931, 12e61627, fa205c82, 04726e78, 0ee9c3d5]
[759f9961, ff871e85, 4a3939a2, 59e796cf, cf4dfbda, bbc62489, 439941c6, 7a003645, fc224a2b, f52573e5, 1144e6c3, 4de11c35, ba2e2d82, 95d9f85e, d4cfdf23, 90dd544b, 59e75219, a3787684, fb5cf421, 9ca37cea, 8cbe36f6, 62494d83, 846001e0, ec1efeaf, 12155931, 12e61627, fa205c82, 04726e78, 0ee9c3d5, 097c1d4e]

从运行结果可以看出，已经解决了问题。但是查看Vector类说明和ArrayList类说明：

/*
- @author Lee Boynton
- @author Jonathan Payne
- @see Collection
- @see LinkedList
- @since JDK1.0
  */
  public class Vector
  extends AbstractList
  implements List, RandomAccess, Cloneable, java.io.Serializable
  {
  …
  }
/*
- @author Josh Bloch
- @author Neal Gafter
- @see Collection
- @see List
- @see LinkedList
- @see Vector
- @since 1.2
  */
  public class ArrayList extends AbstractList
  implements List, RandomAccess, Cloneable, java.io.Serializable
  {
  …
  }
  
  可以看出，Vector是JDK1.0时出现的，ArrayList是JDK1.2出现的。因此，官方肯定不建议使用Vector来解决此问题。
Collections

使用Collections工具类提供的方法来避免ArrayList出现的并发修改问题。代码修改为：

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.UUID;

/**
- 解决方案02-Collections
  *
- @author CL
  *
  */
  public class Solution02 {
  
  public static void main(String[] args) {
  List list = Collections.synchronizedList(new ArrayList());
```
// 启动30个线程
for (int i = 0; i < 30; i++) {
    new Thread(() -> {
        list.add(UUID.randomUUID().toString().substring(0, 8));
        System.out.println(list);
    }, String.valueOf(i)).start();
}
```
  }
}

Set、Map在创建实例时也可以使用此方式：

Set set = Collections.synchronizedSet(new HashSet());
Map map = Collections.synchronizedMap(new HashMap<>());
CopyOnWrite

CopyOnWrite写时复制。CopyOnWriteArrayList.add(…)方法：

/**
- Appends the specified element to the end of this list.
  *
- @param e element to be appended to this list
- @return {@code true} (as specified by {@link Collection#add})
  */
  public boolean add(E e) {
  final ReentrantLock lock = this.lock;
  lock.lock();
  try {
  Object[] elements = getArray();
  int len = elements.length;
  Object[] newElements = Arrays.copyOf(elements, len + 1);
  newElements[len] = e;
  setArray(newElements);
  return true;
  } finally {
  lock.unlock();
  }
  }
  
  可以看出在方法进入时加锁。先创建一个比之前长度大1的数组，并在末尾添加元素，最后再将源容器指向新的容器。这样的好处是源容器不会添加任何元素，这也是一种读写分离的思想。

代码修改为：

import java.util.List;
import java.util.UUID;
import java.util.concurrent.CopyOnWriteArrayList;

/**
 * 解决方案03-CopyOnWrite
 *
 * @author CL
 *
 */
public class Solution03 {

    public static void main(String[] args) {
        List<String> list = new CopyOnWriteArrayList<String>();

        // 启动30个线程
        for (int i = 0; i < 30; i++) {
            new Thread(() -> {
                list.add(UUID.randomUUID().toString().substring(0, 8));
                System.out.println(list);
            }, String.valueOf(i)).start();
        }
    }

}

Set、Map在创建实例时也可以使用此方式：

Set<String> set = new CopyOnWriteArraySet<String>();
Map<Integer, String> map = new ConcurrentHashMap<Integer, String>();

4. 项目地址

collection-thread-unsafe-demo

手机扫一扫

移动阅读更方便

你可能感兴趣的文章

杰哥教你面试之一百问系列:java中高级多线程concurrent的使用

ShardingSphere实战

杰哥教你面试之一百问系列:java多线程