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This article mainly introduces "how to understand the CopyOnWriteArrayList of JUC". In the daily operation, I believe many people have doubts about how to understand the CopyOnWriteArrayList of JUC. The editor consulted all kinds of materials and sorted out simple and easy-to-use methods of operation. I hope it will be helpful to answer the doubts of "how to understand the CopyOnWriteArrayList of JUC"! Next, please follow the editor to study!

CopyOnWriteArrayList is a thread-safe List implementation. The underlying layer relies on arrays as the storage structure and ensures thread safety based on write-time replication (CoW: Copy-on-Write) mechanism. When CopyOnWriteArrayList performs a modify operation, it makes a copy of the underlying array, makes changes on the copy, and finally updates it back to the underlying array. Although this kind of implementation consumes more memory, it brings higher execution efficiency, which belongs to taking space for time.

In addition to CopyOnWriteArrayList, there is another thread-safe component implemented based on the CoW mechanism in the JUC package, namely CopyOnWriteArraySet, but CopyOnWriteArraySet is essentially based on CopyOnWriteArrayList, so I understand the implementation principle of CopyOnWriteArrayList and the implementation principle of CopyOnWriteArraySet at the same time.

Insider implementation of CopyOnWriteArrayList

CopyOnWriteArrayList is implemented from the List interface, so we can use CopyOnWriteArrayList as we do with ArrayList. The fields of the CopyOnWriteArrayList class are defined as follows:

Public class CopyOnWriteArrayList implements List, RandomAccess, Cloneable, Serializable {/ * * reentrant locks that support synchronous operations * / final transient ReentrantLock lock = new ReentrantLock (); / * underlying array * / private transient volatile Object [] array; / /. Omit method definition}

CopyOnWriteArrayList#array array is the underlying storage structure of CopyOnWriteArrayList, and CopyOnWriteArrayList relies on this array to store data. The field CopyOnWriteArrayList#lock corresponds to the ReentrantLock type, which is used to control the modification of the underlying array by multiple threads to ensure that only one thread modifies the underlying array at a time. CopyOnWriteArrayList provides the corresponding CopyOnWriteArrayList#getArray and CopyOnWriteArrayList#setArray methods to access this field.

CopyOnWriteArrayList defines several overloaded constructors to initialize and construct a CopyOnWriteArrayList object, which is relatively simple to implement. Let's focus on the core method implementation declared by CopyOnWriteArrayList in the List interface, that is, add, remove, set, and get operations.

First, let's take a look at the operation of adding elements. CopyOnWriteArrayList defines the method implementation of several overloaded versions of CopyOnWriteArrayList#add, including:

CopyOnWriteArrayList#add (E)

CopyOnWriteArrayList#add (int, E)

CopyOnWriteArrayList#addIfAbsent (E)

CopyOnWriteArrayList#addAll (Collection

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