After reading the copyonwritearraylist, I implemented a copyonwritehashmap

Java Finance 2021-01-05 17:11:14
reading copyonwritearraylist implemented copyonwritehashmap

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interviewer : You look familiar, young man , Did you come here for an interview last year .<br/>
Second fat : ah , No, this is my first time here .<br/>
interviewer : That's ok , Let's start today's interview , Let's start with something simple ,java What do you know about the containers inside , Tell me about it .<br/>
Second fat : well ,java The common containers inside are ArrayList( Thread is not safe )、HashMap( Thread is not safe )、HashSet( Thread is not safe ),ConcurrentHashMap( Thread safety ).<br/>
interviewer : ArrayList Since thread is not safe, is there any thread safe ArrayList Column ?<br/>
Second fat : This ... It seems that I have asked about the blind spot of knowledge .<br/>
interviewer : That's where we're going for today's interview , I have another meeting later , If there is any further notice, the personnel will contact you .<br/>
The above stories are purely fictional. If they are similar, please focus on this article .

What is? COW

stay java When it comes to collection containers, the first thing we usually think of is HashMapArrayListHasHSet These containers are also the most commonly used in development .
These are all non thread safe , If we have a specific business that needs to use thread safe container Columns ,

  • HashMap It can be used ConcurrentHashMap Instead of .
  • ArrayList have access to Collections.synchronizedList() Method (list Every method uses synchronized modification ) Or use Vector( Now it's basically not used , Every method uses synchronized modification )

Or use CopyOnWriteArrayList replace .

  • HasHSet have access to Collections.synchronizedSet Or use CopyOnWriteArraySet Instead of .(CopyOnWriteArraySet Why not call CopyOnWriteHashSet because CopyOnWriteArraySet The bottom layer uses CopyOnWriteArrayList To achieve )

We can see CopyOnWriteArrayList Many times in a thread safe container .
First, let's look at what CopyOnWriteCopy-On-Write abbreviation COW, It's an optimization strategy for programming .

CopyOnWrite Container is the container copied on write . The popular understanding is that when we add elements to a container , Do not add... Directly to the current container , Instead, first, I'll do the current container Copy, Copy out a new container , Then add elements to the new container , After adding elements , Then point the reference of the original container to the new container . The advantage of this is that we can treat CopyOnWrite The container reads concurrently , Without lock , Because the current container will not add any elements . therefore CopyOnWrite Container is also an idea of separation of reading and writing , Read and write in different containers .

Why introduce COW

prevent ConcurrentModificationException abnormal

stay java If we use the wrong circular posture to traverse List When , If you modify and throw while traversing java.util.ConcurrentModificationException FALSE .
If the ArrayList Loop traversal is not very familiar, you can suggest to see this article 《ArrayList Have you mastered all the delete postures of 》

 List<String> list = new ArrayList<>();
list.add(" Zhang San ");
list.add("java Finance ");
Iterator<String> iterator = list.iterator();
String content =;
if(" Zhang San ".equals(content)) {

This chestnut on top is going to happen java.util.ConcurrentModificationException Anomalous , If you put ArrayList Change it to CopyOnWriteArrayList There will be no abnormality .

Thread safe containers

Let's look at the next chestnut, a thread to List Add data , A thread loop list Reading data .

 List<String> list = new ArrayList<>();
list.add(" Zhang San ");
list.add("java Finance ");
Thread t = new Thread(new Runnable() {
int count = 0;
public void run() {
while (true) {
list.add(count++ + "");
for (String s : list) {

What happens when we run the above code ConcurrentModificationException abnormal , If you put ArrayList Instead of CopyOnWriteArrayList It's all right .

CopyOnWriteArrayList The implementation of the

From the two chestnuts above, we can see that CopyOnWriteArrayList It's thread safe , Now let's take a look CopyOnWriteArrayList How to realize thread safety .

public class CopyOnWriteArrayList<E>
implements List<E>, RandomAccess, Cloneable, {
private static final long serialVersionUID = 8673264195747942595L;
/** The lock protecting all mutators */
final transient ReentrantLock lock = new ReentrantLock();
/** The array, accessed only via getArray/setArray. */
private transient volatile Object[] array;

From the source code we can see that CopyOnWriteArrayList This sum ArrayList The underlying implementation is through a Object The array of , It's just CopyOnWriteArrayList The array of is through volatile To decorate , Why volatile You can take a look at 《Java Of synchronized Can I prevent instruction reordering ?》
There are also new ReentrantLock.

add Method :

 public boolean add(E e) {
// Get the lock first
final ReentrantLock lock = this.lock;
try {
Object[] elements = getArray();
int len = elements.length;
// Copy a new array
Object[] newElements = Arrays.copyOf(elements, len + 1);
newElements[len] = e;
// Put the values of the new array To the original array
return true;
} finally {
// Release the lock

The above source code is relatively simple , There are a few points that need a little attention

  • When you add data, it's through ReentrantLock To lock ( stay jdk11 I used synchronized To replace ReentrantLock) Ensure that when multithreading writes, only one thread copies the array , Otherwise, there will be multiple copies of copied data in the memory , Leading to data confusion .
  • Arrays are passed through volatile Embellished , according to volatile Of happens-before The rules , The modification of the array reference by the write thread can be immediately visible to the read thread .
  • Copy while writing to ensure that read and write operations are carried out in two different data containers .

Realize one by yourself COW Containers

Again Java Two uses are provided in the contract CopyOnWrite Concurrent containers implemented by mechanism , They are CopyOnWriteArrayList and CopyOnWriteArraySet, But it didn't CopyOnWriteHashMap We can follow his ideas to achieve a CopyOnWriteHashMap

public class CopyOnWriteHashMap<K, V> implements Map<K, V>, Cloneable {
final transient ReentrantLock lock = new ReentrantLock();
private volatile Map<K, V> map;
public CopyOnWriteHashMap() {
map = new HashMap<>();
public V put(K key, V value) {
final ReentrantLock lock = this.lock;
try {
Map<K, V> newMap = new HashMap<K, V>(map);
V val = newMap.put(key, value);
map = newMap;
return val;
} finally {
public V get(Object key) {
return map.get(key);
public V remove(Object key) {
final ReentrantLock lock = this.lock;
try {
Map<K, V> newMap = new HashMap<K, V>(map);
if (!newMap.containsKey(key)) {
return null;
V v = newMap.get(key);
map = newMap;
return v;
}finally {

So we've implemented a simple CopyOnWriteHashMap, only add、remove、get Methods the rest of the methods can be implemented by themselves , It involves locking as long as the data changes , Read without lock .

Application scenarios

CopyOnWrite The concurrency container is suitable for concurrency scenarios with more reads and less writes , Like the black and white list 、 National City and other basic data cache 、 System configuration, etc . These are basically initialized once when the project starts , The frequency of change is very low . If this scenario of reading more and writing less adopts Vector,Collections These ways of packing are unreasonable , Because although multiple read threads read data from the same data container , But the read thread does not modify the data of the data container , So there's no need to read and lock .

CopyOnWrite shortcoming

CopyOnWriteArrayList Although it's a thread safe version of ArrayList, But every time it changes the data, it copies a copy of the data , therefore CopyOnWriteArrayList It is only suitable for reading more and writing less or reading without lock . If we use it in real business CopyOnWriteArrayList, It must be because the scene is suitable, not to show off .

Memory usage problem

because CopyOnWrite There are two array objects in memory each time a write operation is performed , If this array object takes up a large amount of memory , If you write frequently, it will cause frequent Yong GC and Full GC.

Data consistency issues

CopyOnWrite The container can only guarantee the final consistency of the data , There is no guarantee of real-time consistency of data . The read thread may not read the newly modified data immediately , Because the modification takes place on the replica . But the final modification completes and updates the container, so this is the final consistency .

CopyOnWriteArrayList and Collections.synchronizedList()

A simple test CopyOnWriteArrayList and Collections.synchronizedList() The discovery of reading and writing :

  • When writing with high concurrency CopyOnWriteArray Compared with synchronization Collections.synchronizedList A hundred times slower
  • At high concurrent read performance CopyOnWriteArray Compared with synchronization Collections.synchronizedList Dozens of times faster .
  • When writing with high concurrency ,CopyOnWriteArrayList Why is it so slow ? Because every time add when , Use both Arrays.copyOf Create a new array , frequent add When the memory request is released, the performance consumption is large .
  • High concurrency reading CopyOnWriteArray unlocked ,Collections.synchronizedList There is a lock, so the efficiency of reading is relatively low .


choice CopyOnWriteArrayList When reading, it must be much more than writing . If reading and writing are almost the same, it is recommended to choose Collections.synchronizedList.


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  • Thank you for reading , Thank you for your attention .

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