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
时才发生扩容问题，假设
，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1000
private static class IncreaseTask extends Thread{
@Override
public 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 Collection
final Object mutex; // Object on which to synchronize
SynchronizedCollection(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1000
private static class IncreaseTask extends Thread{
@Override
public 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。
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