【面试专栏】ArrayList 非线程安全案例并提供三种解决方案

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<String> list = new Vector<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：

[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<E>

    extends AbstractList<E>

    implements List<E>, 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<E> extends AbstractList<E>

        implements List<E>, 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<String> list = Collections.synchronizedList(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();

		}

	}

}

Set、Map在创建实例时也可以使用此方式：

Set<String> set = Collections.synchronizedSet(new HashSet<String>());

Map<Integer, String> 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