在copyOnWriteList中，基本底层还是不变：由数组构成的链表，有一个Object[]数组域。
但是在其内部有一个ReentrantLock独占锁，在增删改的时候都是先上锁再操作。
所以它是并发安全的。
①
在实现的时候，都是先先将数组域复制到一个新数组中，然后对这个新数组进行增删改，最后将新数组赋给旧数组。
②
在进行迭代iterator时，实际上是对内部数组域快照的一个迭代，如果这个数组域被修改，迭代还是按照先前的值进行迭代。
详情见demo01.java
package concurrent_list;/*
name: demo01
user: ly
Date: 2020/5/30
Time: 11:11
*/
import java.util.Iterator;
import java.util.concurrent.CopyOnWriteArrayList;
// CopyAndWriteArrayList的迭代弱一致性
public class demo01 {
private static CopyOnWriteArrayList<String> copyOnWriteArrayList = new CopyOnWriteArrayList<String>();
public static void main(String []args) throws InterruptedException{
copyOnWriteArrayList.add("a");
copyOnWriteArrayList.add("b");
copyOnWriteArrayList.add("c");
Thread thread = new Thread(new Runnable() {
public void run() {
copyOnWriteArrayList.remove("c");
copyOnWriteArrayList.set(1,"changed");
}
});
//在启动线程之前获取迭代器
Iterator<String> iterable = copyOnWriteArrayList.iterator();
thread.start(); //现在才启动
thread.join();
while (iterable.hasNext()){
System.out.println(iterable.next());
}
System.out.println("actual:");
iterable = copyOnWriteArrayList.iterator();
while (iterable.hasNext()){
System.out.println(iterable.next());
}
}
}
改进版
很多时候，如何保障你的代码高并发高性能，这的确是个很耐人寻味的话题。高性能意味着你要对采用的每一个容器、集合（数据结构）要非常讲究，而它往往是很多人不太注意的地方，这也行就是你不能专家级的一方面原因！so，基本功很重要，尽可能的接触底层实现。在java圈中，ArrayList 是非线程安全的，难道在多线程场景下我们只有Vector这一种线程安全的数组实现可以选择么？
当然也有List synchronizedList = Collections.synchronizedList(new ArrayList());但是当你不可避免使用contains() 进行搜索的时，它不可避免会锁住整个list，太恐怖了。
Queue 和Deque (基于Linked List)有并发的实现是因为他们的接口相比List的接口有更多的限制，这些限制使得实现并发成为可能。
CopyOnWriteArrayList它规避了只读操作（如get/contains）并发的瓶颈，但是它为了做到这点，在修改操作中做了很多工作和修改可见性规则。 此外，修改操作还会锁住整个List，因此这也是一个并发瓶颈。所以从理论上来说，CopyOnWriteArrayList并不算是一个通用的并发List。
总之，数组ArrayList 虽然insert和get是最快的，但是非线程安全的，CopyOnWriteArrayList虽安全，但insert速度慢
如何优化呢？
其实也不复杂，只需做的这2点
要继续发挥数组ArrayList 的优点
要去掉锁采用高性能的CAS
1. 高性能Unsafe
public class UnsafeUtils {
final static private Unsafe _unsafe;
static {
Unsafe tmpUnsafe = null;
try {
java.lang.reflect.Field field = sun.misc.Unsafe.class.getDeclaredField("theUnsafe");
field.setAccessible(true);
tmpUnsafe = (sun.misc.Unsafe) field.get(null);
} catch (java.lang.Exception e) {
throw new Error(e);
}
_unsafe = tmpUnsafe;
}
public static final Unsafe unsafe() {
return _unsafe;
}
}
public class Unsafe_test {
@Test
public void test() {
Unsafe u = UnsafeUtils.unsafe();
int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
//返回当前数组第一个元素地址(数组起始地址的偏移值)，返回6
int b = u.arrayBaseOffset(int[].class);
//返回当前数组一个元素占用的字节数s，返回4
int s = u.arrayIndexScale(int[].class);
int pos=9;
//获取数组对象某个位置偏移值（b + s * pos)，将intval写入内存
u.putInt(arr, (long) b + s * pos, 1);
for (int i = 0; i < 10; i++) {
//获取数组对象某个位置上偏移值，从而获得元素的值
int v = u.getInt(arr, (long) b + s * i);
System.out.print(v + " ");//1 2 3 4 5 6 7 8 9 1
}
}
}
2. 保持数组ArrayList的优点
/**
* 仅支持get/set，不支持remove
*/
public class ConcurrentArrayList<T> implements RandomAccess {
public static final int MAX_CAPACITY = 1 << 30;
private static final long SIZE_OFFSET;
private static final int ABASE;
private static final int ASHIFT;
private volatile Object[] values;
//unsafe operate
private volatile int size;
static {
try {
Field field = ConcurrentArrayList.class.getDeclaredField("size");
SIZE_OFFSET = UnsafeUtils.unsafe().objectFieldOffset(field);
ABASE = UnsafeUtils.unsafe().arrayBaseOffset(Object[].class);
int scale = UnsafeUtils.unsafe().arrayIndexScale(Object[].class);
if ((scale & (scale - 1)) != 0) {
throw new Error("array index scale not a power of two");
}
ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
} catch (Throwable e) {
throw new Error(e);
}
}
public T get(int index) {
return (T) UnsafeUtils.unsafe().getObjectVolatile(values, offset(ABASE, ASHIFT, index));
}
public void add(T value) {
int index = insertIndex();
set(index, value);
}
private int insertIndex() {
int index = UnsafeUtils.unsafe().getAndAddInt(this, SIZE_OFFSET, 1);
ensureCapacity(index + 1);
return index;
}
public void set(int index, T value) {
final long offset = offset(ABASE, ASHIFT, index);
for (; ; ) {// like cas
final Object[] before = values;
UnsafeUtils.unsafe().putOrderedObject(before, offset, value);
final Object[] after = values;
if (before == after) {
return;
}
}
}
public ConcurrentArrayList() {
this(16);
}
public ConcurrentArrayList(int initialCapacity) {
if (initialCapacity > MAX_CAPACITY) {
throw new IndexOutOfBoundsException("Illegal initial capacity: " + initialCapacity);
}
ensureCapacity(initialCapacity);
}
private void ensureCapacity(int capacity) {
Object[] theArray = values;
if (theArray != null && theArray.length >= capacity) {
return;
}
synchronized (this) {
Object[] finalArray = values;
if (finalArray != null && finalArray.length >= capacity) {
return;
}
int newCapacity = tableSizeFor(capacity);
if (newCapacity > MAX_CAPACITY) {
throw new IndexOutOfBoundsException("" + newCapacity);
}
Object[] objs = new Object[newCapacity];
if (finalArray != null) {
System.arraycopy(finalArray, 0, objs, 0, finalArray.length);
}
values = objs;
}
}
/**
* 成倍扩容
*
* @param cap
* @return
*/
public int tableSizeFor(final int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAX_CAPACITY) ? MAX_CAPACITY : n + 1;
}
/**
* 获取某个元素的offset
*
* @param arrayBase
* @param arrayShift
* @param index
* @return
*/
public long offset(final long arrayBase, final int arrayShift, final int index) {
return ((long) index << arrayShift) + arrayBase;
}
public int size() {
return size;
}
public void clear() {
size = 0;
}
}
基本的测试
synchronizedList:157.6 ms
arrayList:60.30 ms
vector:164.9 ms
concurrentArrayList:86.83 ms
copyOnWriteArrayList:慢到无法统计
public class ConcurrentArrayList_unit {
private static final ArrayList<Boolean> arrayList = new ArrayList<>();
private static final Vector<Boolean> vector = new Vector<>();
private List synchronizedList = Collections.synchronizedList(new ArrayList());
private static final CopyOnWriteArrayList<Boolean> copyOnWriteArrayList = new CopyOnWriteArrayList<>();
private static final ConcurrentArrayList<Boolean> concurrentArrayList = new ConcurrentArrayList<>();
private int maxConcurrent = 200;
private int maxSet = 8000;
private int maxGet = 8000;
@Test
public void arrayList() throws InterruptedException {
Consumer arrayListConsumer = x -> {
for (int i = 0; i < maxSet; i++) {
arrayList.add(Boolean.TRUE);
}
for (int j = 0; j < maxGet; j++) {
arrayList.get(j);
}
};
runTimes(maxConcurrent, "arrayList", arrayListConsumer);
}
@Test
public void concurrentArrayList() throws InterruptedException {
Consumer concurrentArrayListConsumer = x -> {
for (int i = 0; i < maxSet; i++) {
concurrentArrayList.add(Boolean.TRUE);
}
for (int j = 0; j < maxGet; j++) {
concurrentArrayList.get(j);
}
};
runTimes(maxConcurrent, "concurrentArrayList", concurrentArrayListConsumer);
}
@Test
public void vector() throws InterruptedException {
Consumer vectorConsumer = x -> {
for (int i = 0; i < maxSet; i++) {
vector.add(Boolean.TRUE);
}
for (int j = 0; j < maxGet; j++) {
vector.get(j);
}
};
runTimes(maxConcurrent, "vector", vectorConsumer);
}
@Test
public void synchronizedList() throws InterruptedException {
Consumer synchronizedListConsumer = x -> {
for (int i = 0; i < maxSet; i++) {
synchronizedList.add(Boolean.TRUE);
}
for (int j = 0; j < maxGet; j++) {
synchronizedList.get(j);
}
};
runTimes(maxConcurrent, "synchronizedList", synchronizedListConsumer);
}
@Test
public void copyOnWriteArrayList() throws InterruptedException {
Consumer copyOnWriteArrayListConsumer = x -> {
for (int i = 0; i < maxSet; i++) {
copyOnWriteArrayList.add(Boolean.TRUE);
}
for (int j = 0; j < maxGet; j++) {
copyOnWriteArrayList.get(j);
}
};
runTimes(maxConcurrent, "copyOnWriteArrayList", copyOnWriteArrayListConsumer);
}
private void runTimes(int maxConcurrent, String tag, Consumer consumer) throws InterruptedException {
Stopwatch stopwatch = Stopwatch.createStarted();
CountDownLatch latch = new CountDownLatch(maxConcurrent);
for (int i = 0; i < maxConcurrent; i++) {
new Thread(() -> {
consumer.accept(null);
latch.countDown();
}).start();
}
latch.await();
System.out.println(tag + ":" + stopwatch);
}
}
充分利用cpu高性能位运算的，Integer.numberOfLeadingZeros()返回0位的个数，基准选定的分别是2的1、2、3、4次幂