Does Using Volatile with Double-Checked Locking Solve Thread Safety Issues in Singleton Creation?

Double Checked Locking with Volatile

The double checked locking pattern is a common technique used in Java to ensure that a singleton object is only created once. This pattern involves checking if an instance of the singleton already exists before creating a new one. However, without proper synchronization, this can lead to race conditions where multiple threads may try to create the singleton simultaneously.

To address this issue, the volatile keyword is used. The volatile keyword ensures that the instance variable is visible to all threads, even if one thread modifies it. This prevents a thread from seeing a partially constructed instance.

Explanation of the Volatile Usage

In the given code example, the instance variable is declared as volatile because it is shared among multiple threads. When Thread A initializes the instance variable, it assigns memory space to it. However, Thread B may see this assignment and try to use the instance before Thread A finishes constructing it.

Without the volatile keyword, Thread B could see the assignment to instance but not the subsequent construction. This would result in Thread B using a partially constructed instance, leading to errors. The volatile keyword ensures that Thread B sees the most up-to-date value of instance, which includes any modifications made by Thread A.

Addressing the Performance Issue

Some may argue that the use of volatile defeats the purpose of double checked locking, which is to improve performance by avoiding unnecessary synchronization. However, the use of volatile is essential to guarantee thread safety and prevent race conditions.

In reality, the performance overhead of using volatile is typically negligible, especially compared to the cost of synchronizing the entire method. Modern hardware architectures implement memory barriers that ensure that any writes to volatile variables are made visible to other threads in a timely manner.

The above is the detailed content of Does Using Volatile with Double-Checked Locking Solve Thread Safety Issues in Singleton Creation?. For more information, please follow other related articles on the PHP Chinese website!