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This article is about how to parse the principle of Java concurrent CopyOnWrite container, the editor thinks it is very practical, so I share it with you to learn. I hope you can get something after reading this article.

Copy-On-Write, abbreviated as COW, is an optimization strategy used in programming. The basic idea is that everyone is sharing the same content from the beginning, and when someone wants to modify it, they will actually Copy the content to form a new content and then change it, which is a lazy strategy for delay. Starting with JDK1.5, the Java concurrency package provides two concurrency containers implemented using the CopyOnWrite mechanism, which are CopyOnWriteArrayList and CopyOnWriteArraySet. The CopyOnWrite container is very useful and can be used in many concurrency scenarios.

What is a CopyOnWrite container

The CopyOnWrite container is the container that is copied when writing. The popular understanding is that when we add elements to a container, we do not add elements directly to the current container, but first Copy the current container to copy a new container, and then add elements to the new container. After adding elements, we point the reference of the original container to the new container. The advantage of this is that we can read the CopyOnWrite container concurrently without locking, because no elements are added to the current container. So the CopyOnWrite container is also a separate thought of reading and writing, and different containers for reading and writing.

The realization principle of CopyOnWriteArrayList

Before using CopyOnWriteArrayList, let's read its source code to see how it is implemented. The following code adds elements to ArrayList, and you can find that it needs to be locked when it is added, otherwise N copies will be Copy when multithreaded writing.

Public boolean add (T e) {final ReentrantLock lock = this.lock; lock.lock (); try {Object [] elements = getArray (); int len = elements.length; / / copy the new array Object [] newElements = Arrays.copyOf (elements, len + 1); / / add new elements to the new array newElements [len] = e; / / point the original array reference to the new array setArray (newElements); return true;} finally {lock.unlock () }} final void setArray (Object [] a) {array = a;}

There is no need to lock when reading, and if multiple threads are adding data to the ArrayList when reading, the read will still read the old data, because the old ArrayList will not be locked when writing.

Public E get (int index) {return get (getArray (), index);}

CopyOnWriteMap is not provided in JDK. We can refer to CopyOnWriteArrayList to implement one. The basic code is as follows:

Import java.util.Collection;import java.util.Map;import java.util.Set; public class CopyOnWriteMap implements Map, Cloneable {private volatile Map internalMap; public CopyOnWriteMap () {internalMap = new HashMap ();} public V put (K key, V value) {synchronized (this) {Map newMap = new HashMap (internalMap); V val = newMap.put (key, value); internalMap = newMap; return val;}} public V get (Object key) {return internalMap.get (key);} public void putAll (Map)

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