代码文件目录：

 初始版本：

package Chapter5_SingletonPattern.origin;

/**
 * @Author 竹心
 * @Date 2023/8/5
 **/

public class Singleton {
    private static Singleton uniqueInstance;

    private Singleton(){}

    public static Singleton getInstance(){
        if(uniqueInstance == null){
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }

    //问题：多线程中会出现多个实例被创建的情况

    //other functions

}

三种解决多线程问题的方法：

package Chapter5_SingletonPattern.Multithread;

import Chapter5_SingletonPattern.origin.Singleton;

/**
 * @Author 竹心
 * @Date 2023/8/5
 **/

public class Singleton1 {
    private static Singleton1 uniqueInstance;

    private Singleton1(){}

    //使用synchronized迫使每个线程进入此方法之前必须轮到他
    //但是这样会导致性能开销大

    public static synchronized Singleton1 getInstance(){
        if(uniqueInstance == null){
            uniqueInstance = new Singleton1();
        }
        return uniqueInstance;
    }

    //other functions
}

package Chapter5_SingletonPattern.Multithread;

/**
 * @Author 竹心
 * @Date 2023/8/5
 **/

public class Singleton2 {
    //急切创建实例：在静态初始化器中就创建实例
    private static Singleton2 uniqueInstance = new Singleton2();

    private Singleton2(){}

    public static Singleton2 getInstance(){
        return uniqueInstance;
    }

    //other functions
}

package Chapter5_SingletonPattern.Multithread;

import Chapter5_SingletonPattern.origin.Singleton;

/**
 * @Author 竹心
 * @Date 2023/8/5
 **/

public class Singleton3 {

    //用双重检查加锁减少使用同步，降低开销

    private volatile static Singleton3 uniqueInstance;
    //volatile保证uniqueInstance为单实例类的时候多个线程正确处理

    private Singleton3(){}

    public static Singleton3 getInstance(){
        if(uniqueInstance == null){
            synchronized (Singleton3.class){
                if(uniqueInstance == null){//进入同步区块后，再检查一次
                    uniqueInstance = new Singleton3();
                }
            }
        }
        return uniqueInstance;
    }
}

Notes:

单件模式：
    确保一个类只有一个实例，并提供一个全局的访问点