ArrayList为什么会出现并发问题以及相应的解决办法

问题一：ArrayList为什么会出现并发问题？

ArrayList是线程不安全的，在多线程并发访问的时候可能会出现问题，如果想使用线程安全的集合类，java自带有vector,也就是说vector是线程安全的。但是arayList的底层是数组实现的，而且可以自动扩容，获得元素或者在数组尾段插入元素的效率高，所以说ArrayList有其独特的优势。

1.扩容实现

private transient Object[] elementData;public void ensureCapacity(int minCapacity) {modCount++;int oldCapacity = elementData.length;if (minCapacity > oldCapacity) {Object oldData[] = elementData;int newCapacity = (oldCapacity * 3)/2 + 1;if (newCapacity < minCapacity)newCapacity = minCapacity;// minCapacity is usually close to size, so this is a win:elementData = Arrays.copyOf(elementData, newCapacity);}}

elementData数组的初始容量是10，如果需要扩容时，使用 elementData = Arrays.copyOf(elementData, newCapacity)进行复制， elementData = Arrays.copyOf(elementData, newCapacity)底层的核心代码为：

System.arraycopy(original, 0, copy, 0,Math.min(original.length, newLength));
该方法中，original指的是：elementData,旧数组，0指的是复制从elementDate的索引为0开始copy新生成的数组，0，新数组的索引为0开始，最后一个参数Math.min(elementData.length，newCapacity)在上面的源码中，它的意思就是把老数组中的数据复制到一个新生成的容量为newLength的数组中。

扩容的时候，如果在并发操作ArrayList的时候，可能会有数组索引越界的异常产生。

分析上源码可以看出，只有当 $minCapacipity>oldCapacipity$ 时才发生扩容问题，假设 $minCapacipity=oldCapacipity=10$ ，不发生扩容问题，元素是可以被插入的。大家可以看下面的add源码，也就是size=9时可以将元素添加到数组中，多线程示意图如下：

线程A和线程B获取的size都是9，线程A先插入元素e，这个时候elementData数组的大小为10，是正常情况下下次应该是要扩容的，但是线程B获取的size=9而不是10，在线程B中没有进行扩容，而是报出数组index越界异常。

2.add操作

public boolean add(E e) {ensureCapacity(size + 1);  // Increments modCount!!elementData[size++] = e;return true;}

add操作中先进行扩容操作ensureCapacity(size+1)，之后才添加数据到elementData这个数组的末端，但是这样的操作不是线程安全的，多线程操作的时候可能会出现数据覆盖的问题。

线程A执行了ArrayList的add方法，由于线程B获取到的size大小和线程A是一样的，此时的size大小应该是比原来的size要大1，但是B线程不知，所以B线程进行赋值的时候把A线程的值给覆盖，导致添加到数组中元素的个数其实是比逻辑上要少的。

package TestArrayList;import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class Main {private static List<Integer> list = new ArrayList<Integer>();private static ExecutorService executorService = Executors.newFixedThreadPool(1000); //定长线程池1000private static class IncreaseTask extends Thread{@Overridepublic void run() {System.out.println("ThreadId:" + Thread.currentThread().getId() + " start!");for(int i =0; i < 100; i++){list.add(i);}System.out.println("ThreadId:" + Thread.currentThread().getId() + " finished!");}}public static void main(String[] args){for(int i=0; i < 1000; i++){ //开启1000个线程executorService.submit(new IncreaseTask());}executorService.shutdown();while (!executorService.isTerminated()){try {Thread.sleep(1000*10);}catch (InterruptedException e){e.printStackTrace();}}System.out.println("All task finished!");System.out.println("list size is :" + list.size());}
}

结果：list某个随机值是99900，本应该是100000，所以add操作是线程不安全的。

问题二：如何避免ArrayList的并发问题？

（1）使用Collections.synchronizedList()方法对ArrayList对象进行包装

ArrayList<Integer> arraylist = Collections.synchronizedList(new ArrayList());

将问题一中的Arraylist改成Collections.synchronizedList(new ArrayList)生成，问题的结果是100000.

源码：

public static <T> List<T> synchronizedList(List<T> list) {return (list instanceof RandomAccess ?new SynchronizedRandomAccessList<T>(list) :new SynchronizedList<T>(list));}

上述类的继承结构如上所示，SynchronizedList是SynchronizedRandomAccessList的父类，我们现在看下SynchronizedList的源码，了解下为什么SynchronizedList是线程安全的。

 SynchronizedList(List<E> list) {super(list);this.list = list;}SynchronizedList(List<E> list, Object mutex) {super(list, mutex);this.list = list;}public boolean equals(Object o) {synchronized(mutex) {return list.equals(o);}}public int hashCode() {synchronized(mutex) {return list.hashCode();}}public E get(int index) {synchronized(mutex) {return list.get(index);}}public E set(int index, E element) {synchronized(mutex) {return list.set(index, element);}}public void add(int index, E element) {synchronized(mutex) {list.add(index, element);}}public E remove(int index) {synchronized(mutex) {return list.remove(index);}}public int indexOf(Object o) {synchronized(mutex) {return list.indexOf(o);}}public int lastIndexOf(Object o) {synchronized(mutex) {return list.lastIndexOf(o);}}public boolean addAll(int index, Collection<? extends E> c) {synchronized(mutex) {return list.addAll(index, c);}}public ListIterator<E> listIterator() {return list.listIterator(); // Must be manually synched by user}public ListIterator<E> listIterator(int index) {return list.listIterator(index); // Must be manually synched by user}public List<E> subList(int fromIndex, int toIndex) {synchronized(mutex) {return new SynchronizedList<E>(list.subList(fromIndex, toIndex),mutex);}}/*** SynchronizedRandomAccessList instances are serialized as* SynchronizedList instances to allow them to be deserialized* in pre-1.4 JREs (which do not have SynchronizedRandomAccessList).* This method inverts the transformation.  As a beneficial* side-effect, it also grafts the RandomAccess marker onto* SynchronizedList instances that were serialized in pre-1.4 JREs.** Note: Unfortunately, SynchronizedRandomAccessList instances* serialized in 1.4.1 and deserialized in 1.4 will become* SynchronizedList instances, as this method was missing in 1.4.*/private Object readResolve() {return (list instanceof RandomAccess? new SynchronizedRandomAccessList<E>(list): this);}}

关于mutex定义：

final Collection<E> c;  // Backing Collectionfinal Object mutex;     // Object on which to synchronizeSynchronizedCollection(Collection<E> c) {if (c==null)throw new NullPointerException();this.c = c;mutex = this;}SynchronizedCollection(Collection<E> c, Object mutex) {this.c = c;this.mutex = mutex;}

易知，mutex指向的就是当前对象自己，所以SynchronizedList是线程安全的根本原因是使用Synchronized对SynchronizedList的add,delete等操作进行加锁，但是这种锁的力度很大，所以这种方式效率低下。

（2）使用并发容器CopyOnWriteArrayList

CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<String>();

源码：

    private static final long serialVersionUID = 8673264195747942595L;/** The lock protecting all mutators */transient final ReentrantLock lock = new ReentrantLock();/** The array, accessed only via getArray/setArray. */private volatile transient Object[] array;/*** Gets the array.  Non-private so as to also be accessible* from CopyOnWriteArraySet class.*/final Object[] getArray() {return array;}/*** Sets the array.*/final void setArray(Object[] a) {array = a;}

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();}}

从add方法知道：CopyOnWriteArrayList底层数组的扩容方式是一个一个地增加，而且每次把原来的元素通过Arrays.copy()方法copy到新数组中，然后在尾部加上新元素e.它的底层并发安全的保证是通过ReentrantLock进行保证的，CopyOnWriteArrayList和SynchronizedList的底层实现方式是不一样的，前者是通过Lock机制进行加锁，而后者是通过Synchronized进行加锁，至于两者的区别，下次详细描述。

对比实验

将问题一中的测试代码，修改如下，对比SynchronizedList和CopyOnWriteArrayList的执行时间：

package TestArrayList;import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class Main {//private static List<Integer> list = Collections.synchronizedList(new ArrayList<Integer>());private static CopyOnWriteArrayList<Integer> list = new CopyOnWriteArrayList<Integer>();private static ExecutorService executorService = Executors.newFixedThreadPool(1000); //定长线程池1000private static class IncreaseTask extends Thread{@Overridepublic void run() {System.out.println("ThreadId:" + Thread.currentThread().getId() + " start!");for(int i =0; i < 100; i++){list.add(i);}System.out.println("ThreadId:" + Thread.currentThread().getId() + " finished!");}}public static void main(String[] args){long start = System.currentTimeMillis();for(int i=0; i < 1000; i++){ //开启1000个线程executorService.submit(new IncreaseTask());}executorService.shutdown();while (!executorService.isTerminated()){try {Thread.sleep(1000*10);}catch (InterruptedException e){e.printStackTrace();}}System.out.println("All task finished!");System.out.println("list size is :" + list.size());long end = System.currentTimeMillis();System.out.println("operation time:"+(double)(end-start)/1000+"s");}
}

实验结果：

左边是SynchronizedList，右边是CopyOnWriteArrayList，分析表明两者差别不大。

参考：

【1】http://www.cnblogs.com/areyouready/p/6803304.html

【2】https://www.cnblogs.com/vitasyuan/p/8955557.html

【3】https://blog.csdn.net/qing337197645/article/details/51219155

【4】https://my.oschina.net/jielucky/blog/167198