Definition: The singleton pattern is a creational design pattern that ensures a class has only one instance and provides a global point of access to it. This single instance is created by the class itself and is shared throughout the application. A class that follows this pattern is known as a singleton class. There are two main types of singleton patterns: the eager initialization and the lazy initialization.
The eager initialization approach creates the instance at the time the class is loaded, which guarantees thread safety but may consume memory even if the object is never used. On the other hand, the lazy initialization approach delays the creation of the instance until it is actually needed, which can save memory resources but requires careful handling to ensure thread safety.
The code for the hungry (eager) mode is as follows:
The code for the lazy mode is as follows:
The key difference between the two approaches lies in when the instance is created. In the eager mode, the instance is created when the class is loaded, making it thread-safe but potentially less efficient in terms of resource usage. In the lazy mode, the instance is created only when it's first requested, which saves memory but introduces the need for synchronization to prevent race conditions in multi-threaded environments.
To implement a thread-safe version of the lazy mode, several techniques can be used:
1. Use the `synchronized` keyword on the method that returns the instance. While this ensures thread safety, it can lead to performance issues due to the overhead of synchronization on every call.
2. Implement the double-check locking mechanism. This approach reduces the overhead of synchronization by checking the instance status before entering the synchronized block, and again inside the block to ensure the instance is only created once. This method is more efficient and widely used in practice.
The double-check locking mechanism works as follows: When a thread calls the `getInstance()` method, it first checks if the instance is already created without acquiring the lock. If not, it acquires the lock, then checks again before creating the instance. This prevents multiple threads from creating multiple instances while minimizing the use of synchronization.
In addition, the `volatile` keyword is used in the declaration of the singleton instance. This ensures that changes made by one thread to the variable are immediately visible to other threads, preventing potential issues caused by instruction reordering in multi-threaded environments.
The implementation of the double-check locking mechanism is shown below:
By using this technique, you can achieve both thread safety and efficiency, making it a preferred choice for implementing a lazy singleton in multi-threaded applications.
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