Detailed explanation of singleton pattern in Java design patterns

The singleton pattern in Java is implemented through the following steps: Create a private class constructor. Create a private static variable as a reference to the unique instance. Provide a public static method to obtain the instance. Declare static reference variables as final. Declare the implementation class as final.

Java Design Patterns: Singleton Pattern

Introduction

Singleton Pattern Is a way to ensure that there can only be one instance of a class in an application. It helps maintain state consistency, avoids unnecessary object creation, and simplifies management.

Implementation of the singleton pattern in Java

In Java, the singleton pattern can be implemented using the following steps:

1. Create a private constructor: Declare the class constructor as private to prevent the class from being instantiated from outside.
2. Create private static variables: Create private static variables of the class to be used as a reference to the only instance.
3. Provide a public static method: Provide a public static method to get the instance, if it has not been created yet, create the instance first.
4. Declare variables as final: Declare static reference variables as final to prevent them from being reallocated.
5. Declare the class as final: Declare the implementation class as final to prevent subclasses of the class from being created.

Sample code:

public class Singleton {

    private static final Singleton INSTANCE = new Singleton();

    private Singleton() {
        // 私有构造函数
    }

    public static Singleton getInstance() {
        return INSTANCE;
    }

}

Practical case

Consider a messaging application that manages user sessions. The application needs to maintain a unique reference to the current user in order to track their activities and preferences throughout the session. The singleton pattern can be used to maintain this unique user instance, as shown below:

public class UserManager {

    private static final UserManager INSTANCE = new UserManager();
    private User currentUser;

    private UserManager() {
        // 私有构造函数
    }

    public static UserManager getInstance() {
        return INSTANCE;
    }

    public void setCurrentUser(User user) {
        this.currentUser = user;
    }

    public User getCurrentUser() {
        return currentUser;
    }

}

By using the singleton pattern, the UserManager class can ensure that only one copy of the user session state is maintained and managed. This helps simplify user management and ensures consistency across applications.

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